Bremsstrahlung Emission in Strongly Coupled Plasmas

Bremsstrahlung absorption and emission are important in extreme ultraviolet (EUV) lithography during laser absorption and for radiation transport within the plasma itself. Furthermore, plasmas created in EUV lithography can often be in an intermediate coupling regime, where the average kinetic energy of particles is on the order of the potential energy at the average interparticle spacing. Classical bremsstrahlung emission theory consists of a binary collision model and linear-dielectric approach developed for weakly coupled plasmas [1]. In order to test these classical theories in strongly coupled plasmas, we consider a two-component plasma with repulsively interacting species and calculate the emission spectrum from classical molecular dynamics simulations for plasmas with coupling strengths between 0.01-100. The spectra at different coupling strengths differ in two major ways. Firstly, emission greater than the plasma frequency is largely determined by binary collisions. In this regime, we find that binary collision models using Coulomb or Debye-Hückel potentials are no longer adequate to describe the resulting spectra. Instead, we consider particle interactions mediated by the potential of mean force [2]. Secondly, for emission much lower than the plasma frequency, the spectra are dependent on the interspecies collision frequency. We present a model based on the velocity autocorrelation function to capture the spectrum in this low frequency limit.

[1] Befeki, G. Radiation Processes in Plasmas. John Wiley and Sons, 1966.

[2] Scott D. Baalrud and Jérôme Daligault , "Mean force kinetic theory: A convergent kinetic theory for weakly and strongly coupled plasmas", Physics of Plasmas 26, 082106 (2019) https://doi.org/10.1063/1.5095655