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

This paper describes experimental measurements of the velocity ratio (α=ν_⊥/ν_||) 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 experiments by several research groups. A pair of capacitive probes in the drift 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 the intermediate-anode voltage. At low-beam voltage, the peak measured values of α are much lower than those calculated from electron trajectory simulations. The capacitive probe results indicate that electrons reflecting from the peak of the magnetic field are important, increasing the operating voltage of the gun to 50 kV greatly improves the quality of the electron beam.