GYROTRON EXPERIMENTS USING CAVITIES OF DIFFERENT OHMIC-Q

The change in oscillating characteristics of a tunable (20-100 GHz) gy rotron was studied with the change of cavity wall material, all other dimensions and operating parameters remaining constant. The gyrotron w as based on a two-electrode electron gun consisting of a field emissio n, field-immersed cold cathode and a shaped anode cavity. The construc tion material of the cavity was changed from stainless-steel (sigma = 1 . 39 x 10(6) Omega(-1)m(-1)) to oxygen-free, high conductivity (OFHC ) copper (sigma = 5 . 92 x 10(7) Omega(-1)m(-1)) with an associated 6 . 5-fold increase in the cavity ohmic quality factor for all modes. It was possible to determine, by measuring the relative millimetre wave output power and pulse duration, the optimum anode-cathode separation. This experimentally determined that the relativistic electron beam pr oduction was unaltered with the change in cavity construction material . The maser was operated with an anode-cathode separation of 1 . 5 cm and millimetre wave pulse duration of 150 ns at an intra-cavity B fiel d of 1 . 0 T to 4 . 0 T. At an oscillating frequency of 24 GHz the cha nge from stainless-steel to OFHC copper resulted in an increase in the millimetre wave output power from (1 . 15+/-0 . 16) MW to (5 . 23+/-0 . 17) MW. At 29 GHz the power increased from (1 . 52+/-0 . 25) MW to (2 . 63+/-0 . 17) MW with an associated change in the mode of oscillat ion from the resonant, hollow-centred TE(021) mode in the stainless-st eel cavity to a volume mode oscillating in the OFHC copper cavity.