Turbulent transport is studied in 2D MHD turbulence in the
presence of a background shear flow and magnetic field, by
employing a two--scale analysis and the Gabor transform. The
effects of different forcings for a fluid and magnetic
field, as well as different geometry of mean--field
configurations, are investigated. In all cases, the
turbulent viscosity (momentum transport) is shown to be
reduced due to the cancellation of Reynolds stress by
Maxwell stress. The turbulent diffusivity is also
illustrated to be reduced due to the effect of a
large--scale magnetic field. Furthermore, the direction of
the cascade of energy and magnetic vector potential is found
to depend on the properties of forcings. The implication on
the 3D reduced MHD is discussed in a situation where a
strong large--scale magnetic field B=B\hat z is
orthogonal to both a large--scale shear flow U = U(y)
\hat x and its inhomogeneity y.
[UO1.002] Statistical theory of subcritically-excited strong turbulence in inhomogeneous plasmas
Kimitaka Itoh (National Institute for Fusion Science), Sanae -I. Itoh (Research Institute for Applied Mechanics)
Statistical theory of strong plasma turbulence is developed.
Renormalized dielectric is used for coherent plasma
response, and turbulent noise is formulated based on the
random coupling model. Solving the Langevin type equation
for subcritically unstable plasmas, statistical property of
turbulence is obtained. Phase-diagram of turbulence is
given, showing the transition from thermodynamical
fluctuations to turbulent fluctuations. An effective
potential is derived, the minimum of which describes the
most probable state of turbulence. This principle is shown
to be an extension of the minimum entropy production in the
limit of thermodynamical equilibrium. Power law tail of
probability distribution function of fluctuation energy is
obtained. Transition probability between different turbulent
states is derived. This formula is a generalization of
Arrhenius law. Transition probability is obtained as a
function of the distance from stability boundary, and has a
power law dependence on the distance. The balance of
transition and back-transition probabilities gives the
boundary in phase diagram, generalizing Maxwell's
construction.
[UO1.003] Realizability in an EDQNM Closure for MHD Turbulence Possessing Cross-Helicity
Jane Pratt, Leaf Turner (THEORETICAL DIVISION, Los Alamos National Laboratory)
Normally the EDQNM closure is utilized when describing a Navier-Stokes turbulence. In this case, realizability is readily demonstrated. However, when one attempts to demonstrate realizability when utilizing this closure in the two-fluid-like case of MHD turbulence, more must be expected from the EDQNM closure: One must demonstrate that the turbulent fluid energy spectrum remains positive at all times, that the turbulent magnetic energy spectrum remains positive at all times, and that the cross-helicity spectrum satisfies a Schwarz inequality. To do so, one must show that the three evolution equations of the three spectra obtained from the closure yield evolving spectra that always satisfy the three realizability conditions above. In the absence of mean fields, we have done so using the Elsasser field variables, v\pmB.
[UO1.004] Phase mixing and island growth saturation in Hamiltonian reconnection
Daniela Grasso (INFM, Politecnico di Torino, Italy), Francesco Califano (INFM, University of Pisa, Italy), Francesco Pegoraro (INFM and Department of Physics, University of Pisa), Franco Porcelli (INFM and Department of Energetics, Politecnico di Torino, Italy)
The Hamiltonian evolution of magnetic field line reconnection in a two-dimensional fluid plasma [1] leads to a macroscopic equilibrium with a finite size island and fine-scale spatial structures. The latter arise from the phase mixing of the conserved fields. This equilibrium is the analog of the BGK solution for electrostatic Langmuir waves. This result uncovers the underlying unity between different physical phenomena, such as nonlinear Landau damping and dissipationless vortex interaction in 2D fluids on one side, and Hamiltonian magnetic reconnection on the other.
[1] E. Cafaro, D. Grasso, F. Pegoraro, F. Porcelli and A.
Saluzzi, Phys. Rev. Lett. 80, 4430 (1998).
[UO1.005] Numerical Study of Nonlinear Forced Magnetic Reconnection
Kanya Kusano (Hiroshima University), Philippa Browning, Grigory Vekstein (UMIST)
Two dimensional numerical simulations are used to
investigate nonlinear aspects of forced magnetic
reconnection in a low beta highly conducting plasma. This is
representative of the solar corona, where reconnection may
be induced by external perturbations, for example at the
photospheric boundary of the corona. The aim is to
investigate the energy dissipation by the reconnection,
which may provide a mechanism for heating the coronal
plasma. The field is taken to be initially a sheared
force-free equilibrium in a slab, and the effects of
applying a slow deformation to the boundaries are
investigated. Previous analytical studies assuming small
departures from the initial equilibrium have found that a
current sheet forms during an initial ideal phase of
evolution, which subsequently relaxes to a reconnected
equilibrium, releasing some magnetic energy. The linear
theory predicts that the energy release has a singularity
when the field is marginally stable to the tearing mode. The
nonlinear evolution of the field is calculated numerically,
focusing on the energy release. In particular, the strongly
nonlinear behavior is studied in the parameter regime in
which the linear theory breaks down. It is found that
nonlinearities become strong close the marginal stability
point, and that beyond this point an explosive energy
relaxation, which can release greater energy compared with
the pure tearing instability, indeed happens even for weak
boundary deformations.
[UO1.006] Bursting reconnection of the two co-rotating current loops
Sergei Bulanov, Igor Sokolov (General Physics Institute of RAS), Jun-ichi Sakai (Toyama University)
Two parallel plasma filaments carrying electric current (current loops) are considered. The Ampere force induces the filaments' coalescence, which is accompanied by the reconnection of the poloidal magnetic field. Initially the loops rotate along the axii of symmetry. Each of the two loops would be in equilibrium in the absence of the other one.
The dynamics of the reconnection is numerically simulated using high-resolution numerical scheme for low-resistive magneto-hydrodynamics. The results of numerical simulation are presented in the form of computer movies.
The results show that the rotation strongly modifies the
reconnection process, resulting in quasi-periodic (bursting)
appearance and disappearance of a current sheet. Fast
sliding motion of the plasma along the current sheet is a
significant element of the complicated structure of
reconnection (current-vortex sheet). The magnetic surfaces
in the overal flow are strongly rippled by slow magnetosonic
perturbations, so that the specific spiral structures form.
This should result in the particle transport enhancement.
[UO1.007] Current Sheet Thinning Caused by the Lower Hybrid Drift Instability
Gianni Lapenta (Applied Physics Division, MS C305, Los Alamos National Laboratory, Los Alamos, NM 87545), J. U. Brackbill (Theoretical Division, Los Alamos National Laboratory)
Recent simulation results suggest that the Lower-Hybrid
Drift Instability (LHDI) may be an important contributor to
microscopic processes underlying the observed evolution of
macroscopic instabilities in the Earth's magnetotail. The
LHDI grows in regions of strong gradients of the density and
current on the flanks of the magnetotail. It has always been
assumed that the LHDI cannot penetrate the core region of
the magnetotail, but recent results (R. Horiuchi, T. Sato,
Phys. Plasmas, 6, 4565, 1999) suggest the opposite. For
sufficiently high mass ratios (100 in the paper listed
above) the LHDI increases the peak current while flattening
the profile in the flanks. The resulting thinner current
sheet is more unstable to the drift kink instability, and
possibly to other instabilities as well. We present results
for real values of the mass ratio (1836) that show that
current sheet peaking increases dramatically with increasing
mass ratios. The modified profile is indeed observed to be
more unstable to a variety of instabilities (e.g drift-kink
and oblique modes) than the initial profile. These results
may help to explain the puzzling disagreement between
simulations and recent linear theory (W. Daughton, J.
Geophys. Res., 104, 28701, 1999). Since the LHDI grows more
rapidly than other instabilities, these results suggest that
linear theory must be applied to a current sheet that has
been modified by the LHDI, not to the initial current
profile.
[UO1.008] Weak shock wave induced enhanced excitation and ionization in nonequilibrium plasmas
Peter Bletzinger (ISSI, Beavercreek, OH), Biswa Ganguly, Alan Garscadden (Air Force Research Laboratory, Wright-Patterson AFB. OH)
Investigation of the interaction of a weak shock wave with a
N_2 glow discharge has shown that it causes large local
changes in the electric field in the immediate vicinity of
the shock front as well as global changes in discharge
voltage and current. These effects were shown to be
discharge polarity dependent. The observed sharp drop in the
local electric field propagating with the shock front
indicated a local region of enhanced conductivity at the
shock front and a slower decay of this ionization. This
observation, coupled with current continuity, suggested that
a triple- or quadruple space charge layer was connected with
the shock front. The high electric fields then lead to local
excitation and ionization enhancements if the electron Debye
length exceeds the shock layer thickness. We measured the
excitation enhancement of the 2 - 0 vibrational transition
of the B ^3\Pi_g \rightarrow A ^3\Sigma^+_u first
positive band of N_2 for a Mach 1.9 shock wave propagation
in a positive column plasma. We observed five orders of
magnitude enhancement of direct electron impact excitation
rate of the B state. The measurements imply a jump of E/n
from 40 Td to > 200 Td in the shock front suggesting six
orders of magnitude local ionization rate enhancement. These
localized ionization and the electric field modifications
will lead to a redistribution of the Joule heating due to
the shock propagation through a plasma where the Debye
length \ge the shock thickness.
[UO1.009] The Effect of Weak Collisions on Plasma Oscillations
A. Simon, R.W. Short, R. Betti, V.N. Goncharov (LLE, U. of Rochester)
Lenard \textitet al.(A. Lenard \textitet al.),
Phys. Rev. \textbf112, 1456 (1958). studied this using an
approximate collision operator. Observation of plasma echoes
led Su \textitet al.(C. H. Su \textitet al.),
Phys. Rev. Lett. \textbf20, 427 (1968). to study the
effect of weak collisions on the propagation of f_1 from
a localized source. They used boundary layer theory to
obtain the v-dependence of f_1 and found that the
spatial echo is attenuated as exp(-\betax^3).
Recently, this problem has been revisited by Ng \textitet
al.(C. S. Ng \textitet al.), Phys. Rev. Lett.
\textbf83, 1974 (1999). They use a f.t. in x and t
and obtain eigenvalues of the velocity equation by a
numerical procedure and state that the results in Ref. 2 are
in error. We show that this is not the case: they recover
instead the collective oscillations (identical to those in
Ref. 1) but not the dominant f_1 behavior. In addition,
it is incorrect to state that Ref. 2 assumed that the Van
Kampen spectrum is preserved in the presence of collisions.
We use the b.l. method with complex ømega. Our results
are in essential agreement with those of Ref. 2 (and
Landau). This work was supported by the U.S. DOE Office of
ICF under Coop. Agreem. DE-FC03-92SF19460.
[UO1.010] Effects of electromagnetic induced transparency for cyclotron waves in plasmas
Alexander G. Litvak, Michael D. Tokman (IAP RAS, Nizhny Novgorod, Russia)
The parametric effect of electromagnetic induced
transparency (EIT) is formation of the “transparency window”
within the line of resonance absorption in the quantum
three-level system accompanied, at the same time, with
record-breaking deceleration of the reference light wave.
This effect has recently become very popular. This work
describes construction of a classic model of this
phenomenon. Basing on this model, some EIT analogies for
electromagnetic waves at the frequencies of
electron-cyclotron resonance in cold plasma have been
considered. In the first case the build-up of electron
oscillations at the gyrofrequency in the field of the signal
wave is damped due to parametric coupling with collective
electrostatic oscillations. Absolutely similar to the case
of the analogous quantum system, the EIT window in this case
is characterized by group deceleration of the reference
electron-cyclotron wave. In the second case resonance
absorption of electromagnetic modes can be suppressed due to
destructive interference of resonance harmonics of RF
fields. Appearance of these harmonics is caused by strong
oscillations of electrons along wave vectors, which are
excited by electrostatic pumping. There is no group
deceleration in this regime of EIT. The possibilities of
using the above effects for amplification and generation of
waves in plasmas are also discussed.
[UO1.011] Cyclotron stimulated emission without inversion
Mikhail D. Tokman, Maria A. Eroukhimova (Institute of Applied Physics)
We find classical analog of quantum systems capable of
stimulated emission of radiation in the absence of
inversion. We show that cyclotron parametric instability in
low-frequency modulation of the distribution function of
electrons can amplify a bichromatic high-frequency field
when amplification of each spectral component separately is
impossible. We consider different schemes of such cyclotron
stimulated emission in the modulated electron medium with
modulation of reactive or active permeability.
[UO1.012] The classical analog of the Casimir effect in turbulent plasmas
José Tito Mendonça (Instituto Superior Técnico, Lisboa)
It is shown that two parallel plates immersed in a turbulent
plasma can attract each other, due to the difference in the
content of the turbulent spectrum of fluctuations in the
inner and outer regions of the plasma. This attractive force
adds to the usual electrostatic force associated with their
coulombian potential and with the plasma screening. The
present formulation leads to a general expression for the
classical Casimir force. The specific cases of ion acoustic,
electron plasma and electromagnetic turbulence are
considered. The usual expression for the Casimir force in
vacuum can also be derived from our results.
[UO1.013] The analog of Unruh radiation produced by a non-accelerated but superluminal ionization front in a plasma.
Ariel Ricardo Guerreiro, Jose Tito Mendonça, Ana Maria Martins (Instituto Superior Tecnico, Lisboa)
An analog of Unruh radiation is introduced to describe the
effect produced on the electromagnetic field by a
nonaccelerated but superluminal perturbation of the optical
properties of a plasma. The quantum emission of photons from
an initial electromagnetic vacuum is predicted. It is
possible to associate an equivalent temperature and
acceleration to the emitted spectrum which depends on the
perturbation velocity. The possibility of distinguish such
radiation from the background thermal spectrum will be
discussed.
[UO1.014] On Lyapunov stability of nonautonomous mechanical systems
Henri Tasso (Max-Planck-Institut fuer Plasmaphysik, 85748 Garching, Germany), George Throumoulopoulos (Theoretical Physics, University of Ioannina, GR 451 10 Ioannina, Greece)
A sufficient condition for the linear stability of nonautonomous dissipative mechanical systems with circulatory forces is derived. It is applied to autonomous systems transformed to nonautonomous ones by a time-dependent orthogonal transformation. This allows to obtain sufficient stability conditions in case the perturbed potential energy can have negetive values. This is, in particular, of importance for plasma equilibria with sheared flows. The nearness of the sufficient condition to necessity is discussed for a particular example.