Plasma heating by an intense relativistic electron beam: An experimental study

A 3–20 kA, 300–350 kV, 60 nsec electron beam has been axially injected through various anode foils into a 1.6 m long, mirror confined, fully ionized hydrogen plasma, with an initial plasma density variable from 5×1011/cm3 to 2×1013/cm3. Beam propagation and total plasma heating were studied, and special studies were done of return current heating and direct ion heating by the beam. Total energy deposition efficiencies of 4%–30% were observed. Variations with parameters showed stronger heating with thinner anode foils and good agreement with theoretical predictions based on beam energy loss by trapping in the unstable waves generated by the two-stream instability. Return current heating was found to be only 10%–20% of the total heating for beam-to-plasma density ratios ⩽0.02. Anomalous plasma resistivity was observed in all cases; the use of a thinner anode foil strongly increased this resistivity during the beam, probably due to anomalous resistivity induced by the two-stream instability. Rapid ion heating was observed and inferred to be primarily by the return current interaction, by anomalous electron-ion energy transfer, and by radial electrostatic fields induced by the beam.