The extended frequency tuning range of the Strathclyde University gyrotron will be demonstrated. The gyrotron consisted of a field-immersed, field emission cold cathode and a shaped anode cavity. The cavity was oversized with an ill-defined boundary at one end. This system could therefore support the oscillation of a very large number of TEpqs modes. The maser was thus highly tunable via differential mode excitation. A heterodyne technique was used to demonstrate the maser oscillation in the Ka-band (26.5-40GHz) , with the maser oscillating at 35GHz in the TE01 mode. Previous to this, a quasi-optical diffraction grating spectrometer was used to show the maser was oscillating under different conditions respectively, single and multi-mode in the W-band (75-110GHz) and multimode in the G-band(140-220GHz). To control the output beam a quasioptical mode converter has been successfully developed. This converts various TEpq modes into a Gaussian beam. Studying the output of the maser, the mm-wave pulse has been shown to consist of a -100ns envelope containing -2ns pulses spaced -5ns apart. Although this behaviour may have several explanations, some of the most recent code simulations predict this behaviour as a result of mode beating in the complex transient start-up of the maser oscillation. With this gyrotron supporting not only many transverse modes but also several longitudinal modes, such beating is a likely consequence.
S. N. Spark,
"Tunable 35-200GHz gyrotron", Proc. SPIE 1514, 15th International Conference on Infrared and Millimeter Waves, 15146A (1 December 1990); doi: 10.1117/12.2301636; https://doi.org/10.1117/12.2301636