Whistler and electron-cyclotron instabilities in a plasma duct

A gyrating electron beam propagating through a planar plasma duct can excite a whistler wave or an electron-cyclotron wave, depending on the plasma- density profile of the duct and the energy of the beam. The gyrational motion of the electron beam supplies energy to the wave. In the case of a whistler wave the interaction occurs via coupling of the fast cyclotron beam mode (ω ≈ k2vb + ωc/γ0) to the electromagnetic whistler mode through the Weibel instability mechanism, whereas for an electron-cyclotron wave the coupling is through the negative-mass instability mechanism. It is seen that there exist two diflerent beam-energy regimes for excitation of these waves. The growth rate and the efficiency of conversion for both waves are calculated for typical parameters of a gyrotron and a large-orbit gyrotron.