Bremsstrahlung in a Dense Plasma

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Abstract
The bremsstrahlung emitted by an electron scattered in a Coulomb field was first calculated by Bethe and Heitler. The total cross section for production of photons with wave number between k and k + dk by a nonrelativistic electron of kinetic energy {epsilon} is d{sigma}/dk dk = 16/3 Z{sup 2}r{sub 0}{sup 2} (e{sup 2}/hc) (mc{sup 2}/{epsilon})log ({radical} {epsilon}/hck + {radical} {epsilon}/hck -1) dk/k, where Ze is the charge of the (heavy) ion, and r{sub 0} is the classical electron radius. Bremsstrahlung in a plasma has been computed by a number of authors in the approximation of replacing the Coulomb field by a cut-off Coulomb or static Debye potential. It is the purpose of this communication to call attention to another important effect of the medium upon the rate of emission of bremsstrahlung. This may be described as a modification of the relation of the photon's energy to its wave number, due to the index of refraction of the medium. Equivalently, we note that one must include in the calculation of bremsstrahlung in a medium the photon-medium interactions which result in the 'clothing' of a 'bare' photon. The replacement of a particle by a quasiparticle has long been known to be important in the description of strongly interacting systems of massive particles such as liquid helium; the effect can be particularly dramatic for a photon because the medium gives a nonzero effective mass to the quasi-photon

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