Non-uniform emission studies of a magnetron injection gun

Abstract

This thesis investigates the experimental measurement and theoretical simulation of the effects of azimuthal emission non-uniformity of a 96 kV, 40 amp magnetron injection gun (MIG) used in a gyrotron. The accomplishments of this thesis include: Experimental measurement of the azimuthal emission non-uniformity of the MIG gun; Simulation of the beam quality of the MIG gun using MICHELLE 3-D, the first simulation of a MIG electron beam with azimuthal non-uniformity; Benchmarking the MICHELLE 3-D code to other established gun optics codes; Evaluation of the effects on the velocity spread and pitch factor of azimuthal non-uniformity in the MIG gun, showing that the direct effect on the beam quality is very small; Design, fabrication, and testing in the gyrotron of a capacitive probe system divided into four quadrants to measure azimuthal asymmetries of the electron beam; Use of the capacitive probes to measure low-frequency (100 - 160 MHz) oscillations on the beam, the first measurement of such oscillations in a microsecond pulse length gyrotron; First results on testing a new MIG cathode for emission non-uniformity using a special test chamber built by Calabazas Creek Research.

(cont.) This research will contribute to our understanding of the properties of intense electron beams produced by MIG guns in high-power gyrotrons. MIG's are widely used in gyrotron oscillators and amplifiers for fusion applications to create a beam of gyrating electrons generally operating in the temperature limited regime of emission. Due to this dependence on the temperature of the cathode, variation of the emitter surface temperature will result in inhomogeneous emission. Non-uniform emission is attributed to a deviation in the cathode work function as well. Studies have shown this inhomogeneous beam current density can lead to increased mode competition and velocity spread contributing to an overall decreased efficiency of the gyrotron. This research focuses on the effects on velocity spread and in turn the efficiency of the device from non-uniform current emission. Initially, we measured experimentally the detailed azimuthal non-uniformity profile of an existing 110 GHz gyrotron oscillator at MIT. Using a rotating collector current probe the current density of different emitter angles was extracted. These results agreed fairly well with previous measurements of Anderson et al.

(cont.) This non-uniformity profile was then used with a 3-D simulation code to do the first complete 3-D model from the cathode to the cavity of a MIG. In order to investigate these effects of beam non-uniformity with simulation, we use MICHELLE 3-D developed by SAIC. MICHELLE 3-D has been benchmarked to MICHELLE 2-D and EGUN in the case of a uniform beam. The non-uniform beam measurements are entered into MICHELLE and results are computed at four different azimuthal quadrants of different current densities and for the overall beam, giving special attention to the differences in the beam pitch factor and perpendicular velocity spread. MICHELLE found azimuthal non-uniformity to be a fairly small effect on the overall beam quality. Concurrently with the MICHELLE 3-D simulations, segmented pitch factor probes are implemented to measure the pitch factor in the four azimuthal quadrants. In an attempt to compare with MICHELLE's results, these four capacitive probes measure the induced image charge of different azimuthal sections of the electron beam, enabling an estimation of differences in the pitch factor between quadrants.

(cont.) Unfortunately, the experimental error is found to be quite high (±15%) rendering differences in the pitch factor to be contained within the error boundaries. Though the capacitive probes are found to have too much error for adequate resolution of the pitch factor, they are also used to discover the first observations of low-frequency oscillations in a short pulse MW gyrotron. These frequencies, from 100-160 MHz, are found to be dependent on the beam parameters such as the beam voltage, current, magnetic field, and magnetic compression ratio. The frequency range is remarkably close to the frequency of an electron in the adiabatic trap and the experimental as well as the predicted theoretical oscillation behavior of trapped electrons are discussed. Last, initial progress has been made to test three new cathodes on the Calabazas Creek Research cathode tester. This tester is a dedicated test stand for azimuthal non-uniformity able to obtain a measurement directly at the cathode instead of at the collector end of the device. The setup procedure and results on the first cathode test for the 96 kV, 40 amp gun are reported and future tests are summarized.