How synchrotron emission and absorption shape electron distributions in relativistic plasmas

Synchrotron emission by relativistic electrons in large magnetic fields is an important process that drives power losses in aneutronic fusion plasmas and instabilities in astrophysical plasmas. Because the emission spectrum is momentum-dependent, and the plasma opacity is frequency-dependent, modeling the self-consistent evolution of the electron distribution due to radiation emission and reabsorption is challenging, and existing models have usually focused on emission only. Here, starting from the Fokker-Planck (FP) operator for a general radiation spectrum, we derive a simple analytic FP operator for blackbody radiation with a frequency cutoff, which captures the main effects of the synchrotron radiation in the mixed-opacity regime. The operator can be thought of as an “enhanced collisionality” which can affect plasma transport, and also reduces the current drive efficiency in relativistic plasmas. In aneutronic fusion mirror plasmas, significant suppression of the electron distribution occurs for relativistic values of the perpendicular electron momentum, which therefore emit less radiation than predicted under the assumption of a Maxwell-Juttner distribution. In synchrotron-dominated relativistic plasmas, finite opacity can reduce the instability drive.