Preamble

“Bremsstrahlung” is used both as the generic name to describe emission due to an accelerated charged particle and as the specific name for the emission when this acceleration is due to the Coulomb field of another particle. Here we are concerned with bremsstrahlung in the latter more restrictive sense. Bremsstrahlung can result in the emission of waves in any wave mode in a plasma, but most interest is in the emission of transverse waves. Emission of Langmuir waves is of less interest because, unlike transverse waves in a plasma, Langmuir waves are generated efficiently by the Cerenkov process. The absorption process corresponding to bremsstrahlung is called collisional damping or, in some astrophysical literature, free–free absorption.

Qualitative Discussion of Bremsstrahlung

Bremsstrahlung due to Coulomb interactions between electrons and ions is an important emission process in a wide variety of plasmas. We mention only three general applications. (i) For laboratory plasmas and many space plasmas, bremsstrahlung is the basic thermal emission process at radio frequencies. Radio frequency emission results from distant electron–ion encounters in which the motion of the electron is perturbed only slightly by the Coulomb field of the ion. (ii) High-frequency photons, with an energy comparable with the initial energy of the electron, can result from a close encounter between an electron and an ion. So-called non-thermal bremsstrahlung due to energetic electrons is an important source of X-rays from a plasma.