Experimental study of a 1.5-MW, 110-GHz gyrotron oscillator

Abstract

This thesis reports the design, construction and testing of a 1.5 MW, 110 GHz gyrotron oscillator. This high power microwave tube has been proposed as the next evolutionary step for gyrotrons used to provide electron cyclotron heating required in fusion devices. A short pulse gyrotron based on the industrial tube design was built at MIT for experimental studies. The experiments are the first demonstration of such high powers at 110 GHz. Using a 96 kV, 40 A electron beam, over 1.4 MW was axially extracted in the design (TE22,6) mode in 3 us pulses, corresponding to a microwave efficiency of 37 %. The beam alpha, the ratio of transverse to axial velocity in the electron beam, was measured with a probe. At the high efficiency operating point the beam alpha was measured as 1.33. 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. In another phase of the experiment, a second tube was built and tested. This tube used the same gun and cavity but also incorporated an internal mode converter to transform the generated waveguide mode into a free-space propagating beam. The gun was tested to full power and current in the experiment. Preliminary results were obtained. A mode map was generated to locate the region of operating parameters for the design mode, as well as for neighboring modes. Scans of the output microwave beam were also taken using a power-detecting diode. Future work will focus on generating high power, as well as operating the collector at a depressed voltage for even higher efficiency. A study is also presented of the 96 kV, 40 A magnetron injection gun.

(cont.) A critical parameter for the successful application of this electron gun is the uniformity of electron emission. The current-voltage response, at a series of temperatures, is measured for two separate cathodes. Analysis indicates that the work function of the first emitter is 1.76 eV with a (total) spread of 0.04 eV. The second emitter has a spread of 0.03 eV, centered around 1.88 eV. Measurement of the azimuthal emission uniformity with

(cont.) a rotating probe indicates that the work function variation around the azimuth, the global spread, is 0.03 eV for the first cathode, 0.02 eV for the second. The spread due to local (microscopic scale) work function variations is then calculated to be around 0.03 eV for both cathodes. Based on the beam azimuthal measurements, temperature variation is ruled out as the cause of emission nonuniformity. In another part of the current probe experiment, current-voltage curves were measured at azimuthal locations in 30⁰ 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. In addition, a formulation is presented of the irradiance moments 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 intensity measurements. This novel formulation has the important advantage of identifying measurement error, thus allowing for its possible removal. The validity of the irradiance moment approach is shown 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 iteration method. This algorithm was then applied to the design of the phase correcting mirrors used in the internal mode converter experiment.