Experimental Study of a 1 Mw, 170 Ghz Gyrotron Oscillator Experimental Study of a 1 Mw, 170 Ghz Gyrotron Oscillator


A detailed experimental study is presented of a 1 MW, 170 GHz gyrotron oscillator whose design is consistent with the ECH requirements of the International Thermonuclear Experimental Reactor (ITER) for bulk heating and current drive. This work is the first to demonstrate that megawatt power level at 170 GHz can be achieved in a gyrotron with high efficiency for plasma heating applications. Maximum output power of 1.5 MW is obtained at 170.1 GHz in 85 kV, 50 A operation for an efficiency of 35%. Although the experiment at MIT is conducted with short pulses (3 ps), the gyrotron is designed to be suitable for development by industry for continuous wave operation. The peak ohmic loss on the cavity wall for 1 MW of output power is calculated to be 2.3 kW/cm 2 , which can be handled using present cooling technology. Mode competition problems in a highly over-moded cavity are studied to maximize the efficiency. Various aspects of electron gun design are examined to obtain high quality electron beams with very low velocity spread. A triode magnetron injection gun is designed using the EGUN simulation code. A total perpendicular velocity spread of less than 8% is realized by designing a low-sensitivity, non-adiabatic gun. The RF power is generated in a short tapered cavity with an iris step. The operating mode is the TE 28 , 8 , 1 mode. A mode converter is designed to convert the RF output to a Gaussian beam. Efficiencies between 34%-36% are consistently obtained over a wide range of operating parameters. These efficiencies agree with the highest values predicted by the multimode simulations. The startup scenario is investigated and observed to agree with the linear theory. The measured beam velocity ratio is consistent with EGUN simulation. Interception of reflected beam by the mod-anode is measured as a function of velocity ratio, from which the beam velocity spreads are estimated. A preliminary test of the mode converter shows that the radiation from the dimpled wall launcher is a Gaussian-like beam. Acknowledgments This thesis is the culmination of six years of study and research at MIT. Its completion would not have been accomplished without the help of many people. I thank Rick Temkin, for accepting me into this group and providing continual support and guidance throughout the research. I thank Ken Kreischer, for the opportunity to work on such a challenging and rewarding experiment, and the invaluable …

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