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, small orbit gyroklystron electron beam produced by a magnetron injection gun have been performed using a capacitive probe. The probe employed guard rings to reduce uncertainties in measurement and calibration due to fringing electric fields. These studies represent the first measurements of alpha in such a powerful rotating beam, since the current levels in this experiment are two to three times higher than those in previous velocity ratio studies of beams with similar voltage. The effects of changes in voltage, current, and magnetic compression on the measured velocity ratio are investigated. Values of alpha in the range of 0.8-1.3 are typically observed, which are considerably higher than the 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 current limits. Instead, a significant portion of the discrepancy is attributed to a reduction in cathode magnetic field by the self-axial magnetic fields of the electron beam.