Excitation of whistler and slow-X waves by runaway electrons in a collisional plasma

Runaway electrons are known to provide robust ideal or collisionless

kinetic drive for plasma wave instabilities in both the whistler and

slow-X branches, via the anomalous Doppler-shifted cyclotron

resonances. In a cold and dense post-thermal-quench plasma,

collisional damping of the plasma waves can compete with the

collisionless drive. Previous studies have found that for its higher

wavelength and frequency, slow-X waves suffer stronger collisional

damping than the whistlers, while the ideal growth rate of slow-X

modes is higher. Here we study runaway avalanche distributions that maintain the same eigen distribution and increase only in magnitude over time. The distributions are computed from the

relativistic Fokker-Planck-Boltzmann solver, upon which a linear

dispersion analysis is performed to search for the most unstable or

least damped slow-X and whistler modes. Taking into account the

effect of plasma density, plasma temperature, and effective charge

number, we find that the slow-X modes tend to be excited before the

whistlers in a runaway current ramp-up. Furthermore, even when the

runaway current density is sufficiently high that both branches are

excited, the most unstable slow-X mode has much higher growth rate

than the most unstable whistler mode. The qualitative and

quantitative trends uncovered in current study indicate that even

though past experiments and modeling efforts have concentrated on whistler modes, there's a compelling case that slow-X modes should also be a key area of focus in the runaway self-mediation through wave instabilities.