Kinetically unstable distributions as a result of radiative damping in strong electromagnetic fields*

We have identified a novel physical process whereby plasmas can become kinetically unstable when subjected to strong electromagnetic fields and radiative cooling. This can occur under different conditions, such as emittance damping for beams experiencing betatron oscillations in ion-channels and synchrotron cooling for plasmas in strong magnetic fields. Unlike the Lorentz force, the radiation reaction force responsible for cooling the momentum distribution does not conserve momentum space volume and has a preferred cooling direction. This differential cooling can significantly affect the shape of the momentum distribution, resulting in "bunching" in momentum space and strong anisotropies with an energy population inversion under certain electromagnetic field configurations. We analytically examine the case of a collisionless plasma undergoing synchrotron cooling in a strong constant magnetic field and demonstrate a general condition for the development of rings in momentum space, which is fulfilled for many common initial momentum distributions. Such ring momentum distributions are known to be unstable to kinetic instabilities. Specifically, the electron cyclotron maser instability for the case of magnetised plasmas, which leads to coherent radiation emission as it diffuses the ring momentum distribution. This population inversion mechanism and subsequent maser instability are relevant to astrophysical plasmas and coherent radiation mechanisms.

The development of ring distributions and the onset of kinetic instabilities are confirmed with simulations performed with the Particle-in-cell OSIRIS code.