Bump-on-tail instability of twisted excitations in rotating cold atomic clouds

TL;DR

This paper develops a kinetic theory for twisted phonons with orbital angular momentum in rotating cold atomic clouds, revealing conditions under which these excitations become unstable and grow, unlike standard damped waves.

Contribution

It introduces a novel kinetic framework for twisted density waves in magneto optical traps, accounting for orbital angular momentum effects on stability.

Findings

Twisted phonons can become unstable in rotating clouds.

Orbital angular momentum influences the dispersion and damping of density waves.

Instability threshold depends on cloud rotation and structure.

Abstract

We develop a kinetic theory for twisted density waves (phonons), carrying a finite amount of orbital angular momentum, in large magneto optical traps, where the collective processes due to the exchange of scattered photons are considered. Explicit expressions for the dispersion relation and for the kinetic (Landau) damping are derived and contributions from the orbital angular momentum are discussed. We show that for rotating clouds, exhibiting ring-shaped structures, phonons carrying orbital angular momentum can cross the instability threshold and grow out of noise, while the usual plane wave solutions are kinetically damped.