Inertial Fusion Sciences and Applications 2005
J.-C. Gauthier, et al.
J. Phys. IV France 133 (2006) 295-300
Electron temperature anisotropy modeling and its effect on
anisotropy-magnetic field coupling in an underdense laser heated plasmaJ.P. Morreeuw1, A. Sangam2, 3, B. Dubroca1, 2, 3, P. Charrier2, 3 and V.T. Tikhonchuk2 1
CEA-CESTA, BP. 2, 33114 Le Barp, France2
CELIA, University Bordeaux 1, 33405 Talence Cedex, France3
MAB, University Bordeaux 1, 33405 Talence Cedex, France
The laser interaction with an underdense plasma leads to
an anisotropic laser heating of electrons. This temperature anisotropy
gradient in turn is the source of an early magnetic field, which has an
important effect on the plasma evolution, due to the thermal flux reduction.
We describe the temperature anisotropy by an evolution equation
including the anisotropy-magnetic field coupling and observe a
rather efficient magnetic field generation. However at high
anisotropy levels, a small-scale instability emerges, leading to a
serious problem in numerical calculations.
We introduce the kinetics effects, which fix the problem by the
anisotropy diffusion through the heat flux tensor. A
constant-coefficient Fokker-Planck model in the 2-D geometry allows
us to derive an anisotropy diffusion term. The diffusion coefficient
is fitted from the kinetic theory of the collisional anisotropic
(Weibel) instability growth rate.
Such an anisotropy diffusion term wipes out the unphysical
instability without any undesirable smoothing. This diffusion along
with the viscosity term leads also to a quite good restitution of
the Weibel instability growth rate and to the short wavelength
cutoff, even in a weakly collisional situation. This allows us to
use such a model to predict emergence the Weibel instability as well
as its saturation.
© EDP Sciences 2006