Non-Maxwellian electron distributions and continuum X-ray emission in inverse Bremsstrahlung heated plasmas

Abstract

The heating of a completely ionized plasma by inverse Bremsstrahlung absorption (IB) is studied by numerically integrating the time varying kinetic equation for electrons, and the resulting distribution functions are used to calculate the continuum X-ray emission. The shape of the distribution functions is determined by the competition between IB and electron-electron collisions, and, in all the uniform plasma simulations, the authors find that the distributions are well fitted by the formula Cmexp(-( nu / nu m)m) where m is between 2 and 5. They find the dependence of m on alpha =Z2 nu osc/ nu th2, where Z is the atomic number, nu osc is the oscillation velocity in the laser field and nu th is the thermal velocity: m( alpha )=2+3/(1+1.66/ alpha 0.724). More importantly, in the realistic case of a space and time dependent kinetic simulation of a Be plasma heated by a Nd laser, including heat flow and ion motion, this fit remains valid for electron densities below 0.75 Nc (with constant m), although other effects come into play closer to the critical surface. Thus, these simple results can be used to determine the continuum X-ray spectrum due to Bremsstrahlung (BR) and to direct radiative recombination (DRR).