68 research outputs found
Astrophysical Gyrokinetics: Basic Equations and Linear Theory
Magnetohydrodynamic (MHD) turbulence is encountered in a wide variety of
astrophysical plasmas, including accretion disks, the solar wind, and the
interstellar and intracluster medium. On small scales, this turbulence is often
expected to consist of highly anisotropic fluctuations with frequencies small
compared to the ion cyclotron frequency. For a number of applications, the
small scales are also collisionless, so a kinetic treatment of the turbulence
is necessary. We show that this anisotropic turbulence is well described by a
low frequency expansion of the kinetic theory called gyrokinetics. This paper
is the first in a series to examine turbulent astrophysical plasmas in the
gyrokinetic limit. We derive and explain the nonlinear gyrokinetic equations
and explore the linear properties of gyrokinetics as a prelude to nonlinear
simulations. The linear dispersion relation for gyrokinetics is obtained and
its solutions are compared to those of hot-plasma kinetic theory. These results
are used to validate the performance of the gyrokinetic simulation code {\tt
GS2} in the parameter regimes relevant for astrophysical plasmas. New results
on global energy conservation in gyrokinetics are also derived. We briefly
outline several of the problems to be addressed by future nonlinear
simulations, including particle heating by turbulence in hot accretion flows
and in the solar wind, the magnetic and electric field power spectra in the
solar wind, and the origin of small-scale density fluctuations in the
interstellar medium.Comment: emulateapj, 24 pages, 10 figures, revised submission to ApJ:
references added, typos corrected, reorganized and streamline
Propagation of Electron Magnetohydrodynamic structures in a 2-D inhomogeneous plasma
The fully three dimensional governing equations in the electron
magnetohydrodynamic (EMHD) regime for a plasma with inhomogeneous density is
obtained. These equations in the two dimensional (2-D) limit can be cast in
terms of the evolution of two coupled scalar fields. The nonlinear simulations
for the two dimensional case are carried out to understand the propagation of
EMHD magnetic structures in the presence of inhomogeneity. A novel effect
related to trapping of dipolar magnetic structures in the high density plasma
region in the EMHD regime is observed. The interpretation of this phenomena as
well as its relevance to the problem of hot spot generation in the context of
fast ignition is presented
Conservation of canonical circulation and its relation to finite Hall term magnetohydrodynamics
Kinetic formulation and global existence for the Hall-Magneto-hydrodynamics system
This paper deals with the derivation and analysis of the the Hall
Magneto-Hydrodynamic equations. We first provide a derivation of this system
from a two-fluids Euler-Maxwell system for electrons and ions, through a set of
scaling limits. We also propose a kinetic formulation for the Hall-MHD
equations which contains as fluid closure different variants of the Hall-MHD
model. Then, we prove the existence of global weak solutions for the
incompressible viscous resistive Hall-MHD model. We use the particular
structure of the Hall term which has zero contribution to the energy identity.
Finally, we discuss particular solutions in the form of axisymmetric purely
swirling magnetic fields and propose some regularization of the Hall equation
Spectral features of solar wind turbulent plasma
Spectral properties of a fully compressible solar wind Hall
Magnetohydrodynamic plasma are investigated by means of time dependent three
dimensional Hall MHD simulations. Our simulations, in agreement with spacecraft
data, identify a spectral break in turbulence spectra at characteristic
length-scales associated with electromagnetic fluctuations that are smaller
than the ion gyroradius. In this regime, our 3D simulations show that turbulent
spectral cascades in the presence of a mean magnetic field follow an
omnidirectional anisotropic inertial range spectrum close to . The
onset of the spectral break in our simulations can be ascribed to the presence
of nonlinear Hall interactions that modify the spectral cascades. Our
simulations further show that the underlying charachteristic turbulent
fluctuations are spectrally anisotropic, the extent of which depends critically
on the local wavenumber. The fluctuations associated with length scales smaller
than the ion gyroradius are highly compressible and tend to exhibit a near
equipartition in the velocity and magnetic fields. Finally, we find that the
orientation of velocity and magnetic field fluctuations critically determine
the character of nonlinear interactions that predominantly govern a Hall MHD
plasma, like the solar wind.Comment: This paper is accepted for publication in Monthly Notices of the
Royal Astronomical Society Main Journa
Electron density evolution during a fast, non-diffusive propagation of a magnetic field in a multi-ion-species plasma
Rapid dissipation of magnetic fields due to Hall current
We propose a mechanism for the fast dissipation of magnetic fields which is
effective in a stratified medium where ion motions can be neglected. In such a
medium, the field is frozen into the electrons and Hall currents prevail.
Although Hall currents conserve magnetic energy, in the presence of density
gradients, they are able to create current sheets which can be the sites for
efficient dissipation of magnetic fields. We recover the frequency,
, for Hall oscillations modified by the presence of density
gradients. We show that these oscillations can lead to the exchange of energy
between different components of the field. We calculate the time evolution and
show that magnetic fields can dissipate on a timescale of order
. This mechanism can play an important role for magnetic
dissipation in systems with very steep density gradients where the ions are
static such as those found in the solid crust of neutron stars.Comment: 9 pages, changed fig.
Gyrokinetic turbulence: a nonlinear route to dissipation through phase space
This paper describes a conceptual framework for understanding kinetic plasma
turbulence as a generalized form of energy cascade in phase space. It is
emphasized that conversion of turbulent energy into thermodynamic heat is only
achievable in the presence of some (however small) degree of collisionality.
The smallness of the collision rate is compensated by the emergence of
small-scale structure in the velocity space. For gyrokinetic turbulence, a
nonlinear perpendicular phase mixing mechanism is identified and described as a
turbulent cascade of entropy fluctuations simultaneously occurring at spatial
scales smaller than the ion gyroscale and in velocity space. Scaling relations
for the resulting fluctuation spectra are derived. An estimate for the
collisional cutoff is provided. The importance of adequately modeling and
resolving collisions in gyrokinetic simulations is biefly discussed, as well as
the relevance of these results to understanding the dissipation-range
turbulence in the solar wind and the electrostatic microturbulence in fusion
plasmas.Comment: iop revtex style, 14 pages, 1 figure; submitted to PPCF; invited talk
for EPS Conference on Plasma Physics, Crete, June 2008; Replaced to match
published versio
Relativistic Laser-Matter Interaction and Relativistic Laboratory Astrophysics
The paper is devoted to the prospects of using the laser radiation
interaction with plasmas in the laboratory relativistic astrophysics context.
We discuss the dimensionless parameters characterizing the processes in the
laser and astrophysical plasmas and emphasize a similarity between the laser
and astrophysical plasmas in the ultrarelativistic energy limit. In particular,
we address basic mechanisms of the charged particle acceleration, the
collisionless shock wave and magnetic reconnection and vortex dynamics
properties relevant to the problem of ultrarelativistic particle acceleration.Comment: 58 pages, 19 figure
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Recyclable transmission line (RTL) and linear transformer driver (LTD) development for Z-pinch inertial fusion energy (Z-IFE) and high yield.
Z-Pinch Inertial Fusion Energy (Z-IFE) complements and extends the single-shot z-pinch fusion program on Z to a repetitive, high-yield, power plant scenario that can be used for the production of electricity, transmutation of nuclear waste, and hydrogen production, all with no CO{sub 2} production and no long-lived radioactive nuclear waste. The Z-IFE concept uses a Linear Transformer Driver (LTD) accelerator, and a Recyclable Transmission Line (RTL) to connect the LTD driver to a high-yield fusion target inside a thick-liquid-wall power plant chamber. Results of RTL and LTD research are reported here, that include: (1) The key physics issues for RTLs involve the power flow at the high linear current densities that occur near the target (up to 5 MA/cm). These issues include surface heating, melting, ablation, plasma formation, electron flow, magnetic insulation, conductivity changes, magnetic field diffusion changes, possible ion flow, and RTL mass motion. These issues are studied theoretically, computationally (with the ALEGRA and LSP codes), and will work at 5 MA/cm or higher, with anode-cathode gaps as small as 2 mm. (2) An RTL misalignment sensitivity study has been performed using a 3D circuit model. Results show very small load current variations for significant RTL misalignments. (3) The key structural issues for RTLs involve optimizing the RTL strength (varying shape, ribs, etc.) while minimizing the RTL mass. Optimization studies show RTL mass reductions by factors of three or more. (4) Fabrication and pressure testing of Z-PoP (Proof-of-Principle) size RTLs are successfully reported here. (5) Modeling of the effect of initial RTL imperfections on the buckling pressure has been performed. Results show that the curved RTL offers a much greater buckling pressure as well as less sensitivity to imperfections than three other RTL designs. (6) Repetitive operation of a 0.5 MA, 100 kV, 100 ns, LTD cavity with gas purging between shots and automated operation is demonstrated at the SNL Z-IFE LTD laboratory with rep-rates up to 10.3 seconds between shots (this is essentially at the goal of 10 seconds for Z-IFE). (7) A single LTD switch at Tomsk was fired repetitively every 12 seconds for 36,000 shots with no failures. (8) Five 1.0 MA, 100 kV, 100 ns, LTD cavities have been combined into a voltage adder configuration with a test load to successfully study the system operation. (9) The combination of multiple LTD coaxial lines into a tri-plate transmission line is examined. The 3D Quicksilver code is used to study the electron flow losses produced near the magnetic nulls that occur where coax LTD lines are added together. (10) Circuit model codes are used to model the complete power flow circuit with an inductive isolator cavity. (11) LTD architectures are presented for drivers for Z-IFE and high yield. A 60 MA LTD driver and a 90 MA LTD driver are proposed. Present results from all of these power flow studies validate the whole LTD/RTL concept for single-shot ICF high yield, and for repetitive-shot IFE
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