Bremsstrahlung

Remarks

Example: let an electron of charge $-e$ and mass $m_e$ pass an ion of charge $Ze$ at distance $b$. Near closest approach its acceleration has size about $Z e^2 / (4\pi\varepsilon_0 m_e b^2)$, where $\varepsilon_0$ is the vacuum permittivity. By the Larmor formula it radiates power proportional to the square of this acceleration. Non-example: radiation from an electron captured by an ion into a bound state is recombination radiation (free-bound), not bremsstrahlung, because the electron is bound after the event. Consider thermal bremsstrahlung of electrons of density $n_e$ and ions of density $n_i$ and charge $Ze$. Its emissivity per unit frequency is proportional to $Z^2 n_e n_i T^{-1/2} e^{-h\nu/(k_B T)}$ times a slowly varying Gaunt factor. Its total power per unit volume is proportional to $Z^2 n_e n_i T^{1/2}$. Collisions between identical particles, such as electron–electron, give no dipole radiation, so in a non-relativistic plasma electron–ion encounters dominate.