Plasma Distribution Function in a Superthermal Radiation Field

A plasma which is immersed in superthermal radiation suffers velocity-space diffusion which is enhanced by the photon-induced Coulomb-field fluctuations. This enhanced diffusion universally produces a power-law distribution ${(\frac{E}{{E}_{0}})}^{\ensuremath{-}\ensuremath{\kappa}}$ at energy $E$ larger than a critical energy ${E}_{0}$ where the transition energy ${E}_{0}$ and the power $\ensuremath{\kappa}$ are inversely proportional to the photon-field intensity.