MEASUREMENTS OF VELOCITY RATIO IN A 90 MW GYROKLYSTRON ELECTRON-BEAM

Measurements of the ratio of perpendicular to parallel velocity (alpha = upsilon(perpendicular to)/upsilon(z)) in a 390-420 kV, 160-240 A, s mall orbit gyroklystron electron beam produced by a magnetron injectio n gun have been performed using a capacitive probe. The probe employed guard rings to reduce uncertainties in measurement and calibration du e to fringing electric fields. These studies represent the first measu rements of alpha in such a powerful rotating beam, since the current l evels in this experiment are two to three times higher than those in p revious velocity ratio studies of beams with similar voltage. The effe cts of changes in voltage, current, and magnetic compression on the me asured velocity ratio are investigated. Values of alpha in the range o f 0.8-1.3 are typically observed, which are considerably higher than t he values predicted by theoretical simulations. Errors in the modeling of space charge effects are ruled out as the cause of the discrepancy by the agreement between theoretical and experimental space charge cu rrent limits. Instead, a significant portion of the discrepancy is att ributed to a reduction in cathode magnetic field by the self-axial mag netic fields of the electron beam.