Second azimuthal mode of the electron cyclotron instability

Electrostatic oscillations are excited in a cylindrical shell of plasma by an electron beam which passes through the plasma with a helical trajectory. Oscillations of the first azimuthal mode ( l = 1) result from a non-convective instability and have been reported previously. In this paper oscillations of the l = 2 azimuthal mode, resulting from a convective instability, are reported. The frequencies and wavelengths of the l = 2 oscillations, and the range of plasma density over which they occur, are compared with theoretical predictions.