J.P. Anderson

Design and emission uniformity studies of a 1.5-MW gyrotron electron gun

We present the design and initial operation of a 96-kV 40-A magnetron injection gun for a 1.5-MW 110-GHz gyrotron. A critical parameter for the successful application of this electron gun is the uniformity of electron emission. The current-voltage curve of emission, at a series of temperatures, is measured. Analysis indicates that the work function of the emitter is 1.6 eV with a (total) spread of 0.07 /spl plusmn/ 0.01 eV. Measurement of the azimuthal emission uniformity with a rotating probe indicates that the work function variation around the azimuth, the global spread, is 0.04 /spl plusmn/ 0.02 eV. The spread due to local (microscopic scale) work function variations is then calculated to be 0.06 /spl plusmn/ 0.02 eV. Temperature variation can be ruled out as the cause of the observed emission nonuniformity.

Studies of the 1.5-MW 110-GHz gyrotron experiment

Results of a 1.5-MW 110-GHz short-pulse (3 /spl mu/s) gyrotron experiment are reported. The gyrotron magnetron injection gun operated at full voltage (96 kV) and current (40 A), producing up to 1.4 MW at 110 GHz in the TE/sub 22,6/ mode. The operation of the TE/sub 22,6/ mode, as well as nearby modes, was measured as a function of magnetic field at the cavity and at the electron gun to produce a mode map. Significant mode competition was found, but the measured efficiency of 37% in the TE/sub 22,6/ mode, without a depressed collector, is close to the design value of 39%. The beam alpha, the ratio of transverse to axial velocity in the electron beam, was measured with a probe. The alpha value was found to be 1.33 when the gyrotron was operating at conditions for achieving the highest output power level (1.4 MW.) This value of alpha is less than the design value of 1.4, possibly accounting for the slightly reduced experimental efficiency. The output power and efficiency, as a function of magnetic field, beam voltage, and beam current, are in good agreement with nonlinear theory and simulations with the MAGY code. These results are promising for the development of an industrial version of this gyrotron capable of long pulse or continuous-wave operation.

Phase retrieval of gyrotron beams based on irradiance moments

We present the formulation of the moment method applied to the determination of phase profiles of microwave beams from known amplitudes. While traditional approaches to this problem employ an iterative error-reduction algorithm, the irradiance moment technique calculates a two-dimensional polynomial phasefront based on the moments of weighted intensity measurements. This novel formulation has the very important advantage of quantifying measurement error, thus allowing for its possible reduction. The validity of the irradiance moment approach is tested and confirmed by examining a simple case of an ideal Gaussian beam with and without measurement errors. The effectiveness of this approach is further demonstrated by applying intensity measurements from cold-test gyrotron data to produce a phasefront solution calculated via the irradiance moment technique. The accuracy of these results is shown to be comparable with that obtained from the previously developed iteration method.

Experimental studies of local and global emission uniformity for a magnetron injection gun

We report measurements of the azimuthal emission nonuniformity of a gyrotrons thermionic cathode as determined from two experiments; one which determines the total emission, and one which scans the emitted beam using a rotating probe. In the total emission experiment, which measured the total collector current over a range of voltages, the cathodes total work function spread was found to be 0.033 eV at T=1000/spl deg/C, centered around 1.88 eV. In the subsequent experiment, which used a mechanically rotatable current probe, global, and local effects were investigated. Voltage was varied to examine emission nonuniformities in different regions of cathode operation. While emission became almost completely uniform in the low-voltage space-charge limited regime, emission varied as much as 50% at high voltages. In another part of the current probe experiment, current--voltage curves were measured at azimuthal locations in 30/spl deg/ increments for several cathode temperatures. From this extensive set of data the work function distribution parameters were identified over small sections of the cathode for the entire cathode surface.

An overmoded coaxial buncher cavity for a 100-MW gyroklystron

An overmoded abrupt transition coaxial buncher cavity has been designed and experimentally cold tested for use in a second-harmonic 17.136-GHz three-cavity 100-MW gyroklystron. Circuit efficiencies of 41% can be achieved with a buncher cavity that has a quality factor of 389 in the TE/sub 021/ mode. Scattering matrix and finite-element codes were used to design and model the cavity theoretically and to determine that the cavity would be stable to oscillation. The experimental cold testing confirmed these results and refined the final dimensions from the theoretical models.

Recent results for the 1.5-MW, 110-GHz gyrotron experiment

High power gyrotrons used for ECH heating of the DIII-D Tokamak at General Atomics have been able to produce 1 MW at 110 GHz for close to 10 s. The next generation of these tubes is being designed and built to produce 1.5 MW. The gyrotron design includes a 96 W, 40 A MIG electron gun to achieve a microwave efficiency of 39% in the TE/sub 22,6/ mode. The physics and engineering features of the 1.5 MW, 110 GHz gyrotron are being examined at MIT, where a short (3 /spl mu/s) pulse experimental version of the gyrotron is used for testing the gun and cavity design. In this study we present the most recent results of these experiments.

Synthesis of gyrotron phase-correcting mirrors using irradiance moments

A new approach using the moments of field amplitudes has been applied for phase correcting mirror synthesis of a 110 GHz gyrotron internal mode converter. The synthesized mirrors have smooth profiles in contrast to the mirrors synthesized using the previously employed iteration method. The design has been checked using a physical optics propagation code with the result of a Gaussian output amplitude at the gyrotron window position.

Design and cold testing of a radial extraction output cavity for a frequency-doubling gyroklystron

Research in the University of Maryland at College Parks Gyroklystron (GKL) Project has recently centered around the development of a high-power high-gain frequency-doubling 17.136-GHz system. The current tube is a four-cavity (input, buncher, penultimate, and output) coaxial frequency-doubling system that will be used to drive a linear accelerator structure. This paper presents the design, simulation, optimization, cold test methodology, and performance data of a proposed radial extraction output cavity in which the microwave energy is extracted through an inner coaxial conductor in the TE/sub 01/ circular mode. The positioning of dielectrics in the drift spaces and the effect of axial and radial misalignments between the inner and outer walls of the cavity were studied in depth. One advantage of this topology is that it reduces the size and complexity of the output waveguide chain otherwise needed to convert the TE/sub 02/ circular mode from the GKL into the standard rectangular waveguide mode for injection into the Haimson Research Corporation accelerator structure. Cold test results show that this new cavity, which has a Q of 458 and a resonant frequency of 17.112 GHz, is a viable replacement for the output cavity currently in the system, as long as the cavity is well aligned.

Operation of a 1.5-MW, 110-GHz gyrotron depressed collector experiment

We report results from a 1.5 MW, 110 GHz gyrotron experiment. Operating in three microsecond pulses at 96 kV and 40 A, 1.4 MW was obtained in the TE/sub 22,6/ mode when power was taken along the axis. New results with an internal mode converter and depressed collector are presented.

Initial results from the 1.5 MW, 110 GHz gyrotron experiment at MIT

We present preliminary results from the 1.5 MW, 110 GHz gyrotron oscillator experiment. Power levels of greater than 1 MW in the design (TE/sub 22,6/) mode are obtained using an 82 kV, 50 A triode magnetron injection gun. Results are also reported for the experiment using the 96 kV, 40 A diode gun.