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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 (E{{E₀}) }^- at energy E larger than a critical energy E₀ where the transition energy E₀ and the power are inversely proportional to the photon-field intensity.
Hasegawa et al. (Mon,) studied this question.