Population inversion in other radiatively cooled plasmas (betatron cooling in laboratory beams)

Determine whether radiatively cooled plasmas other than synchrotron-cooled systems—specifically high-energy particle beams undergoing betatron cooling in ion channels under laboratory conditions—develop momentum-space population inversions (i.e., ring-shaped distributions with positive perpendicular gradient ∂f/∂p⊥ > 0) analogous to those predicted for synchrotron cooling in a uniform magnetic field within the Landau–Lifshitz radiation reaction framework.

Background

This work shows that, in a strong uniform magnetic field, Landau–Lifshitz radiation reaction drives anisotropic synchrotron cooling which compresses momentum-space volume and generically produces ring-shaped momentum distributions with inverted Landau populations in sufficiently hot relativistic plasmas. Thresholds and timescales for ring formation are derived and compared with astrophysical and laboratory regimes.

The authors suggest that analogous non-conservative cooling mechanisms, such as betatron radiation in ion channels where radiated power depends nonlinearly on the oscillation amplitude, may produce similar momentum-space inversions. They explicitly conjecture that laboratory high-energy beams undergoing betatron cooling are prime candidates, but this remains to be established quantitatively.