Bragg Resonator Cyclotron Resonance Maser Experiments Driven by a Microsecond, Intense Electron Beam Accelerator

The cyclotron resonance maser (CRM) has proven to be attractive for many high power microwave applications such as fusion plasma heating, radar/communications, and high gradient RF accelerators. Most of the previous CRM experiments with MV electron beams have been conducted with short (<0.1 musec) pulses. The present work contains the first comprehensive experimental study on mode competition in a high-Q Bragg resonator CRM employing a microsecond, relativistic electron beam. We have designed and fabricated a high-Q sinusoidal Bragg resonator designed to excite high frequency CARM oscillation of the TE_{31} cylindrical cavity mode at 18.9 GHz. The measured reflectivity of the TE_{31} mode is consistent with the prediction of uncoupled single mode theory. A high quality annular electron beam with low velocity spread and energy spread is produced through an apertured mask-anode. The apertured electron beam has been characterized by the use of glass plate diagnostics. The measured beam velocity ratio, v_{| }/v_{|}, was shown to be in agreement with computer simulation results and the theoretical predictions. Experiments have been performed for 4 cases: (1) Bragg resonator with ripples half-inward, (2) large diameter smooth tube without Bragg resonator, (3) Bragg resonator with ripples fully-outward, and (4) small diameter smooth tube without Bragg resonator. The Bragg resonator with ripples half-inward generated high power microwave radiation from TE_ {11} gyro-BWO interactions, TE _{21} absolute instability, and high harmonic gyrotron modes. Considerably less power from the TE_{11} gyro -BWO was observed for the Bragg resonator with ripples fully -outward. The microwave emission from the TE_ {21} absolute instability in the Bragg resonator with ripples fully-outward was successfully suppressed by lowering the cavity magnetic field. These three undesired oscillations, (TE _{21} absolute instability, TE _{11} gyro-BWO, TE _{51} second and third harmonic), were the most serious competing modes in the present Bragg resonator CRM experiments, apparently suppressing the TE _{31} CARM oscillation. For the Bragg resonator with ripples half-inward, we have performed gyrotron experiments with a high current electron beam. In these experiments, we have observed mode competition between the TE_{21 } absolute instability and the TE_ {11} gyro-BWO interaction by the use of frequency measurements and gas breakdown diagnostics.