Observation of electron cyclotron harmonic emissions due to electrostatic instabilities in mirror-confined plasma

Abstract

Electron cyclotron instabilities are of fundamental importance in space and laboratory plasmas having nonthermal electron distribution functions. Observations are reported of electromagnetic emissions at electron cyclotron harmonics in a mirror-confined electron cyclotron resonance plasma. The emissions are attributed to electrostatic instabilities involving a warm 200–400eV mirror-confined population of electrons and a much colder ∼1 eV component confined by the positive plasma potential. Due to electron-ion Coulomb collisions, electrons with energy ≲50eV are scattered into the loss cone, leading to a warm ring distribution with depleted low-energy components. The combination of a warm ring distribution and a cold distribution is susceptible to electrostatic cyclotron instabilities, with the unstable modes having frequencies near the upper hybrid frequency of the cold component, which can be significantly lower than the overall upper hybrid frequency. Coulomb collisions are shown to be capable of forming the ring distribution for typical experimental parameters and a linear stability analysis is undertaken using a model thermal ring distribution with different fractions of cold electrons. The observations are supported by Vlasov simulations that are used to study the nonlinear evolution of the instabilities.