Tunable gyrotrons

Progress in the design, theoretical modeling and operation of prototype tunable gyrotrons as sources for coherent submillimeter-millimeter radiation is surveyed. Low power, highly tunable gyrotrons (electron cyclotron resonance masers) are of great potential worth for spectroscopic studies of materials and for plasma diagnostics. Attention is focused on three generations of gyrotrons which were built at the University of Sydney, and the associated theoretical work and experimental results. Analytical models are defined for cavity modes, diffraction Q's and the parameters controlling the conversion of electron beam energy to RF emissions. Linear and nonlinear analyses are carried out of the energy transfer and frequency detuning processes. Experimental results are provided to demonstrate techniques for broadband tuning from one mode to another by varying the magnetic field, narrowband tuning by varying the gyrofrequency over a mode, and the requirements of mechanical tuning. Tests have verified that a split-cavity design can serve the same purpose as using a cavity diameter change for continuous tuning.