Free-Free Emission in the Radio-Frequency Range.

Classical and quantum-mechanical calculations for the free-free emission (bremsstrahlung) of electrons in a completely ionized plasma are critically investigated. Avoiding altogether the sometimes misleading assumptions found in the literature, it is concluded that the several classical computations are in complete agreement with the quantum-mechanical approach. Numerical coefficients are derived for a Maxwellian distribution of electron velocities. It is shown that for temperatures below, roughly, 550 0000K the classical results can be safely used; however, for higher temperatures quantum corrections are required. Finally, examination of the problem of a cutoff of the ion's Coulomb potential shows that this cutoff can be neglected for all frequencies above the plasma frequency in the range where the refractive index is effectively unity.