Velocity ratio measurements on a 20-60 kV quasi-optical gyrotron electron beam

This paper describes experimental measurements of the velocity ratio (alpha = v(perpendicular to)/v(parallel to)) of an annular electron beam for use in a quasi-optical gyrotron (QOG), The electron gun was originally designed for 80-kV 35-A operation, and identical guns have been used in gyrotron ex periments by several research groups. A pair of capacitive probes in the dr ift region are used to characterize the beam over a wide range of voltage ( 20-60 kV) and current (0-6 A) while varying the magnetic compression and th e intermediate;anode voltage. At low-beam voltage, the peak measured values of ct are much lower than those calculated from electron trajectory simula tions. The capacitive probe results indicate that electrons reflecting from the peak of the magnetic field are important. Increasing the operating vol tage of the gun to 50 kV greatly improves the quality of the electron beam.