Interactions of a Plasma with a Spiraling Electron Beam

A hollow, spiraling electron beam with energies along and across the applied magnetic field (B0) was injected into argon (7 × 10−5 Torr) to produce a plasma. With increasing plasma density oscillations were observed at frequencies above integral multiples of the electron cyclotron frequency. A theoretical study of the coupling between electrostatic plasma waves and waves on a spiraling beam is shown to predict the observed frequencies and wavelengths. The theoretical analysis accounts for the finite thickness of the beam and for finite Larmor-radius effects in the plasma. The latter are shown to explain the sudden jumps in the observed frequencies which resulted when the plasma density increased. The results show that a spiraling beam with energy across B0 is able to transfer some of this energy to the plasma even when the electrons are hot.