Initial Operation of a High Power Cusp Gun*

A cusp gun, designed to generate an axis encircling beam with low velocity spread for high harmonic, high power (70 kV, 3.5 A) operation in gyrotrons and peniotrons, has been successfully fabricated and hot tested at reduced voltage, in a beam tester designed to measure beam perveance, size, thickness, velocity ratio (beam alpha), and later, beam ripple and velocity spread. The correct beam perveance was observed, and a relatively thin, round, hollow beam, with approximately the correct dimensions, was imaged on the Cerium glass witness plate. The data indicated, however, that there was substantial current intercepted on the anode and excessively high beam alpha measured through the capacitive probes. Maxwell magnetic field simulations and EGUN trajectory simulations showed these results to be consistent with a substantial fraction of the electrons incident on the glass probe generating reflecting secondaries and/or re-reflected primaries, which back-stream through the capacitive probes and then are intercepted at the anode.