Experimental Study of a 200-300 GHZ Megawatt Gyrotron Oscillator
Abstract
A detailed experimental study is presented of a pulsed megawatt gyrotron oscillator operating in the 200-300 GHz range whose design is consistent with continuous operation for ECRH of fusion plasmas. A cylindrical waveguide cavity over 20 free space wavelengths in diameter was designed to limit ohmic wall losses in the copper cavity to less than 2 kW/cm^2. The frequency spacing between TE waveguide modes in this highly overmoded cavity is less than 2%. The cavity is positioned at the peak magnetic field of a 14 T Bitter magnet. Two different radii beams produced by magnetron injection guns (MIGs) were used to excite the cavity. The large and small MIG guns produced annular beams of 0.75 cm and 0.45 cm radius respectively. The guns operate with beam currents approaching 60 A and voltages as high as 100 kV. The voltage is produced by a line-type modulator with a pulse length of 3 mu s at a repetition rate of up to 4 Hz. Experimental results have shown that megawatt power levels can be generated in CW gyrotron oscillators at 200-300 GHz with efficiencies approaching 20%. The emission is single mode, single frequency with a single rotation which can easily be mode converted for transmission. No multimoding was observed at the high powers and efficiencies. The highest power reached with the large MIG gun was 0.97 MW at 230 GHz in the TE_{34,6} mode with an efficiency of 18% and beam parameters of 59 A and 90 kV. This was the peak efficiency which was also obtained at 290 GHz in the TE_{41,8 } mode with a power of 0.89 MW and beam parameters of 54 A and 93 kV. The highest power reached with the small MIG gun was 0.78 MW at 280 GHz in the TE _{25,13} mode with an efficiency of 17% and beam parameters of 51 A and 92 kV. The small MIG gun peak efficiency was also 18% at 0.72 MW, 290 GHz in the TE_{25,14} mode. Efficiencies obtained in this experiment are about half of less highly overmoded gyrotrons. Analysis of the experiment shows mode competition is the main cause of the low efficiency, with voltage depression, beam thickness and velocity spread contributing only a fraction to the decrease. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.) (Abstract shortened by UMI.).
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1992
- Bibcode:
- 1992PhDT.......156G
- Keywords:
-
- MAGNETRON INJECTION GUNS;
- Physics: Fluid and Plasma; Physics: Electricity and Magnetism; Engineering: Electronics and Electrical