Non-relativistic electron-ion bremsstrahlung: an approximate formula for all parameters

TL;DR

This paper presents a simple, accurate approximate formula for non-relativistic electron-ion bremsstrahlung cross sections applicable across a wide range of velocities and photon energies, simplifying calculations in plasma physics.

Contribution

It introduces a new approximate formula valid for all relevant parameters, reducing computational complexity while maintaining high accuracy.

Findings

Error remains below 3% across most regimes.

Thermally averaged spectra are accurate to better than 2%.

The formula spans from quantum to classical regimes.

Abstract

The evaluation of the electron-ion bremsstrahlung cross section exact to all orders in the Coulomb potential is computationally expensive due to the appearance of hypergeometric functions. Therefore tabulations are widely used. Here, we provide an approximate formula for the non-relativistic dipole process valid for all applicable relative velocities and photon energies. Its validity spans from the Born- to the classical regime and from soft-photon emission to the kinematic endpoint. The error remains below 3% (and widely below 1%) except at an isolated region of hard-photon emission at the quantum-to-classical crossover. We use the formula to obtain the thermally averaged emission spectrum and cooling function in a Maxwellian plasma and demonstrate that they are accurate to better than 2%.