Generation of Bernstein modes in space and laboratory plasmas by injection of an electron beam

Abstract

The theory of electron-beam interaction with Bernstein plasma modes is developed for conditions of active space plasma experiments as well as experiments in large vacuum chambers. Translational movement of the beam along a magnetic field and Larmor rotation of the beam are considered as sources of free energy for wave build-up. Part of the energy of translational movement is lost to the excitation of Langmuir plasma oscillations. Bernstein modes then slowly grow owing to a ‘fan’-type instability. In this case the emission spectrum consists of lines near the half-harmonics of the electron gyrofrequency, ≈(n+½)ωc. Larmor gyration of the beam is not destroyed by the excitation of Langmuir turbulence, and thus there is strong hydrodynamic interaction of the rotated beam with the Bernstein modes. Integer harmonics of gyrofrequency nωc are generated in this case, with the spectral maximum near the upper-hybrid plasma frequency.