THE DESIGN AND STUDY OF THE SUB-MILLIMETER WAVE LENGTH GYROTRON AND FUNDAMENTAL AND SECOND CYCLOTRON HARMONICS

Abstract

This dissertation documents research activities directed toward designing high power, high efficiency gyrotrons to operate in the sub-millimeter wavelength region. The gyrotron is to produce pulsed RF power at 670 GHz, with possible application to a novel scheme for detecting concealed radioactive materials. High efficiency is to be achieved by designing a cavity resonator in which the electrons interact with the high-order TE31,8 mode. The choice of resonating mode helps to alleviate Ohmic losses in the cavity walls, and simulation results show that the output efficiency could be more than 30%. The design study takes into account a variety of known effects that could affect efficiency, such as orbital velocity spread, voltage depression and after-cavity interaction. The 670GHz gyrotron was built using the resonator design; operation confirmed that record high efficiency was achieved at an output power level of about 200 kilowatts. In addition, the issue of radial spread in electron guiding centers, which is related to the design of the magnetron injection gun used in the 670GHz gyrotron, was also examined. This spread degrades the interaction between the electrons and the RF field. This often overlooked issue is important for future electron gun designs; this thesis presents analytical methods for estimating how much the degradation affects gyrotron efficiency. The analytical method was verified with numerical simulation, showing that the efficiency's sensitivity to spread in guiding centers is highly dependent on the location of an annular electron beam: when the beam is injected in the inner peak of the desired mode, the radial spread should be kept to less than 1/3 of the RF wavelength.

Finally, the dissertation investigates the possibility of further extending the operating parameters of the gyrotron by using the second harmonic of the electron cyclotron resonance. An average output power could be increased by operating the gyrotron continuously rather than in pulses. Using the second cyclotron harmonic allows the magnetic field requirement for resonance condition to be reduced by a factor of two, so that in 670 GHz gyrotrons the pulsed solenoid can be replaced with a cryo-magnet. The investigation shows that for the TE31,8 mode at the second cyclotron harmonic, the operating mode has only one competing mode at the fundamental cyclotron harmonic that could present a mode stability issue. Numerical simulation shows that this mode is TE11,6, the operating mode can suppress this mode, while achieving 20% interaction efficiency. Results also reveal that the resonator for the operating mode at second cyclotron harmonic must be modified to increase the Q-factor. Continuously operating gyrotrons using cryo-magnets have been used for plasma heating in controlled thermonuclear fusion research, albeit at lower frequency than the 670 GHz of the current study.