Investigation of beam generated, ion heating RF waves in a non-beam formed cylindrical plasma



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An unmodulated, 3-4 kV, electron beam was injected axially into mirror confined, > 10[raised 12] /cm[cubed] He+ and H[lowered 2]+ plasmas. The plasmas had a continuously decreasing radial density and the experiment was performed in the high density regime, [Pi][lowered e, raised 2]/[Omega][lowered e, raised 2] > 1 ([Pi][lowered e] is the electron plasma frequency and [Omega][lowered e] is the electron cyclotron frequency). Energetic ions above 300 eV were observed demonstrating ion heating in the high density regime. The energetic ions were associated with a strong standing wave radial field inside the plasma column. Cold plasma theory does not predict reflections from a declining density boundary; however, the observed standing waves imply the existence of a reflecting boundary. The wave frequency was near the ion plasma frequency and was found to be ion mass independent. This is consistent with a resonant plasma column model based on a cold, magnetized, high density plasma with waves propagating nearly transverse to the magnetic field.