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

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, it is found that the distributions are well fitted by the formula Cmexp[-(υ/υmm)] where m is between 2 and 5. The dependence of m on α =Z2υosc/υth2, is found where Z is the atomic number, υosc is the oscillation velocity in the laser field and υth is the thermal velocity: m(α) = 2 + 3/(1 + 1.66/α0.724). 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 0.75Nc (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). The contributions of BR (valid for average and large atomic numbers) and DRR into each shell are given in a parametric form as functions of m and Tc. The implications for the plasma diagnostics based on continuum X-ray emission are discussed. (Author)