Session VP1 - Poster Session X.
POSTER session, Thursday afternoon, October 26
Exhibit Hall AB, Qu\'{e}bec City Convention Centre

[VP1.001] Waves (DPP)

This abstract not available.

[VP1.002] Two-dimensional measurements of ion temperature in a helicon plasma source

Laser Induced Fluorescence (LIF) is used to measure the perpendicular velocity distribution of metastable ions in a helicon source plasma. In the WVU Hot hELIcon eXperiment (HELIX), an XY translation stage for LIF optics is used to obtain 2D measurements of ion temperature, density of metastable species, and transverse plasma flow. HELIX is a steady state helicon plasma source that can be operated under a wide range of conditions: RF power (0-2 kW), RF frequency (6-14 MHz), magnetic field (0-1.4kG) and pressure (1-10 mTorr). Under certain conditions, ion temperatures as high as 1.0 eV are observed in argon plasmas. We will present measurements of the ion temperature and metastable density profiles for the high ion temperature conditions. For these HELIX parameters, the metastable ion density is strongly dependent on electron density and weakly dependent on electron temperature. Thus, the metastable ion density profiles can provide qualitative measurements of the electron density profile.

[VP1.003] Rogue waves in plasmas

A basic problem in the study of nonlinear waves is to develop an understanding of the full range of behavior that is possible in a given physical circumstance. As a specific example: consider nonlinear wave trains subject to modulational instabilities. What is the long-term behavior of such systems? How likely are they to develop high amplitude localized transient pulses (which we call `rogue' waves)? Can these pulses be predicted in advance? Can they be controlled (e.g. can they be suppressed or shaped)? In this poster we report on recent developments in the analytical theory of unstable nonlinear waves. We focus attention on the nonlinear Schrodinger (NLS) equation (iq_t + q_xx + |q|^2q = 0) which is solvable via the Inverse Scattering Transformation (IST). Using the IST, a new class of `rogue wave' solutions of the NLS has been found (Osborne and Tracy, in preparation). Here we discuss the physical interpretation of these new solutions, the statistical distribution of rogue waves, and their possible control.

[VP1.004] Extraordinary/ordinary wave conversion via gyroballistic modes

Previous studies of gyroresonant damping of electron waves have overlooked the conversion mediated by the gyroballistic waves representing the perturbed electron distribution. When an extraordinary (X) electron wave (of given frequency), in a nonuniformly magnetized plasma, crosses the fundamental gyroresonance layer, it converts a fraction of its energy into a continuum of gyroballistic waves. These waves propagate primarily in k-space; thus they typically intersect the dispersion surface of the ordinary(O)-mode. They then undergo a second conversion to that mode. Thus there is a net X/O conversion, a two-step process mediated by gyroballistic waves. Using techniques developed by D R Cook et al [PhysLettA175(1993)326] for minority-ion processes, we derive an analytic expression for this conversion, valid when the coupling is weak. Analogous expressions are obtained for O/X conversion and for reflection. These results should be used to supplement the standard expressions [M Bornatici et al, NucFus23(1983)1153] for gyroresonant damping. Similar results can be obtained for harmonic gyroresonance and for conversion via the upper-hybrid Bernstein wave.

[VP1.005] Noise-driven instabilities in systems exhibiting pulse formation via passive mode-locking

We consider the stability and control of systems exhibiting pulse formation via passive mode-locking. Such pulse formation arises due to the combined effects of a spatially localized saturable gain region, cavity resonance effects that provide a mode selection mechanism, saturable absorption, and dispersion. Spontaneous pulse formation is exploited, for example, in the Colliding Pulse Mode-locked (CPM) laser which can generate ultra-fast sub-picosecond optical pulses. This combination of physical effects can also occur in plasmas (e.g. ion-cyclotron emissions in DT plasmas under fusion burn conditions) implying the potential for pulse formation in certain parameter regimes. In this poster we will discuss a simple model which captures many of the qualitative aspects of the dynamics, the effects of noise on instabilities, and symbolic time series methods for detecting precursors of the mode-locking/mode-unlocking transition.

[VP1.006] Ion acoustic wave damping with hybrid kinetic-hydrodynamic closure

A semi-analytic linear expression for the ion sound damping rate \gamma is developed that is valid over the whole range of collisionality regimes. We use a kinetic and fluid hybrid approach. Dissipative effects of ion-ion collisions and Landau wave-particle interactions are included through Chapman-Enskog like closures for parallel viscosity and heat flux. The complete linearized collision operator is truncated with 21 moments to ensure the correct Braginkii expression in the collisional limit. For practical purposes an approximate formula is obtained from the general expression for \gamma.

[VP1.007] Vlasov-Maxwell simulations of the strong wave-plasma interaction in a magnetized plasma

The interaction of a finite amplitude electrostatic mode with a collisionless magnetized plasma is investigated by means of a 1D2V Vlasov-Maxwell code which solves the Vlasov equations for electrons and ions, coupled self-consistenly with the field equations. The externally applied pump wave is a monochromatic mode propagating at right angle with the ambient uniform magnetic field, thus modeling the excitation of Bernstein waves during the electron cyclotron heating of a magnetized plasma.

The wave frequency is varied around the upper-hybrid resonance f_0 = \sqrtf_pe^2 + f_ce^2, in order to test the role of the finite electron temperature on the dispersion. Since the wave propagation is strictly perpendicular to the magnetic field , i.e. k_\parallel = 0, and the temperature is much lower than m_e c^2, no resonant electrons are present at the initial stage of the interaction.

The time evolution of the e.d.f., initially an isotropic Maxwellian, is investigated paying particular attention to the formation of transverse beams of accelerated electrons in the phase space, and to the corresponding hydrodynamic manifestations of the distortions of the e.d.f. The dynamics of the wavevector spectral densities of the electric field and of the fluid variables will be also investigated.

[VP1.008] Exact Damping for Relativistic Plasma Waves

The damping coefficient for a relativistic plasma may be reduced to a single integral with no approximations through use of the Newberger sum rules when k_z=0. Expanding the integral in a series, the leading term agrees with the leading term of the weak relativistic function F_7/2(z), but the remaining terms are not alike. The single expansion parameter is proportional to \lambda z, indicating that the result may NOT be accurately expressed as a series involving products of Bessel functions of argument \lambda times functions F_q(z). Expressions for the imaginary parts of all dielectric tensor elements will be presented. The real parts of the tensor elements are not as simple, but because the elements are analytic, they must likewise be modified.

[VP1.009] Study of electromagnetic wave propagation in a hot plasma in a 2-dimensional nonuniform magnetic field

The objective of this research is to develop a feasible method for the direct solution of the linearized Vlasov-Maxwell equations in an inhomogeneous magnetic field with one ignorable spatial coordinate in the ion cyclotron range of frequencies. A method of solving these equations by solving them as an initial value problem is studied. In this method the em waves are generated by a periodic source starting at t=0. Within a small Larmor radius approximation, it is computationally feasible to calculate the evolution of em fields up to t=10-20 ion cyclotron periods. The analysis is concentrated on the model with a cylindrical plasma in a tokamak-like magnetic field. The results of the study will be presented.

[VP1.010] Optimal Distributed Excitation of Surface Wave Plasmas

Surface wave sustained plasmas are an emerging technology for next generation sources for material processing. There is promise of producing high density, uniform sheath plasmas at low neutral pressures over large target surface areas. Such plasmas are being produced by distributed arrays of slot antennas by numerous groups. However, work remains to obtain the optimal surface wave frequency and wave vector for sustaining a plasma.

In this work, the optimal phase shift between slot antennas in a surface wave plasma is being sought using 2d3v PIC-MCC simulation. A long plasma loaded planar metal waveguide with a distributed exciting structure along one wall is modeled in these simulations. Of particular interest is the wave-particle interaction of electrons in the high energy tail of the velocity distribution (responsible for ionization in low pressure discharges) with driven low phase velocity (v \ll c) surface waves.

[VP1.011] Three Wave Interactions in a Weakly Collisional Plasma

Three wave interactions have been studied in a linear magnetized RF discharge plasma device. Laser induced flourescence is used to monitor the Ar II ion response to electrostatic ion cyclotron waves launched by a four-ring phased antenna. Waves are launched at parent frequencies ømega_1 and ømega_2 and measurements are taken of the plasma response at the daughter frequency ømega_3 = ømega_1 + ømega_2. We present analyses of the structure of the second order perturbed distribution function f_2 and of the influence of f_2 on wave-particle interactions.

[VP1.012] Nonlinear Magnetic Beach

The ion response to the rf-field in the magnetic beach problem can be essentially nonlinear. This paper presents a self-consistent theory of the rf-wave propagation and ion motion through the ion cyclotron resonance. An important ingredient of the problem is the ion flow along the magnetic field. The flow velocity limits the time the ions spend at the resonance, which in turn limits the ion energy gain. A feature that makes the problem nonlinear is that the flow accelerates under the effect of the grad B force and rf-pressure. This acceleration can produce a steep decrease in the plasma density at the resonance, resulting in partial reflection of the incident wave.

*Work supported by VASIMR project at NASA and by U.S. DOE Contract DE-FG03-96ER-54346.

[VP1.013] Waves (ICPP)

This abstract not available.

[VP1.014] Whistler Wave Emission by Modulated and Pulsed Electron Beams

[VP1.015] Global Drift Waves in Weakly Ionised Magnetized Plasmas: Confronting Theory and Observations

[VP1.016] Ion-Acoustic Waves Generated by Drift Instability in a Weakly Magnetized Plasma

Spontaneous excitation of ion acoustic waves has been investigated in a magnetic multi-pole plasma. The magnetic multi-pole devices are widely used in plasma experiments such as double plasma devices and negative ion source chambers. Such devices, using multi-cusp magnetic confinement at the chamber walls, can effectively produce quiescent steady plasma except for the vicinity of the multi-pole. It is known that the high-level density fluctuations are generated near the dipole or the multi-pole magnets due to the drift instability.

[VP1.017] Dielectric Characteristics of an Axisymmetric Low Aspect Ratio Tokamak with Circular Magnetic Surfaces

Kinetic wave theory in the low aspect ratio tokamaks should be based on the solution of Vlasov-Maxwell's equations. This problem is not simple even in the scope of linear theory since to solve the wave equations we should use a correct dielectric tensor valid for concrete plasma models. In this paper, transverse and longitudinal dielectric tensor elements are evaluated for radio-frequency waves in an axisymmetric tokamak with circular magnetic surfaces, without the smallness assumption of the inverse aspect ratio and poloidal magnetic field. Vlasov equation is solved separately for untrapped and trapped particles. Coordinate system with the ''straight'' magnetic field lines was used. Contributions of untrapped and trapped particles to the dielectric tensor components are expressed by the summation of bounce-resonant terms, which include a double integration in velocity space, resonant denominators, phase coefficients, standard elementary and elliptic functions. Permittivity elements, presented in the paper, are suitable to analyze the electromagnetic fields and their interaction with charged particles (e.g., during the plasma heating)in the frequency range of Alfvin, fast magnetosonic, ion-cyclotron and lower hybrid waves, for both the large and low aspect ratio tokamaks.

[VP1.018] Ship Waves Driven by the Poloidal Component of a Sheared Flow in a Tokamak Plasma

[VP1.019] Stability of Two Counterstreaming Ion Beams with Inhomogeneous Density Profiles

Inhomogeneity in plasma parameters can provide free energy to drive instabilities. We have evaluated the stability properties of plasma with two ion populations drifting oppositely along the magnetic field. The perpendicular density-profile maxima of the two populations are separated so that the density gradients in the overlapping region are of opposite sign. The goal is to sufficiently characterize the stability so that relevant laboratory experiments can be interpreted quantitatively and so that space-plasma modelers are aware of the distinction between using a single Maxwellian ion population whose drift speed varies perpendicular to the magnetic field, i.e., a case of parallel-velocity shear, and using two Maxwellian ion populations whose parallel drift speeds are uniform but whose density (zeroth moment) varies perpendicular to the magnetic field, i.e., a case of no parallel-velocity shear. We investigate the unstable modes over a range of beam parameters in the linear and nonlinear regimes. Applications to the experiments of Koepke, Zintl, Reynolds, Wang, and Good [Eos Trans. AGU, 81, S377, 2000] will be discussed.

[VP1.020] Inhomogeneous Plasma Parametric Decay Instability Driven by Frequency Modulated Pump

In the paper the inhomogeneous plasma decay instability suppression by frequency modulated pump is studied numerically in the framework of coupled equations for unstable wave amplitudes. It is shown that convective (spatial) amplification coefficient is non-sensitive to the stochastic pump frequency modulation, which leads only to broadening of the decay region. In the case of fast harmonic modulation the amplification coefficient is not affected as well, which is shown also analytically. However for slow modulation the enhancement of amplification is observed, which is interpreted in terms of suppression of convective losses. In the case of absolute decay instability, which was introduced into the system by the spatial harmonic variation of the phase mismatch, the saturation of instability was observed, but at pump frequency deviation much higher than the instability growth rate. It took place when the size of decay region became comparable to the size of feedback loop, leading to the absolute instability onset. In the case of slow harmonic pump frequency modulation the instability growth rate was enhanced. The obtained results of numerical computations are in agreement with recent experimental observations (Arkhipenko V.I. et al Proc. 25th EPS Conf. 1998, 22C, 2350).

[VP1.021] Importance of Neutral Dynamics in Partially Ionized Plasma

This paper deals with instabilities in partially ionized, inhomogeneous and magnetized plasma with neutral dynamics. It is shown that the neutrals must not be considered to form only a passive scattering background but instead, being a ``storehouse of momenta", they play an important role either to onset new instabilities or modify existing instabilities in partially ionized plasma. We derive the most general dispersion relation and have enunciated how significant the neutral dynamics in partially ionized plasma is. We analyze the dispersion relation for two extreme frequency windows with and without magnetic field for the cases of both loose and tight coupling between charged and neutral species. We discuss specific results for limiting cases that may be appropriate for applications to space and laboratory plasma phenomena.

[VP1.022] Dynamic Stabilization of Plasma Disruption Precursors on the HT-7 Superconducting Tokamak

[VP1.023] Drift-Alfven Instability in the Presence of Sheared Flows

In the present report the temporal evolution of the drift-Alfven instability in a sheared plasma flow is considered. The analysis is based on a system of four equations for the perturbed electrostatic and magnetic potentials and for the electron and ion pressure perturbations. The temporal evolution of the spatial Fourier modes of the perturbations is studied directly without the use of spectral expansions in time. The results provide a novel view into the dynamics of the drift-Alfven instability in plasmas with shear flow.

[VP1.024] Electromagnetic Instability Study in Nonuniform Streaming Electron Magnetohydrodynamics

We report on a local dispersion relation for electromagnetic modes in a non-uniform collisional magnetized electron plasma with fixed ion background, taking into account equilibrium magnetic field and pressure gradients, as well as impurity radiation losses. The dispersion relation is then analyzed both analytically as well as numerically. We found that for a low-b plasma, the principal source for the generation of unstable modes is the impurity radiation loss; whereas for a high-b plasma, the various effects such as the electron streaming, the electron-ion collisions, finite electron thermal conductivity, and impurity radiation losses are shown to be responsible for unstable perturbations. The results should be useful in the interpretation of nonthermal electromagnetic fluctuations in non-uniform collision-dominated magneto-plasmas with impurities.

[VP1.025] Observation of low frequency instability in a magnetised plasma

The instability excited in magnetised plasma is fundamental topic of interest in plasma science. These inherent fluctuations may grow due to the gradients in different plasma parameters such as density, temperature and magnetised field. In the present study an observation has been made of the low frequency instability in magnetised plasma with the density gradient in the radial direction while the external magnetic field is applied in the axial direction of a cylindrical plasma. Plasma is produced by hot cathode discharge between cathode filaments and a grounded grid. The instability is measured in front of the grid in the radial direction as well as axial direction with the help of a plane Langmuir probe. The instability is excited as soon as the magnetic is applied. A density gradient is found to exist in the radial direction. The frequency of the instability shows small variation with the magnetic field strength. The amplitude of the instability slowly increases toward the wall where the plasma density is minimum and the plasma space potential is higher. The instability occurs only in the uniform magnetic field region. Measured ion energy distribution shows the presence of an energetic ion beam in axial direction in the central region of the plasma. The excitation of this instability may be attributed to the combined effect of density gradient and magnetic field in perpendicular direction. A theoretical model is also developed to explain the observed phenomena.

[VP1.026] Instability in a Bounded Discharge Plasma: Simulation and Experiment

[VP1.027] Influence of Secondary Electrons on the Stability of the Plasma-Wall Transition

The one-dimensional unmagnetized plasma sheath and collisional presheath are investigated with respect to their stability in the presence of secondary electrons [1]. It is shown that in practically all current regimes the secondary electrons can cause strong oscillations. The amplitude of these oscillations depends on the damping (collision) processes in the plasma presheath and on the current to the wall (or, equivalently, the voltage drop in the sheath and presheath regions). Analytical results obtained are checked against Particle-in-Cell simulations.

[VP1.028] One Dimensional Model for Radiative Collapse of a Z-Pinch

The dynamics of radiative collapse for a Z-pinch is studied, under the Hall-magnetohydrodymanic approximation, for a one-dimensionalmodel. The plasma is assumed to be formed by fully ionized deuterium, and thermal bremssthralung to be the only source of radiation. Rather than assuming Bennet equilibrium, the model relies on equilibria obtained from integration of the MHD equations, for given current density profiles, under the assumption that the mass density remains radially constant. The evolution of the radius of the column is compared for parabolic and Bessel model current density profiles. Since the existence of current density profiles is subject to having consistent resistivity profiles, the evolution of such profiles is also studied.

[VP1.029] Turbulence and Transport (DPP)

This abstract not available.

[VP1.030] Suppression of runaway electron avalanches by radial diffusion

Runaway electrons are frequently generated in tokamak disruptions. In future, large tokamaks it is thought that close Coulomb collisions between thermal electrons and existing runaways can lead to catastrophic exponential multiplication of the latter -- a so-called runaway avalanche. The existing kinetic theory of this process assumes that there is no loss of runaway electrons. In practice, these particles undergo radial transport due to magnetic fluctuations and are thus imperfectly confined. We have calculated the reduction of the avalanche growth rate that this causes, both by an analytical approximation and by 3D Monte Carlo simulation of the avalanche kinetics in full toroidal geometry. In these calculations, it is important to account for the fact that the confinement of relativistic electrons progressively improves as they are accelerated by the electric field. As the poloidal magnetic flux that is available to induce an electric field is finite, avalanches are prevented altogether by sufficiently strong radial diffusion. The requisite magnetic fluctuation level is sensitive to the mode structure and the speed of the disruption, and can be estimated to be around \delta B/B \sim 10^-3 for JET and ITER-like parameters.

[VP1.031] Modifications to Neoclassical Theory in the H-mode Edge for Bootstrap Current Calculations

The bootstrap current in the boundary of low collisionality plasmas can play a significant role in MHD stability. This is particularly the case in H-mode plasmas that have strong edge pressure gradients. However, there are many physics effects in the plasma boundary that can modify the bootstrap current but are not included in standard neoclassical theory. We examine, using the NCLASS code(W.A. Houlberg, K.C. Shaing, S.P. Hirshman, M.C. Zarnstorff, Phys. Plasmas 4 (1997) 3230.), the effects of models for ion orbit loss, charge exchange damping, and non-linear corrections to the plasma viscosity as the poloidal Mach number approaches or exceeds unity. Bootstrap current measurements near the plasma edge provide a means of validating the modifications to the theory, which are also key elements in the L-H transition model based on ion orbit loss. We present a summary of the models and example applications.

[VP1.032] Neoclassical Radial Electric Field and Transport with Finite Orbits

A low noise \delta f simulation is applied to study the self-consistent electric field and its effect on neoclassical transport. A considerable self-collision driven ion flux is found in the region within the inner half radius, resulting in the broken ambipolarity. Enforcing the ambipolarity condition requires the development of a radial electric field. Small orbit width analysis finds that the time scale for the development of the neoclassical electric field is \tau_ii(L/\rho_i\theta)^2. In improved confinement modes where the average ion orbit width (\rho_i\theta) is comparable to the plasma gradient length (L), this could be as fast as an ion collision time. By following the electric field dynamics, an equilibrium state with vanishing ion flux is established and the self-consistent steady-state electric field is obtained. Because of the finite orbit effect, the electric field is non-locally dependent on density and temperature profiles. With the electric field, ion energy flux near magnetic axis is largely reduced due to vanishing ion flux. Ion heat flux seems to be less affected. The comparison to the neoclassical theory will be presented and the difference between present results and previous work will be discussed.

[VP1.033] A Scaling Theory for Effect of Strong Flow Shear on Cross-Phase

Theories that calculate the effect of flow shear on the cross phase between transport-causing fluctuations have generally dealt with multiple inhomogeneities of undetermined scale [1]. To treat this complication, the theories have restricted themselves to shear values that are significantly lower than those of experiment. We address here the strong shear limit for cross phase effects by constructing a scaling theory for the conditions of the original scaling theory of amplitude suppression [2], i.e., a passive scalar advected by ExB flow in a plasma whose only inhomogeneity is the mean flow. We show that correlation between the scalar and the potential has a stronger dependence on inverse shear strength than does the product of the amplitudes, indicating that the cosine of the phase angle is reduced by flow shear. We also examine a scalar with a mean gradient and calculate self-consistently the flux that drives transport of the mean scalar. 1. A.S. Ware, et al., Phys. Plasmas 5, 173 (1998). 2. H. Biglari, et al., Phys. Fluids B 2, 1 (1990).

[VP1.034] Theory of Multifield Bispectral Deconvolution

Bispectral deconvolution is a method for inferring linear growth rates from third-order moments (bispectrum) of a turbulent system. This allows a direct comparison of theoretical instability growth rates with experimental data. However, in systems with multiple fields of comparable importance, the linear growth rate may bear little resemblance to the rate at which energy is injected into turbulent modes. This nonlinear instability can arise from distortion of linear eigenmode structure by common nonlinearities. Thus, a linear growth rate inferred from single field measurement under bispectral deconvolution may have little relationship to either the turbulent spectrum and transport rates, or to the linear growth rate of models with multifield dynamics. Multifield bispectral analysis addresses this problem by using third-order moments from multiple-field measurements to infer the full linear coefficients of the system. From these we show that we can derive both linear and nonlinear growth rates and identify different nonlinear coupling mechanisms by their field signature. The method for inferring these quantities will be presented and assessed.

[VP1.035] The Hurst Exponent and Long-time Correlation

Important aspects of plasma transport in magnetically confined plasmas remain a mystery in modern plasma physics research. Self-organized-criticality (SOC) has been suggested recently as an important contributing mechanism. To identify the existence of transport by avalanches, which is characteristic of SOC systems, the rescaled-range-statistics (R/S) method is invoked [B. A. Carreras et al., Phys. Plasmas 10, 3632 (1998)] to detect the long-time correlation induced by an avalanche-type transport. In their work, the Hurst exponent (H) in several devices is calculated to be well above 0.5, which is interpreted to indicate the existence of long-time correlation in the plasma edge turbulence. We apply the R/S method to the ion-saturation-current fluctuations at the edge of the tokamak Tore Supra to evaluate H. Data-block randomization is carried out on the data set in order to investigate the relationship between the Hurst exponent and long-time correlation. It is observed that H is well above 0.5 in the long-time self-similar range. However, the information which leads to H>0.5 is found to be entirely contained in the short time correlation and no link to long times is found. This brings into question the use of the SOC mechanism to interpret plasma edge turbulence.

[VP1.036] Anomalous Ion Loss from Stochastic Collisional Ripple Transport in Tokamaks

The ripple-induced banana tip motion is subject not only to a standard mapping but also to a web mapping. Since the web mapping yields greater radial excursions to the banana tip motion, the existence of the web structure has a significant influence on the collisional radial transport. Above the collisionless stochasticity threshold, the radial confinement is lost since the ion orbit becomes stochastic due to magnetic shear. Below the collisionless stochasticity threshold, the Coulomb pitch angle scattering can yield an enhanced radial random walk of the banana tips over the mixed web and standard mapping structure. The resulting enhancement in the radial ion transport is much greater than those previously realized by the ripple-plateau phenomenon [Boozer, Kolesnichenko]. There are two immediate impacts on the tokamak fusion plasmas. The first one is the enhanced helium ash removal rate through the plasma edge where the existence of the ripple is natural and where an H-mode can suppress the turbulence transport. The hot heliium ash particles transported to the edge are below the collisionless stochasticity threshold but can still be subject to the enhanced collisional ripple loss. The scond one is the enhanced main ion thermal transport rate toward the tokamak edge, due to the collisional ripple transport of the tail ions. Comparison with existing experimental data will also be presented.

[VP1.037] Shear Flow Generation by Finite Beta Drift Waves and Kinetic Shear Alfven Waves

The generation of shear flow is an important issue for understanding the variety of good confined modes observed in tokamak plasmas. We have developed a linear theory to investigate the instability which leads to the generation of shear flow by finite beta drift and kinetic shear Alfven waves. The finite beta effect is found to significantly modify the stability boundary for the shear flow instability for modest plasma betas. The finite beta effect introduces three new features: (1) a modification to the electrostatic drift wave dispersion relation (2) coupling to zonal magnetic fields besides the zonal flow and the side bands, and (3) introduces the kinetic shear Alfven branches. We will present results which address the consequence of each of these effects on the generation of shear flow. A low-dimensional nonlinear model will also be presented. The implication of our results to recent simulations of L-H transitions will be discussed.

[VP1.038] Initiation and Controlled Sustainment of Dynamic Transport Barriers

A wide variety of magnetic confinement devices have seen transitions to an enhanced confinement regime. A simple model incorporating the nonlinear interactions between the turbulent fluctuations and the sheared radial electric field coupled to a transport model is able to capture much of the observed dynamics[1]. In addition to using beams and flows to trigger and maintain the barriers, pellet injection, alone or with RF heating, can be used, as can pulses from the edge such as H-L transitions or cold pulses. This, added to evolving flows and beam deposition, allows a wider variety of dynamics to be investigated and more initiation/control schemes to be explored. The transition dynamics are seen to be impacted by the alignment of the various components in the radial electric field, such as the competition between the flow components and the pressure gradient component. Implications of these calculations for barrier initiation and control are discussed. 1 D. E. Newman, B. A. Carreras, D. Lopez-Bruna, P. H. Diamond, and V. B. Lebedev, Phys. Plasmas, 5 (4) 938-952 (1998).

[VP1.039] Comparison of Observed Toroidal Rotation with Neoclassical Transport Theory

Toroidal rotations have been observed in Ohmic and ICRF discharges(J.E. Rice et al.), Nucl.\ Fusion 39 (1999) 1175.\ which have little overall momentum input. They are found to correlate with the thermal energy content and the magnitude of the plasma current and change sign relative to the plasma current in different conditions. Existing comparisons with neoclassical transport theory either focus on the relation of the rotation with the radial electric field or fail to use the full expression of the angular momentum flux. We seek to remedy this by invoking the correct expressions(M.N. Rosenbluth et al.), Plasma Phys.\ Contr.\ Nucl.\ Fusion Research (IAEA, Vienna, 1971), Vol.~1, p.~495.^,(R.D. Hazeltine, Phys.\ Fluids 17) (1974) 961.^,(F.L. Hinton and S.K. Wong, Phys.\ Fluids 28) (1985) 3082.\ which contain both diffusive and non-diffusive terms. Developmental work is performed to consider such issues as the presence of impurity ions, the occurrence of near-sonic flows, and the lack of up-down symmetry of flux surfaces. Comparison with experiments will be presented.

[VP1.040] Transport Barrier Under The Conservation Of Total Angular Momentum

Transport barriers in axisymmetric systems such as tokamaks have been investigated under the conservation of total angular momentum. The results [1] show that a plasma profile is strongly constrained by the conservation of total angular momentum when a plasma rotation is large. The profiles do not change unless the total angular momentum decays. A plasma prefers to take a profile which correspond to the maximum total angular momentum for a given rotation energy. This paper first discuss the meaning of this constraint. Then, we report comparison of actual tokamak profiles and the profiles dictated by the theory, and discuss possible reasons for some discrepancies. We also describe effects of radial electric fields on the profiles and the conditions that the constraint due to the total angular momentum conservation is effective. [1] Paper CN-77 TH4/3 presented at the 18th IAEA Fusion Energy Conference (Sorrento, Italy, October 2000).

[VP1.041] Taming drift wave turbulence: experiment and simulation

The dynamics of drift waves in magnetized plasmas is mostly more or less turbulent. Drift waves are candidates for causing anomalous cross-field particle transport. It is thus an appealing goal to achieve active control over the turbulent drift wave dynamics. This is a spatiotemporal problem, and an open-loop control scheme is chosen acting both in space and in time. Experiments have been performed in the linear, magnetized, low-\beta plasma experiment MIRABELLE (LPMI Nancy, France). For the synchronization of drift waves, an azimuthal arrangement of eight electrodes (octupole exciter) is placed in flush-mounted geometry in the edge region of the plasma column. The electrodes of the octupole exciter are driven by sinusoidal signals with a fixed phase angle between neighbouring electrodes. This makes it possible to preselect the azimuthal mode structure (m=1,2,3) of drift waves. It is demonstrated that the periodically forced octupole exciter is able to synchronize both regular and turbulent drift wave states. The efficiency of the spatio-temporal synchronisation depends on the frequency and on the phase shift of the exciter signals. The experimental observations agree very well with computer simulations of a Hasegawa-Wakatani model in cylindrical geometry, where the octupole exciter is assumed to create a azimuthally rotating, mode structured current profile.

[VP1.042] Initial Results from the Controlled Shear Decorrelation Experiment (CSDX)

The controlled shear de-correlation experiment (CSDX) has been designed to study the effect of sheared flows on: (1) known linear pressure gradient-driven drift and/or effective gravity-driven flute eigenmodes; (2) the nonlinear three-wave coupling of a finite number of large amplitude coherent modes; and (3) on the rate of electrostatic turbulent fluctuation energy cascades. In CSDX the plasma state (i.e. quiescent, single small-amplitude drift wave, nonlinearly coupled modes, or strongly turbulent) is controlled by varying the magnetic field strength, collisionality, parallel current, and/or effective gravity due to solid-body plasma rotation driven by azimuthal ExB drifts. The radial electric field strength and shear rate is controlled independently of the plasma state by the application of externally controlled voltages to concentric annular rings which will form the ends of the experimental region. Machine construction has been completed and initial results demonstrating all of these capabilities are summarized in this paper. From the fluid equations we show that bispectral analysis of turbulence-scaled processes can yield insight into the key nonlinear physics of shear flow-fluctuation interactions. Initial results and plans using this technique on CSDX data will be discussed.

[VP1.043] Experimental study of the effect of velocity shear upon small amplitude coherent waves in a quiescent cylindrical plasma

A new experimental investigation of the effect of velocity shear upon linear resistive drift waves has begun on the Controlled Shear Decorrelation Experiment (CSDX) at UCSD. In this experiment a quiescent helicon plasma source generates a cylindrical plasma which is ~2.5 meters long and 0.2 meters in diameter. As the axial magnetic field is increased, finite larmor radius viscosity becomes small and, as a result, electron-ion and electron-neutral collisions can destabilize small amplitude coherent modes which appear to be resistive drift waves. A radially distributed set of concentric biased semi-transparent meshes are then used to impose a plasma potential profile. The effect of the resulting azimuthal ExB plasma drift upon the linear resistive wave will then be studied. In this paper we will compare experimental measurements of small amplitude coherent oscillations with the linear resistive drift eigenmode. The effects of an externally imposed radially sheared azimuthal flow on these fluctuations will be discussed.

[VP1.044] Experimental Studies of Nonlinear Shear Flow-Turbulence Interactions Using

Transport barrier formation in the core region of magnetized fusion plasmas is thought to arise from turbulence-generated ExB shear. A similar process is thought to govern the generation of shear flows in rotating planets and stars. Thus studies of nonlinear shear flow-wave dynamics may yield important results for a broad range of 2-D fluid dynamical problems. In this paper a nonlinear bispectral signal processing techniques are used to study the nonlinear dynamics of shear flow-wave interactions. These techniques are being applied to archived data from the L-H transitions of several tokamaks. First results from this analysis will be presented in this paper. In addition a small-scale experiment has been built at UCSD (The Controlled Shear De-correlation Experiment: CSDX) in which the fluctuation drive and amplitude, the flow shear, and the magnetic shear can be independently varied. Experiments on CSDX, in which an externally driven shear flow is used to modify the nonlinear interaction between large-amplitude coherent modes, are being performed. An initial examination of these nonlinear interactions using bispectral techniques will be presented.

Word supported by US-DOE DE-FG0399ER-54553 and DEFG-03-95ER-54924.

[VP1.045] Sheared Toroidal Flow Effects on Microturbulence in Electric Tokamak Plasmas

The effect of sheared toroidal flow on ion temperature gradient driven turbulence (ITGDT) in a large aspect ratio tokamak such as the Electric Tokamak at UCLA is being investigated using global nonlinear toroidal three-dimensional gyrokinetic particle-in-cell calculations which incorporate the entire plasma cross section. This study complements previous work on the effect of sheared poloidal flows on ITGDT [1,2] and the same radial dependence for the flow is used in both cases. Preliminary results indicate that toroidal flow magnitudes on the order of 1/10 of the Alfven speed are needed to globally affect the turbulence and that local suppression occurs where velocity shear is largest. Results of systematic scans with various global and local flow profiles and magnitudes will be reported and analyzed.

[VP1.046] Helimac -- A Model Device for Plasma Turbulence

The Helimac is a simplified model system for the study of drift-wave plasma turbulence. It approximates an infinite cylinder with an MHD equilibrium that depends on a single radial variable. The magnetic field lines are helices of tight pitch. The configuration is thus 1-D with magnetic curvature and shear, and flow shear can be externally applied and controlled. It is analogous to typical SOL plasmas and a model for collisional drift-wave regimes. The device to be built will feature continuous operation and full electrostatic probe diagnostics for complete characterization of the turbulence. The experimental design and plans for turbulence measurement and flow-shear suppression will be presented. With an average radius of 1 m and height of 2 m, the size is very large compared with all scale lengths at the field of 0.1 T, temperatures of 10 eV, and densities of 10^17 m^-3. Previous experiments on ECH breakdown for tokamak discharges have established the typical plasma parameters and presence of turbulence.

[VP1.047] Status of the National Transport Code Collaboration Modules Library

A library of code modules is being developed under the auspices of the National Transport Code Collaboration (NTCC). Code modules are software packages which implement numerical physics models, or, perform ancillary functions such as i/o or graphics, or, provide tools for dealing with common issues in scientific programming, such as portability of fortran codes. Code modules are submitted by researchers in the plasma physics community, and then reviewed for adherance to programming and documentation standards. All code modules come with source code and clear instructions for compilation of binaries on a variety of target architectures. All modules library codes and ancillary information such as current standards are available from the NTCC Modules Library website: http://w3.pppl.gov/NTCC. The goal of the project is to develop a resource of value to builders of transport codes and indeed to plasma physics researchers generally. This poster reports on the status of the project, including a complete inventory of available codes and a list of new codes added this year. [...work funded by U.S. DoE]

[VP1.048] New Features Implemented in National Transport Code Collaboration Java Client

The NTCC Demonstration Project Java client provides a user interface to the NTCC data and physics servers. In our recent work we have enhanced its features by providing capabilities to save the physics configuration in a local file, print plots, and cut and paste text, as well as improved the style and ease-of-use for the user. The configuration file specifies the modeled machine, database, shot number, run id, variables chosen to be advanced, choices in transport models, grid parameters, attributes of the solver, radial and time plots. This configuration is saved in XML format and can be read upon the start of a new session. The user is also now able to save the time-slice information in NetCDF format, allowing for easy benchmarking of the results. In addition, new style features such as combo boxes, modal dialogs preventing overlapping changes, and button disabling forcing users to take valid paths, have been implemented.

[VP1.049] Documentation and Testing of the NTCC Physics Server

The transport equations and their connection to transport models in the National Transport Code Collaboration (NTCC) physics server are described and tested. Documentation is presented for the flux-surface-averaged transport equations that have been implemented in the physics server to predict the time evolution of electron and ion temperature, toroidal velocity, and turbulence energy density as a function of radius.(G.~Bateman, ``US National Transport Code Collaboration,'' 17th Int. Conf. on Numerical Simulation of Plasmas, Banff, Canada, May 2000.) The documentation also describes the connection between these transport equations and a variety of theory-based transport models such as IFS/PPPL, GLF23, Multi-Mode (MMM95), and NCLASS (for neoclassical transport). Options are described, including flow shear stabilization, which is believed to be responsible for internal transport barriers. The numerical stability of the physics server is examined. Simulation results are compared with other codes and with experimental data for a variety of tokamak discharges.

[VP1.050] Benchmarking and Application of NTCC Python Physics Server

The Python-based physics server, PYNTD, a component of the National Transport Code Collaboration's Demonstration Project, advances transport equations for any combination of electron and ion pressure, rotation, a turbulent energy density field, and (by the meeting date) density, with the remaining fields read in from a database. PYNTD has undergone benchmarking tests: simple analytic source and diffusivity models for which analytic solutions exist, as well as comparisons with other codes and experimental data for a selection of specific shots and anomalous transport models. In addition the code is being exercised to model experiments in DIII-D in which neon injection has been observed to facilitate transport barrier formation. Impurity effects on the turbulent transport, ExB velocity shear, and radiative cooling are modeled. Results from the benchmarks and impurity injection simulations will be presented, as well as a report on code status.

[VP1.051] Improvements to the National Transport Code Collaboration Data Server

The data server of the NTCC Demonstration Project provides a universal network interface to interpolated or raw transport data accessible by a universal set of names. Data can be acquired from a local copy of the Iternational Multi-Tokamak (ITER) profile database as well as from TRANSP trees of MDS Plus data systems on the net. Data is provided to the user's network client via a CORBA interface, thus providing stateful data server instances, which have the advantage of remembering the desired interpolation, data set, etc. This paper will review the status and discuss the recent improvements made to the data server, such as the addition of data file preprocessing and data caching that significantly speeds up data access, the addition of more missing data rules, and the improved interpolation flags.

[VP1.052] Lower Hybrid Current Drive and Heating for the National Transport Code Collaboration

The Lower hybrid Simulation Code LSC was originally written as a subroutine to the Toroidal Simulation Code TSC (Jardin, Pomphrey, Kessel, et al) and subsequently ported to a subroutine of TRANSP\@. Modifications to simplify the use of the LSC both as a callable module, and also independently of larger transport codes, and improve the documentation have been undertaken with the goal of installing LSC in the NTCC library. The physical model, which includes ray tracing from a Brambilla spectrum, 1D Fokker-Planck development of the electron distribution, the Karney-Fisch treatment of the electric field, heuristic diffusion of current and power and wall scattering, has not been changed. The computational approach is to suppress or remove from the control of the user numerical parameters such as step size and number of iterations while changing some code to be extremely stable in varied conditions. Essential graphics are now output as \tt gnuplot commands and data for off-line post processing, but the original outputs to \tt sglib are retained as an option. Examples of output are shown.

[VP1.053] Tearing Mode Stability of Evolving Toroidal Equilibria

There are a number of toroidal equilibrium (such as JSOLVER, ESC, EFIT, and VMEC) and transport codes (such as TRANSP, BALDUR, and TSC) in our community that utilize differing equilibrium representations. There are also many heating and current drive (LSC and TORRAY), and stability (PEST1-3, GATO, NOVA, MARS, DCON, M3D) codes that require this equilibrium information. In an effort to provide seamless compatibility between the codes that produce and need these equilibria, we have developed two Fortran 90 modules, MEQ and XPLASMA, that serve as common interfaces between these two classes of codes. XPLASMA provides a common equilibrium representation for the heating and current drive applications while MEQ provides common equilibrium and associated metric information needed by MHD stability codes. We illustrate the utility of this approach by presenting results of PEST-3 tearing stability calculations of an NSTX discharge performed on profiles provided by the TRANSP code. Using the MEQ module, the TRANSP equilibrium data are stored in a Fortran 90 derived type and passed to PEST3 as a subroutine argument. All calculations are performed on the fly, as the profiles evolve.

[VP1.054] Efficient Computational Techniques for Toroidal Plasma Geometry in Transport Simulations

Efficient algorithms for flux surface averaging and line tracking through a plasma are essential in many applications, e.g., pellet injection, beam deposition, current drive,geometry for transport equations and unfolding diagnostic measurements. These calculations may be fairly complicated and time consuming for more complex geometries.

In this paper, an extensively revised version of the line tracking code TRACK( Attenberger, S., Houlberg, W., and Hirshman, S., J.~Comput.~Phys., 72), 435 (1987) is presented. Much of the original functionality has been improved with significant gains in speed. An efficient 3d flux surface averaging module, using an optimized grid, has been added to the suite of routines. The implementation, testing and application of the routines to the PELLET, BEAM and THRIFT (magnetic flux evolution) codes, in both 2d and 3d is discussed and illustrated. In addition, the related issue of obtaining flux surface averages from a \Psi(R,Z) representation is discussed in terms of a coordinate transform optimizing the averaging process.

The PELLET and the modified TRACK modules are being prepared for submission to the NTCC module library.

[VP1.055] A Quasi-linear Hyper-resistivity Theory with Applications to the RFP and Spheromak

Hyper-resistivity, i.e., a current-diffusion term in Ohm's law, is a frequently used model for describing the effects of magnetic perturbations due to magnetic islands or low-amplitude turbulent fields on current transport across mean magnetic surfaces. This process is important in the magnetic-field evolution of toroidal devices such as the spheromak and the RFP and can provide a mechanism for sustained operation near relaxed states (flat J||/B). In a recent paper we presented a quasi-linear formalism for calculating the hyper-resistivity coefficient due to magnetic islands in a cylinder [1]. Here we report further progress with that formalism, including a treatment of various collisional regimes which recovers appropriate scalings in limiting cases. In addition this hyper-resistive effect is being added to Ohm's law in LLNL's transport code Corsica. We will attempt to present comparisons of results obtained with this code at large aspect-ratio with a new resistive MHD code using a spectral representation for a cylinder. We plan to compare these results with simulations using the DEBS code. Finally we present Corsica applications to the MST RFP, and we present edge-current driven current transport simulations of the SSPX spheromak experiment at LLNL. [1] Berk et al., Int. Sherwood Fus. Theory Meeting (March, 2000), paper 2C41.

[VP1.056] Turbulence and Transport (ICPP)

This abstract not available.

[VP1.057] Coupled Electron and Ion Heat Fluxes in the Magnetized Plasmas

[VP1.058] Modulational Instability of Drift Wave Turbulence in the Presence of MARFE in a Tokamak Edge

[VP1.059] Nonlinear Simulations of Alpha-Particle-Driven TAE

Alpha-particle-driven toroidicity-induced Alfven eigenmodes (TAEs) were first observed in the Tokamak Fusion Test Reactor (TFTR) [Phys. Rev. Lett. 78, 2976 (1997)]. It should be noted that the estimated amplitudes of the TAEs were very small, and they persisted much longer than the typical damping time. We carry out five dimensional Fokker-Planck simulation of the alpha-particle-driven n=4 TAE in TFTR shot #103101. The parameters and the initial alpha-paritcle distribution are taken to be consistent with the experiment. The simulation is carried out with realistic collisional rates, the pitch-angle scattering rate of 1.0 [1/s] and the slowing-down rate of 2.8 [1/s]. The mode damping rate is chosen to be half of the linear growth rate. The linear growth rate is in good agreement with the NOVA-K analysis result [Phys. Plasmas 5, 4284 (1998)]. We confirm that the distribution function in the linear growth stage is consistent with the side-band resonance condition. In the simulation results the amplitude decreases monotonically after saturation. The TAE never stays at steady amplitude. This is inconsistent with the experimental results and the particle simulation results [Phys. Plasmas 6, 226 (1999)]. The reason of this discrepancy is discussed.

[VP1.060] Aspect Ratio Effect in Collisional Transport for Toroidal Configurations

In previous papers, collisional transport has been carried out for plasma configurations of any shape. In this paper, we calculate the average velocity around a magnetic surface of elliptical form with triangularity and Grad-Shafranov shift as a function of the aspect ratio. Large, medium and tight aspect ratio have been considered in our numerical and theoretical model, with the same ellipticity, triangularity and Grad-Shafranov shift. Our results show the advantage of compact toroids with respect to tokamaks of tight and large aspect ratio. If we keep fixed the aspect ratio, triangularity and Grad-Shafranov shift, and we change the elongation K, an optimum value of the elongation can be reached, where a minimum of the average velocity appears. This happens only for large aspect ratio. For values smaller than 3.5, no minimum appears. We have also analyzed the contribution due to the inductive electric field. This contribution has been also studied numerically. We have compared the diffusion due to the toroidal electric field with that depending on the pressure gradient, which is usually much larger. The regions where this field contribution can be neglected have been determined, and its importance in each case has been established.

[VP1.061] Drift Eigenmodes in Tokamak Plasma Rotating both Toroidally and Poloidally

This abstract not available

[VP1.062] Radial Electric Field Generation During Anomalous ExB Impurity Diffusion

[VP1.063] Visualization of the Ponderomotive Force During Electron Cyclotron Resonant Wave Absorption and the Resulting Poloidal Plasma Rotation

High power EC plasma heating is being used in several experiments, and it is reasonable to expect nonlinear effects to be of importance at these high wave amplitudes, such as the poderomotive force. On the other hand, plasma rotation has been recognized as a key element in producing improved confinement in toroidal plasmas and it has been observed in some experiments that poloidal plasma rotation often arises during EC heating. We have proposed [1] a mechanism that produces poloidal plasma rotation under the action of the ponderomotive force associated with EC waves absorbed at the resonant surface. In the present work we show, by directly following particle motion, how the required ponderomotive force originates in a magnetized plasma. We analize both O- and X-mode waves, near the resonant surface and determine the magnitude and properties of the ponderomotive force, by doing the appropriate time-averaging. Using these results we can estimate the required wave amplitude for the ponderomotive force to be important in producing plasma rotation. Our numerical computations provide a visualization of the poderomotive force through its effect on particle motion. [1] J.J. Martinell, C. Gutierrez-Tapia, 27th EPS Conference, Budapest, P2.076 (2000).

[VP1.064] ICRH Induced Neoclassical Poloidal Plasma Rotation in Tokamaks

In the paper, we shall consider the neoclassical poloidal rotation of a tokamak plasma heated by microwaves in the range of ion cyclotron frequency (ICRH). In this case, the results from the usual neoclassical transport theory are no more valid since the external source might be strong enough to compete with collisions thus leading to a local equilibrium distribution significantly distorted from the Maxwellian. We show that the global approach based on the moment expansion can be adapted to cover the case of non-Maxwellian local distribution function solution of the bounce averaged Fokker-Planck kinetic equation including collision and quasilinear RF operators. An explicit expression of the poloidal plasma rotation is derived. The new features are discussed in detail.

[VP1.065] General Spherical Torus (DPP)

This abstract not available.

[VP1.066] Ion behaviour in MAST plasmas

Ion heating and the energy distribution of the fast ions (0.5<A(amu)E(keV)<600) are investigated using a 78 channel E\|B dual mass Neutral Particle Analyzer (NPA) diagnostic. In a representative NBI heated discharge, T_i increases significantly, approximately doubling its value in a time \Delta t~20-30ms. In plasmas where T_i \sim T_e, \tau_E,i may be estimated from the decay of T_i when the beam is terminated and the fast ion density is much reduced. To determine \tau_E,i during NBI the LOCUST Monte-Carlo code is used which includes full treatment of orbits, Coulomb scattering and all known relevant particle loss mechanisms. High energy ion tail formation is observed and usually accompanied by occurrence of sawteeth and modes during NBI. Results from the NPA were compared with those from a 20 chord charge-exchange Doppler spectrometer (C^5+ 529nm). There is some evidence of higher Ti from NPA measurements during ion tail formation which is being investigated

[VP1.067] Characteristics of High Performance OH Plasmas and Program Plans for Pegasus

The present campaign in the Pegasus Toroidal Experiment concentrates on the equilibrium and stability characteristics of ultra-low aspect ratio plasmas at high toroidal beta. At A < 1.2, high beta is readily achieved with ohmic heating while auxiliary heating provides access to lower collisionality and higher normalized beta. Discharges studied are formed with the first stage of the ohmic heating power supply only. For A = 1.1-1.4, I_p \leq 0.1 MA, and B_t \leq 0.07 T, magnetic equilibrium reconstruction gives \beta_t up to 22%, \beta_N to 5, \beta_p to 0.6, and q(a) down to 7. No limits in stability are yet evident; these discharges are limited in current and pulse length by the OH power supply. Stored energies are consistent with values expected from the ITER98pby1 confinement scaling which best described the START plasmas. Projections to full operation (0.3 MA) indicate access to \beta_t > 50% with ohmic heating only. The OH power supply upgrade is complete and the next campaign will extend to full pulse length and current. Initial tests of the 1-2 MW RF heating system and an increased diagnostic set are also scheduled.

[VP1.068] Operational Limits and MHD Activity at Near-Unity Aspect Ratio

Experiments on the PEGASUS Toroidal Experiment are focusing on the limits of plasma operation at ultralow aspect ratio (A<1.2). Plasmas have been produced with I_p/I_TF=1, with q_98 \approx 4-6; no external kink stability boundary is evident at that level. Studies of the q-limit have been constrained by the incomplete status of the OH power supply; the recent completion of this supply allows further exploration of this limit. Electron densities approaching the Greenwald limit have been observed. An n=1 mode is often observed during startup and is commonly associated with fast current ramps (>30 MA/s). This mode often limits the discharge evolution. There is also evidence that double tearing modes are present during the current ramp. Internal reconnection events have been observed with characteristics similar to those observed on other ST experiments.

[VP1.069] Magnetic Reconstruction of PEGASUS Equilibria

Magnetic equilibrium reconstruction on the PEGASUS Toroidal Experiment is a crucial tool to determine macroscopic plasma parameters such as geometry, l_i, beta, and edge q. To date, plasmas on the order of 0.1 MA with aspect ratios from 1.1 to 1.4, elongations from 1 to >3, l_is in the range 0.3 to 0.7, and \beta_ts on the order of 0.2 at toroidal fields less than 0.07 T have been analyzed with the TokaMac magnetic reconstruction code. To support a wide variety of diagnostic measurements and plasma profile models, a code using a nonlinear least-squares fitting routine coupled to a Grad-Shafranov solver has been developed. Induced currents are estimated using a time-evolving current filament model and are constrained during the reconstruction using wall mounted flux loops. A scan of model plasma parameters has been employed to determine a minimal array of magnetic diagnostics for accurately characterizing the plasma equilibrium and induced wall currents. This set includes a poloidal array of 13 magnetic pickup coils , 9 plasma flux loops, and a Rogowski loop for the toroidal plasma current.

[VP1.070] The Soft X-ray Flux Surface Shape Diagnostic on the PEGASUS Toroidal Experiment

Flux surface shape information can be used as a constraint for reconstruction of the plasma current and q profiles for shaped, low-aspect ratio toroidal devices. To exploit this sensitivity of shape to plasma profiles, an equilibrium code was developed to incorporate flux surface shape information from linearized chordal soft X-ray (SXR) measurement inputs from a 2-D tangentially viewing pinhole camera. This equilibrium code uses a nonlinear least squares fitting to facilitate the inclusion of the SXR measurement constraints. The pinhole camera consists of a 2mm diameter pinhole projection onto a 150mm diameter viewport coated with a high-efficiency phosphor, P43. A 40mm diameter MCP image intensifier is lens coupled to the phosphor for maximum sensitivity, resulting in a signal to noise ratio of <1% from photon statistics. The MCP is optically coupled to a fast framing CCD camera allowing for frame rates of 1000 frames/sec. and exposure times < 100 \mus. First results from a prototype system will be achieved with a commercial intensified camera.

[VP1.071] Equilibrium and Stability Measurements via Neutral Beam Spectroscopy

An optical neutral beam spectroscopy system is being designed to provide plasma density, local temperature, internal structure of large-scale MHD instabilities, and magnetic field structure for the PEGASUS Toroidal Experiment. Time resolved, spatially localized measurements of the plasma density are determined by the intensity gradient of the beam fluorescence. Ratios of line intensities of a helium beam provide the electron temperature profile. Spectrally resolved measurements of the charge-exchange recombination emission of impurities determine the local ion temperature. Plasma stability is studied with localized MHD measurements via the beam emission spectroscopy (BES) technique. Motional Stark broadening of deuterium beam emission provide the magnitude (mod-B) and direction of the total field by analyzing the amplitude and phase delay of an oscillating spectral linewidth driven by a rotating polarizer. Present efforts are focused on refurbishing the beam hardware (25 kV, 4 A) and exploring the feasibility of a compact pencil beam.

[VP1.072] The Helicity Injected Torus (HIT) Program

The purpose of the Helicity Injected Torus (HIT) program is to develop current drive techniques for low-aspect-ratio toroidal plasmas. The present HIT-II spherical tokamak experiment is capable of both Coaxial Helicity Injection (CHI) and transformer action current drive. The HIT-II device itself is modestly sized (major radius R = 0.3 m, minor radius a = 0.2 m, with an on-axis magnetic field of up to B_o = 0.5 T), but has demonstrated toroidal plasma currents of up to 200 kA, using either CHI or transformer drive. An overview of ongoing research on HIT-II plasmas, including recent results, will be presented. An electron-locking model has been developed for helicity injection current drive; a description of this model will be presented, as well as comparisons to experimental results from the HIT and HIT-II devices. Empirical results from both the HIT program and past spheromak research, buttressed by theoretical developments, have led to the design of the upcoming HIT-SI (Helicity Injected Torus with Steady Inductive helicity injection) device (T.R. Jarboe, Fusion Technology 36, p.~85, 1999). HIT-SI will be able to form a high-beta spheromak, a low aspect ratio RFP or a spherical tokamak using constant inductive helicity injection. The HIT-SI design and construction progress will be presented.

[VP1.073] Edge Plasma Characteristics in the HIT-II Spherical Tokamak

We have studied edge plasma characteristics in the HIT-II spherical tokamak, in both CHI and Ohmic mode of operation. In the high current CHI mode of operation, the edge plasma has a large negative floating potential (\sim -150 V) and a large plasma density (\sim 10^14 cm^-3). Radial profile measurements of floating potential indicate that the closed magnetic field lines exist at a distance of about 10 cm inside from the outer wall. The Ohmic mode of operation has two phases. During the current-rise phase, edge plasma density is low and floating potential is large-magnitude negative (\sim -50 V), suggestive of a slide-away regime. During the current flat-top phase, the edge plasma density increases by a factor of ten while the floating potential decreases in magnitude. Detailed results on edge plasma behavior as well as edge fluctuation characteristics in HIT-II discharges will be presented.

[VP1.074] Multi-Point Thomson Scattering First Results in HIT-II Plasmas

A multi-point Thomson scattering diagnostic has recently been implemented on the Helicity Injected Torus (HIT-II). The HIT-II device is a low-aspect-ratio (R_o=0.3m, a=0.2m) torus with an on-axis toroidal field of up to 0.5 Tesla. The machine configuration allows both inductive current drive and Coaxial Helicity Injection (CHI) current drive operations and has demonstrated plasma currents on the order of 200 kA in either mode. Electron temperature and density profile measurements for both inductive- and CHI-driven plasmas are presented. The Thomson scattering data is discussed in the context of complimentary diagnostics on the HIT-II device, including: a two-chord FIR interferometer; a 16-channel ion Doppler spectrometer; two tangentially-viewing VUV spectrometers; and, finally, a triple Langmuir probe for direct internal measurements.

[VP1.075] Alignment and Calibration of the Multi-Point Thomson Scattering Diagnostic for the Helicity Injected Torus

A multi-point Thomson scattering system has been designed and built to determine electron temperature and density at 11 radial positions at one time during the plasma discharge within the Helicity Injected Torus (HIT-II). The system includes collection optics and a Littrow mount spectrometer on loan from Princeton, and a 1 GW laser and multi-anode microchannel plate detector from the University of Texas. A method for alignment and calibration of the Thomson scattering system is presented along with a description of the physical device.

[VP1.076] Multi-chord Interferometry and Particle Balance on the HIT-II Experiment

A two-chord tangentially-viewing heterodyne FIR (185 microns) interferometer has been implemented on HIT-II and has been used to obtain density measurements on plasma discharges employing ohmic and CHI (coaxial helicity injection) current drive. The two-chord system is in a double-pass Martin-Puplett configuration which minimizes plasma refractive effects on interferometer alignment. The two chords are scannable through several tangential impact parameters which enables profile studies to be performed. The results of a particle balance study using data from interferometry and other diagnostics will also be presented. The Martin-Puplett system uses a polarizing beam-splitter to separate two frequency-shifted orthogonal polarizations in the beam. One polarization (the scene beam) is transmitted through the polarizer and is sent through HIT-II. Outside the plasma, a roof-mirror reflects the beam back along its incident path, which corrects for refraction caused by plasma density gradients. The roof-mirror also rotates the scene beam's polarization so that it undergoes reflection when incident upon the polarizer a second time. The reference beam, which does not traverse the plasma, experiences a similar process. The two beams are recombined at the polarizer and the interference signal is measured.

[VP1.077] Surface Magnetics and Numerical Simulations for the HIT Program

Thirty-two three-direction magnetic probes are mounted in the outer shell of the HIT-II vacuum vessel, along with forty-two inside the central column. An overview of these is presented, as well as an explanation of the use of outer-shell toroidal-direction probes to measure directly the enclosed poloidal current, F. The relevance of these measurements for incorporation into equilibrium reconstruction on HIT-II using EFIT is discussed. Plans for surface magnetics on the upcoming HIT-SI device are presented, along with their potential importance for use with the NIMROD code for that device.

[VP1.078] Measurements of Ion Velocity Distributions in the HIT-II

Measurements using a 1m, resolution of 0.09 ÅIon Doppler Spectrometer (IDS) have been made on plasmas employing coaxial helicity injection (CHI) current drive on the HIT-II experiment. The data from the IDS has been used to determine toroidal ion flow velocites (10 \sim 20 km/s) and temperatures for various ion species (\sim 30 eV CIII at the plasma edge, \sim 100 eV OV in the core, etc.). The measured axial and toroidal flows are correlated with a high-frequency (50 kHz) n=1 mode. The IDS instrument uses a broad-spectrum optical fiber linked to a telescope which is mountable on several mid-plane and axial ports to image the plasma at various toroidal positions and impact parameters on a shot-to-shot basis. Light exiting the fiber passes through the entrance slit, is incident upon a diffraction grating, and then directed onto a 16 channel PMT. The diffraction grating can be rotated to image different spectral domains/impurity lines, and the grating-PMT distance is adjustable to change the wavelength resolution per channel. The instrument is capable of resolving fluctuations up to 200 kHz.

[VP1.079] Design of the Helicity Injected Torus with Steady Inductive Helicity Injection (HIT-SI)

Steady Inductive Helicity Injection (SIHI) is an inductive current drive method that injects helicity at a nearly constant rate, without open field lines, and without removing any helicity or magnetic energy from the plasma(T.R. Jarboe, Fusion Technology 36), p. 85, 1999. SIHI directly produces a rotating magnetic field structure, and the current profile is nearly time independent in the frame of the rotating field. The Helicity Injected Torus with SIHI (HIT-SI) is a ``bow tie'' spheromak designed to implement SIHI so that the current profile in the rotating frame is optimized. SIHI is accomplished using two inductive helicity injectors that operate 90^o out of phase with each other. Each helicity injector is a 180^o segment of a ZT-P size (a \approx 8cm, R \approx 32cm) RFP. The presence of a spheromak equilibrium will be readily apparent on several diagnostics, including the surface magnetic probes. The design of HIT-SI is presented, including the manufacturing techniques and metallurgical processes being used in the construction of the one-meter diameter close-fitting flux conserver. Several small prototype tests have been performed to prove the vacuum seal and electrical insulation capabilities of the design, and a finite element stress analysis of the flux conserver will be presented.

[VP1.080] Design and Development of the Liquid Lithium Limiter (L3) for CDX-U

--- This poster describes experiments with liquid Li that informed the design of a Liquid Lithium Limter (L3) built by UCSD for installation on the CDX-U spherical torus at PPPL. It was necessary to resort to wetting liquid Li to textured structures in order for the limiter to intercept 2-3 density e-folding lengths of the scrape off layer (3 cm) of the CDX-U plasma. Since Li is chemically active and corrodes rapidly in all but the driest air, we carried out wetting experiments in vacuum (10^-7 - 10^-8 torr) and also in Ar at near atmospheric pressure. Wetting of steel occurred reliably at substrate temperatures near 500 ^oC under all conditions, but this high temperature presented special problems of rapid material loss through evaporation, especially under vacuum. Once the surface is wetted, however, lost Li can be replenished at \sim 200 ^oC (just above the melting temperature) where evaporation is negligible. A wetted limiter can even be cooled to room temperature and then reheated many hours later as long as clean conditions are maintained. Surface textures, heating techniques, effective seal materials for piston-driven liquid Li reservoirs, and other aspects of the limiter system design will be presented.

Work supported by US DOE grant DE-FG03-95ER54301

[VP1.081] Liquid lithium experiments in CDX-U

The research goal of the Current Drive eXperiment - Upgrade (CDX-U) is to investigate the use of liquid lithium as a plasma-facing component in an operating spherical torus. A liquid lithium rail limiter system has been constructed at UCSD and will be installed in CDX-U in late July 2000. A liquid lithium toroidal belt limiter is to follow in October 2000. Modifications to CDX-U to accommodate in-vessel inventories of approximately 1 liter of liquid lithium, techniques for loading lithium onto the limiters, and other preparations will be described. Results of CDX-U experiments with lithium limiter systems on lithium influx, core and edge plasma modifications associated with very low recycling edges, and the effect of forced disruptions and halo currents on the liquid metal will be presented as available.

[VP1.082] Measurement of Electron Temperature Profile through Electron Bernstein Wave (EBW) Detection

The electrostatic EBW provides a promising avenue for rapid electron temperature profile measurement in an ``overdense" spherical torus plasma (ømega_pe\ggØmega_ce). Data are presented for electromagnetic measurements of EBW emission, mode-converted near the plasma edge, on the CDX-U spherical torus (B_0\sim2 kG, \left\langle n_e\right\rangle\sim10^13 cm^-3, T_e\sim10-200 eV). Depending on plasma conditions, the absolutely calibrated, mode-converted EBW radiation temperature T_rad was less than or equal to T_e measured by Thomson scattering. Observed T_rad/T_e ratios are compared to theoretical estimates of mode conversion efficiency calculated using measured edge density profiles. Modulated gas puffing experiments indicate EBW emission is localized at the electron cyclotron harmonic resonance layer. RF electron heating was observed using a fast scanning radiometer. Results of mode-converted polarization measurements are presented.

[VP1.083] Impurity transport studies in ohmically heated plasmas in CDX-U spherical torus.

Intrinsic impurity and radiated power profiles have been measured in MHD dominated and quiescent CDX-U discharges with T_e \le 100 eV. The peaked C V and O VI profiles and the radiated power profiles are modeled with the MIST impurity transport code. The inferred values of the diffusion and inward convection velocity point to the neoclassical nature of impurity transport. Neoclassical transport estimates are presented for the case of several impurity species. Ion temperature and impurity density gradients are examined as the causes for the observed impurity accumulation under various plasma conditions. Pending the completion of the lithium experiments, the analysis of lithium transport in CDX-U will be presented. This work is supported by U.S. DoE Grant DE-FG02-86ER53214 at JHU and DoE Contract No. DE-AC02-76-CHO-3073 at PPPL.

[VP1.084] Formation of Compact RFP, Spheromak and ST in TS-3 Device and Their Merging Magnetic Reconnection

A comparative experiment of compact RFP, spheromak and ST has been demonstrated in the TS-3/4 devices[1]. The compact RFP had smaller numbers of unstable modes and higher q-value in sharp contrast with the conventional RFPs. When those CTs were sustained by the OH coil (poloidal flux injection), an oscillation of toroidal mode n=3 was dominant in the RFP and that of n=2 in the spheromak. An important finding is that the mode amplitudes were reduces by factor 2 when CT merging was used for current drive together with the OH coil. The merging / magnetic reconnection process injected toroidal flux (or edge poloidal current) into the target CT, while the OH coil injected poloidal flux. It experimentally demonstrated a new balanced flux injection of poloidal and toroidal, eliminating a need for the flux conversion (dynamo) from poloidal (core) to toroidal (edge). This dynamo reduction effect was observed in magnetic fluctuation and loop voltage and was found the largest in the compact RFP with the lowest q-value. The edge-current-drive by merging has several advantages over the inductive poloidal current drive by TF coil: (1) intermittent / continuous edge current drive, (2) wide current profile control by means of size and field of colliding CT and (3) large ion heating effect of reconnection. [1] Y. Ono et al., Phys. Plasmas 7, 1863 (2000); M. Inomoto et al., Fusion Energy 1998 2, 927 (1999).

[VP1.085] Ultra-high-beta Spherical Tokamak formed by Plasma Merging and Its Ballooning Instability in the TS-3 Device

Ultra-high-beta (> 50%) spherical tokamak has been produced in TS-3 merging experiment using two types of CT/ST merging. Co-helicity merging of two STs was observed to heat ions significantly and to increase \beta value of a merged ST 2-3 times as large as that of a single ST. Counter-helicity merging of two spheromaks formed an FRC with ion temperature up to 200[eV] and further fast application of external toroidal magnetic field transformed an FRC into an ultra-high-\beta ST. A new finding is that the ultra-high-\beta ST had an ``absolute minimum B" configuration together with diamagnetic toroidal field and hollow toroidal current profile. Both high-\beta STs have largest pressure gradient and magnetic shear around the plasma edge, indicating the formation of ultra-high-\beta ST in the second stability regime for ballooning mode. Heating power was varied by factor 10 by varying reconnecting magnetic field angle and strength. It was found that the high-\beta ST decayed quickly due to large magnetic fluctuation if its pressure gradient exceeds the threshold value. Stability limit in s-\alpha diagram against ballooning mode will be reported based on experimental and computational analyses.

[VP1.086] Construction and Initial Operation of the large-scale merging facility: TS-4

Novel merging experiments of CTs and STs: merging formation of FRC, magnetic reconnection experiment, and ultra-high-beta ST formation, have been demonstrated in TS-3. These successful results lead us to construction of an large-scale merging device TS-4. For the past two years, TS-4 has been operated as an fixed-boundary experiment: proto-TS-4, by installing a coaxial magnetized plasma gun and a flux conserver. We have now prepared for the full-setup of TS-4 and will started an initial operation around August 2000. It has two flux cores(R=1m) for inductive CT/ST formation, a center coil(external toroidal field coil and OH coil) and DC external field coils. It can produce two CTs/STs with the following parameters: q= 0-30 R=0.45-0.55m, A=1.2, elongation=0.8-3, pulse length\approx5ms, B_t=0-0.5T and I<0.3MA. Its research subjects will be (1) comparative transport/kinetic/stability study of STs, spheroamks, compact RFPs and FRCs in a single device, (2) transport/kinetic/stability study of high-beta boundary regime between ST and FRC, (3) study of ultra-high-beta ST in the second stability regime produced by oblate FRC, (4) high-power initial heating and current-drive/amplification of ST by use of axial merging.

[VP1.087] A new electron density profile measurement for comparative study of law-aspect torus plasmas

A CO2 laser interferometer system with time and spatial resolution has been developed in TS-3 (university of Tokyo, Spherical torus-3) experiment to measure electron density profile of merging CT/STs.@The new Michelson type interferometer system has the probe composed of a reflection mirror and glass tube which is installed axially inside the cylindrical vacuum vessel. Its spatial scan enables the single channel interferometer to measure 1-D electron density profile in z-direction. A CW CO2 laser is used to measure the time evolution of electron density profile and heterodyne system using infrared acousto-optic modulator is introduced to realize a precise measurement in plasma with high electron density. The TS-3 device can produce various kinds of law-aspect torus plasma such as ST (spherical tokamak) and spheromak, RFP (reversed field pinch) and FRC (field reversed configuration) inside a single vacuum vessel. In this study, comparative study of ST, spheromak and RFP by electron density profile measurement will be reported. It was found that merging RFPs and spheromaks have higher electron density at the mid-plane than the merging tokamaks. The electron density profile is flat and has a gradient near the separatrix in the high q formation such as tokamak, on the contrary. These phenomena will be investigated in details by measuring merging speed, thermal pressure and flux distribution. A particle confinement time of measured law-aspect ratio torus plasma will be discussed based on those data.

[VP1.088] Non-axisymmetric Electrostatic Helicity Injection into the HIST Spherical Torus

Studies of helicity injection physics including the magnetohydrodynamic (MHD) dynamo and self-organizing phenomena are very important in the spherical torus (ST) and spheromak research. In the HIST experiment, we have found that the intermittent generation of plasma current on ST by coaxial helicity injection (CHI) is responsible for repetitive plasmoid injection from the coaxial gun. We have verified that helicity balance is satisfied during the axisymmetric plasmoid injection process. In order to investigate furthermore the important role of helicity by varying the topology of the system, i.e. symmetry breaking, we perform non-axisymmetric electrostatic helicity injection experiments on FACT and HIST using Compact Torus (CT) injector. CT injector can inject the spheromak with both particle and helicity into the ST plasma from the outboard side. A long-lived spheromak tends to relax to the m=1 helical state in the entrance/drift tube of the CT injector. If we can maintain the m=1 helical configuration there in a steady state, so helicity is continuously transported from the injector toward the outer edge of ST, resulting in current drive through MHD relaxation. In the FACT-ST experiment, we observed that the toroidal current is amplified during spheromak injection, and also investigated the behavoir of the spheromak injected in the ST plasma.

[VP1.089] General Spherical Torus (ICPP)

This abstract not available.

[VP1.090] RF Wave Experiments on the TST-2 Spherical Tokamak

TST-2 is a new spherical tokamak with R = 0.36 m and a = 0.23 m (A = 1.6). Expected parameters are toroidal field of up to 0.4 T and plasma current of up to 0.2 MA. Reliable plasma initiation is achieved with 1 kW of ECH power at 2.45 GHz. Inductively driven plasma currents of up to 90 kA have already been obtained. Plasma currents of up to 1.2 kA were generated by 1 kW of ECH power alone. The direction of the vertical magnetic field around the midplane of the center column (-0.4 m < z < 0.3 m) reversed, indicating formation of a closed magnetic surface. These currents are believed to be pressure-driven currents. RF wave physics experiments, concentrating on detailed studies of wave excitation, propagation, and absorption, have begun. High-harmonic fast wave (HHFW) in the frequency range 20-30 MHz is launched from a 6-element combline antenna at 1 kW level. As the plasma current increases and the plasma grows to its full size, the loading resistance increases as expected. RF magnetic probes show that the wave field detected on the center post is rather uniform both toroidally and vertically, but are much weaker than the wave field detected on the outboard limiter. This result is consistent with the wave field distribution calculated by the TASK/WM full-wave code.

[VP1.091] Global Motions of a Highly Elongated Low Aspect Ratio Tokamak Produced by Negative Bias Theta Pinch

A Low Aspect Ratio Tokamak (LART) configuration with the elongation from 5 to 10, is produced by a negative bias theta pinch device. Safety factors at a plasma edge and a magnetic axis are about 40 and 10. Global motions, which are a shift and a tilt motions, a vertical displacement and an elliptical deformation of toroidal cross section are observed. A wall stabilization effect of the theta pinch coil is expected to be very small, because the plasma is produced away from the coil.In order to suppress these motions,a multipole field, which is effective for the control of the similar motion in FRC experiments, is applied. A magnetic field strength for the suppression will be experimentally defined and compared with a simple analytic model.

[VP1.092] Dynamics on Relaxation Phenomena in Spherical Tokamak

This abstract not available.

Part V of program listing