Tunable 35?200GHz, high power, gyrotron

Abstract

The extended frequency tuning range of the Strathclyde University gyrotron is 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 multi-mode in the G-band (140–220GHz). To control the output beam a quasi-optical 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. Since this gyrotron not only supports many transverse modes, but also several longitudinal modes, such beating is a likely consequence.