Observation of Frequency Locked Coherent Transition Radiation

Measurements of frequency locked, coherent transition radiation (CTR) were performed at the 17 GHz high-gradient accelerator facility built by Haimson Research Corporation (HRC) at the Massachusetts Institute of Technology Plasma Science and Fusion Center. CTR produced from a metallic foil placed in the beam path was extracted through a window, and measured with a variety of detectors, including: WR6 diode detector, and double heterodyne receiver system. The angular energy distribution measured by the diode agrees with calculations for a 15 MeV, 150 mA, 110 ns beam of 1 ps bunches. Heterodyne receiver and frequency meter measurements were able to show frequency locking, namely inter-bunch coherence at integer multiples of the accelerator RF frequency of 17.14 GHz. At the locked frequencies the power levels are enhanced by the number of bunches in a single beam pulse. The CTR was measured as a comb of locked frequencies up to 377 GHz, with a bandwidth of 50 MHz.