Session UO1 - Space and Astrophysical Plasmas, and Gas-Puff Z-pinch Experiments.
ORAL session, Friday morning, October 31
Galisteo/Aztec, ACC

[UO1.001] Unexpected property of ion phase space holes

We present analytical solutions for 3D ion phase space holes, taking into account both ion and electron density perturbations. In the limit that the potential energy due to the solitary potential is much smaller than the plasma thermal energy, the plasma can sustain solitary structures even when the electron thermal energy is comparable to the ion thermal energy, and the form of the potential is only weakly constrained. This unexpected result comes from the fact that in small amplitude limit the nonlinear response actually dominates over the linear response, unlike the usual expectation that nonlinearity tends to be less important at small amplitudes. Our results account for the previously unexplained observations of ion holes in the Earth's auroral ionosphere when the ion and electron thermal energies are comparable.

[UO1.002] Fully kinetic simulations and linear theory of E-region instabilities

We present new results of continuing efforts to simulate and understand turbulent E-region plasma driven by strong ambient DC electric field. Such processes, including the Farley-Buneman (FB) and gradient instabilities, have been studied experimentally and theoretically for four decades. Our recent 2- and 3-dimensional simulations have revealed a significant role played by thermal effects, which were disregarded by the majority of E-region investigators. In the last decade, two new thermally driven instabilities have been described along with some supporting observational evidence. Linear theory based on simplified kinetic and fluid models of plasma behavior predicts that ion thermal effects may play an important role at higher altitudes, while electron thermal effects may be of importance at lower E/upper D region. These effects should be especially strong at sufficiently strong DC electric field, well above the threshold of the FB instability. We simulate the FB and thermal instabilities in homogeneous plasma by using a highly parallelized electrostatic particle-in-cell (PIC) code describing both electron and ion dynamics. A fully kinetic PIC algorithm allows us to correctly simulate electron and ion temperature variations and other kinetic effects. Our new results show that the ion and electron thermal driving mechanisms strongly modify the linear and nonlinear behavior of the FB instability. We have identified two different regimes of the dynamical behavior of E-region irregularities. At a moderate driving electric field, a highly turbulent mode-coupling regime develops presumably caused by the FB driving mechanism. In the regime of a stronger electric field, a thermal mechanism of modulated ion frictional heating results in coherent density perturbations. These perturbations have the form of a quasi-plane nonlinear wave with asymmetrically shifted direction of the preferred wavevector. Such peculiar behavior may have serious implications for radar observations.

[UO1.003] Collimation of Extragalactic Jets

Extragalactic jets are created in accretion disks around supermassive black holes and retain their collimation for distances that can reach the order of the megaparsec. Understanding the physical causes of collimation for such remarkable distances proves to be a great challenge. Many authors have attempted to understand the processes in terms of MHD equilibria, basing the analysis on the Grad-Shafranov (GS) equation. We propose a new approach in this direction of investigation. We consider two mathematical analogies: the analogy of the GS equation with the Helmholtz equation and the analogy of the GS equation with the waveguide propagation equation. Based on the second analogy we investigate a class of solutions mathematically similar to optical solitons. We show the properties of the soliton solutions and their possible application to extragalactic jets.

[UO1.004] Polarization of synchrotron emission from relativistic magnetized jets.

The direction of the electric vector of synchrotron emission of ultrarelativistic particles is perpendicular to a magnetic field and to the direction of propagation of a photon. If the emitting volume of plasma moves relativistically, as it is the case for almost all extragalactic and galactic jets, Lorentz transformations of electromagnetic field and the direction of the propagation of the photon result in that the electric vector of the wave is no longer orthogonal to the magnetic field in the laboratory frame. This changes the interpretation of the astronomical polarization observations. Here we demonstrate the effect by calculating the polarization properties of synchrotron radiation emitted by a cylindrical relativistically moving jet with large scale ordered spiral magnetic field. Integrated polarization is bimodal, which is favoured by the observations. Small changes in the velocity and observing angle can produce a flip of polarization. Our simulated polarization maps are in agreement with observations, indicating that the jets are likely to posses spiral magnetic fields.

[UO1.005] Cyclotron Maser Radiation from Astrophysical Shocks

We present a model of coherent emission directly connected to the process of particle acceleration to high energies due to collisionless shock waves, including the full relativistic effects. We consider acceleration by either plasma wave turbulence or quasi-perpendicular shocks. The particular plasma waves we consider propagate mainly perpendicular to the magnetic field, which can accelerate electrons by the surfatron process (Katsouleas amp; Dawson 1983). It is not the field that accelerate it is the longitudinal wave. Both surfatron and the shock acceleration provide us with velocity space ring type distribution functions, which we demonstrate are ideal for generating cyclotron maser radiation. A coherent emission mechanism would explain the high brightness measurements without having to rely on special geometrical effects. The model we propose is directly linked to the injection and acceleration of particles at shocks and does not need a second stage process since it cannot be separated from the acceleration process. The radiation is a result of the relaxation process whenever an anisotropic distribution is formed. A laboratory experiment to simulate the effect will also be discussed.

[UO1.006] Classical Mode Conversion Description of Neutrino Oscillations in Dense Magnetized Plasmas

The evidence for neutrino oscillations and therefore neutrinos with mass is now firmly established. Direct evidence from the observations of solar and atmospheric neutrinos have established neutrino oscillations as the explanation of the solar neutrino deficit. Solar neutrinos above a certain minimum energy may be converted into different neutrino flavors on their way through the sun, the MSW effect. This is a matter effect and is dependent on the electron density. The presence of large magnetic fields such as those produced in supernovae can significantly alter the MSW effect and may be important in the evolution of supernovae. We describe neutrino oscillations in dense magnetized plasmas similar to those found in supernovae. The model that we adopt is similar to the classical mode conversion problems found in fluid dynamics and plasma physics. This approach allows for the derivation of analytical results for the neutrino conversion probability for different combinations of background density profile and magnetic field profile.

[UO1.007] Neutrino Beam Interactions in Dense Plasmas

Employing the relativistic kinetic equations for neutrinos interacting with dense plasmas via the weak interaction force, we explore collective plasma instabilities driven by neutrinos. We examine the anomalous energy transfer between the neutrinos and the background plasma via excitation of electron plasma waves. We present the relativistic equations including the inclusion of external magnetic fields. Solutions of the dispersion equation describing the coupling between a neutrino beam and a plasma wave demonstrates the existence of two regimes a) the kinetic regime and b) the hydro dynamical regime. We demonstrate that the hydro dynamical regime has a growth rate many orders of magnitude larger than the kinetic regime. A nonlinear equation describing the coupling of electron plasma is derived from the relativistic kinetic equations where the neutrino pondermotive force is explicitly written. We show the similarity between a neutrino beam driven plasma wave and an electron beam driven plasma wave, there is also a striking similarity to photon driven plasma wakes. We also derive an expression for the ponderomotive force in the presence of a magnetic field, which may be responsible for the birth velocity of pulsars.

[UO1.008] A Review of the Total Radiated Output of an Argon Z-Pinch Using the Z Radiation Simulator

Sze et.al.(Ref. 1) described the use of an 8 cm diameter double shell nozzle that produced over 270 kJ of argon K-shell emission (>3 keV) on the Z generator at >15 MA peak current. A striking feature seen in those tests was a sharp decrease in K-yield with increasing load mass, but very weak changes in total yield. We will report additional measurements made on those tests that address the non-K-line output of the z-pinches. On the one hand, >5 keV photons due to the free-bound continuum can constitute a significant fraction of the nominal K-shell emission (Ref. 2). On the other hand, the sub-keV L-shell fluence seems to vary little as the K-emission changes substantially. We examine whether the latter data shed light on the cause of the drastic reduction of the K-shell yield for high mass loads. Comparisons will be made between the observations, analytical models and 1D radiation-hydrodynamics MHD code predictions.

1. H. Sze, et.al., Phys. Plasmas, 8, 3135 (2001).

2. A. Velikovich, et.al., Phys. Plasmas 8, 4509 (2001).

[UO1.009] Initial Z-Pinch Results With a 12 cm Diameter Nozzle Using Argon on the Decade Quad

We will report on the initial tests of a large diameter double shell nozzle using the 300 ns rise-time current pulse (> 6 MA) of the DECADE QUAD (DQ) pulsed power machine. With a large 4 cm recess, the gas flow at the pinch location is well collimated and nearly a uniform fill, a flow condition that has been shown (Ref.1) to give better output than shell-like flows for gas z-pinches. Results from a limited set of shots on DQ will be presented.

Ref 1. P. L. Coleman, et.al., “A review of recent z-pinch research at Maxwell Physics International”, Laser and Particle Beams, 19, 2001, pp. 409-441.

[UO1.010] Long Implosion Time (240 ns) Z-Pinch Experiments with a Large Diameter (12 cm) Double-Shell Nozzle

Recently, an 8 cm diameter double-shell nozzle produced argon z-pinches with high K-shell yields with an implosion time of 210 ns. To produce even longer implosion time z-pinches for facilities such as Decade Quad (9 MA short circuit current at 300 ns), a larger nozzle (12 cm outer diameter) was designed and fabricated. During initial testing on Double-EAGLE, 9 kJ of argon K-shell radiation in a 6 ns FWHM pulse was produced with a 240 ns implosion. This is the longest implosion time multi-MA argon z-pinch for x-ray production that has been reported. The initial gas distributions produced by various nozzle configurations have been measured and their impact on the final radiative characteristics of the pinch are presented. The addition of a central jet to increase the initial gas density near the axis is observed to enhance the pinch quality. Results from higher current experiments on Decade Quad will be presented, if available.

[UO1.011] ALEGRA MHD 2D calculations for Ar gas puff Z-pinches

We report on 2D rz MHD calculations performed using the Sandia code ALEGRA for Argon gas puffs. Similar calculations were conducted with other MHD codes in the past, such as DELTA and MACH2, and reported in various conferences. We selected to run the case of a gas puff shot taken on the DTRA machine Double Eagle, for which we can compare with the experimental results and previous DELTA computations. This shot #4428 is a puff-on-puff gas load with total imploded mass of the order of 0.4 mg of Ar, of about 4 cm radial extent and 4 cm axial length. The resulting current pulse peaks at 3.6 MA and the implosion time is about 180 ns. The observed Ar K-shell radiation was 12 kJ with peak power of 1TW. For this application ALEGRA is run in 2T with tabular EOS and emission radiation from a steady state Collisional Radiation Equilibrium (CRESS) model, which was the approach taken for the DELTA calculation. The initial density distribution for the Ar gas is a fit of actual interferometric measurements of the puff. We compare ALEGRA results to establish the ability of this code to reproduce two main observables of the shot: load current and K-shell radiation emitted power.

[UO1.012] Characterization of 12 cm Diameter Triple-shell Gas Puff Z-pinch Loads

To mitigate the Rayleigh-Taylor instability and thus increase the x-ray yield, shell-on-shell gas puff loads have been studied on high current z-pinch drivers such as Double-EAGLE, DQ and Z. A triple-shell gas puff has been developed, where a gas jet is introduced in the center and the outer and inner gas shell radii are 5.5 and 2.5 cm, respectively. We report on the characterization of the triple-shell argon gas puff obtained using Planar Laser Induced Fluorescence, Rayleigh and/or Raman scattering. A 266nm, 5ns, 5cm line focused laser beam passes through the Ar gas puff, which is mixed with few percent acetone by pressure. Acetone fluorescence and laser scattering from the gas are imaged using a gated intensified CCD camera. From the fluorescence images captured coincident with or delayed in respect to the laser pulse, the gas density or the gas flow velocity profiles are derived, respectively. The PLIF measurements are compared with the Rayleigh and/or Raman scattering results and the 2-D gas flow modeling. Comparisons are also made with the z-pinch implosion experiments.

[UO1.013] Polar Faculae Are Faculae Of Old Age, Ascending To Photosphere From The Suns Upper Magnetic Toroid Levels

We present arguments that Sun’s polar magnetic fields, near 1 gauss strength, ascend to photosphere in polar facular increment, in accord with Fig. 1, ^1 thru secular meridional circulation of both 3,600 ± 25 gauss mag. toroids (as obs. at upper level) of each hemisphere, reversing polarities every 11 yr. Having been submerged to just below lower mag. toroid and traveling slightly faster there in circulatory mer. motions of Sun, which drive toroids, incipient polar faculae do not ascend with toroid at \theta_c = 40^o,, but continue to higher latitudes >= 50^o, and now having aged by as much as one solar cycle plus \sim 3 yr., they begin their appearance at photosphere. Ascent to photosphere requires greater travel time the greater is their lat. of vertical ascent, owing to reduced mer. circulation velocity with inc. in lat. above 40^o, (Confer sketch of fluid motions, Fig. 1. ^1) This polar faculae reaching photosphere at 63^o,, where surface density increases strongly and reaches an almost constant value at \theta_c = 70^o,, and that possibly extends over whole polar cap when near polar facular max., would be expected to have been formed at 40^o, or at lesser lat. as faculae in previous sunspot cycle and would thus possess a significantly reduced observed lifetime compared to faculae in photosphere newly found below 40^o,.

Part U of program listing