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

Electron cyclotron instabilities are of fundamental importance in space and laboratory plasmas having non-thermal electron distribution functions. Observations are reported of electromagnetic emissions at electron cyclotron harmonics in a mirror-conned electron cyclotron resonance (ECR) plasma. The emissions are attributed to electrostatic instabilities involving a warm 200-400 eV mirror-conned population of electrons and a much colder ~ 1 eV component conned by the positive plasma potential. Due to electron-ion Coulomb collisions, electrons with energy ≲ 50 eV 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 signicantly lower than the over-all 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 non-linear evolution of the instabilities.