High-power traveling wave tubes powered by a relativistic electron beam

This thesis presents the results of a high power traveling wave tube (TWT) amplifier experiment. The experiment tested the feasibility of a high power, rippled wall waveguide TWT powered by a relativistic electron beam. Initially, the amplifiers consisted of a single section of slow wave structure. Two TWTs of this type were used, with lengths of 11 and 22 periods. These single stage tubes were linear and operated in the TM{dollar}sb{lcub}01{rcub}{dollar} mode at maximum gains of 33 dB and bandwidths on order of 20 MHz centered at 8.76 GHz. The maximum efficiency was 11% corresponding to an output power of 110 MW. Below beam currents of 1.4 kA, the single stage tubes were monochromatic and phase stable to within {dollar}pm{dollar}8{dollar}spcirc{dollar}; however, above this current, a "sideband"-like structure developed in the frequency spectrum. The two "sidebands" were unequally displaced from the center frequency. As the current was increased still further to 1.6 kA, the single stage amplifier oscillated due to positive feedback arising from reflections at the exit taper of the TWT. At this point, the TWT was no longer useful as an amplifier.; To reduce the positive feedback and stop the oscillations, we severed the amplifier. Two different lengths of sever were used, the shorter of the two having the highest gain. The shorter sever saturated at 975 A, with a total gain of 37 dB and total power output of 410 MW. Beyond this current, the tube was no longer linear with respect to the input power. The bandwidth of the severed tube was about 100 MHz centered at 8.76 GHz. The severed amplifiers showed the "sideband"-like behaviour at all the beam currents used. The "sideband" frequency separation from the center frequency increased with beam current. The relative importance of the "sidebands" increased with increasing beam current and the total power in the amplifier. As much as 32% of the total power could be in any one "sideband". When the "sidebands" were taken into account, the total power at the magnetron frequency was 209 MW, giving an efficiency of 24%.