Absorption of laser light in overdense plasmas by sheath inverse bremsstrahlung

Abstract

The original sheath inverse bremsstrahlung model [P. J. Catto and R. M. More, Phys. Fluids 20, 704 (1977)] is modified by including the v×B term in the equation of motion, as the evanescent magnetic field in an overdense plasma is greater than the corresponding electric field. It is shown that the present results are significantly different from those derived without the v×B term. The v×B term is also important in interpreting the absorption mechanism. If the v×B term were neglected, the absorption of the light would be incorrectly interpreted as an increase in the transverse components of the canonical momentum, in the case of a normally incident laser light. It is also shown that both the sheath inverse bremsstrahlung and the anomalous skin effect are limiting cases of the same collisionless absorption mechanism. Results from particle-in-cell (PIC) plasma simulations are compared with the absorption coefficient calculated from the linear theory. Finally, the effects of finite density gradients are investigated by PIC simulations.

Anomalous skin effect for linearly and circularly polarized intense laser light

“…The absorption coefficient and skin depth are calculated by following the usual treatment of the anomalous skin effect~Yang et al., 1995;Ichimaru, 1973!. The first term in Eq.~25!…”

Ultra intense laser/plasma interaction at normal incidence: Relativistic mirrors effects, high harmonics generation and absorption

“…At such relativistic laser intensities, the dominant absorption mechanisms are collisionless [3]. However, even in this "simple" case of a laser pulse at normal incidence interacting with a steep plasma gradient, a plethora of absorption mechanisms exists in the literature: the ponderomotive J x B heating [4], different sort of skin effects [5], vacuum heating and many other mechanisms. Due to the complex mixing of all these processes, the basic physics is poorly understood.…”

“…The "2&> electron bunch" or J x B heating, is thus negligible. In this latter case (close to a linear analysis), the main absorption mechanisms are the well-known SIB (nolo < 1) or anomalous skin effect (nolo > 1) [5], it is to say, the absorption comes from the imaginary part of the dielectric plasma function (see Eq. (42) in [5]), our results agree with that for ao/no ~0.01.…”

“…In this latter case (close to a linear analysis), the main absorption mechanisms are the well-known SIB (nolo < 1) or anomalous skin effect (nolo > 1) [5], it is to say, the absorption comes from the imaginary part of the dielectric plasma function (see Eq. (42) in [5]), our results agree with that for ao/no ~0.01. Increasing arj, in regime A, electrons can be trapped during many laser cycles before leaving the laser/plasma interaction region due to the electro-static field oscillation at the plasma frequency.…”

“…Secondly, the numerical analysis we have performed of the EDF in a Vlasov description, using both the fields of the fluid model and PICS, shows the so-called chaotic scattering [10] of electron particles in momentum space, which explains partially the new results concerning the non-monotonic behavior of absorption with laser intensity. Thirdly, some physical arguments are given in order to explain the transition between the sheath inverse bremsstrahlung absorption regime [5] and the J x B heating, responsible for the 2&> electron bunches [4].…”