Absolute scale power measurements of frequency-locked coherent transition radiation

We report the absolute power measurement of frequency-locked terahertz coherent transition radiation (CTR) from a train of electron bunches produced by a 17.14 GHz accelerator. Frequency measurements were obtained by two independent techniques: a wave meter with a video diode detector, and a double heterodyne system. Good agreement was seen between these two techniques. The emitted radiation was observed to be a comb of frequencies that are harmonics of the 17.14 GHz accelerator frequency. The heterodyne receiver system showed that each of the emitted harmonics has a very narrow bandwidth, about 25 MHz, which corresponds to the transform-limited bandwidth for the bunch train length of 40 ns. The highest observed frequency was 377.08 GHz, the 22nd harmonic of the 17.14 GHz accelerator frequency. The emitted CTR power was measured on an absolute scale to test the theory of CTR. The power was compared with calculations made using an electric field integral equation (EFIE) formulation and excellent agreement was obtained. This agreement serves as a valuable benchmark of the EFIE code, proof of both its validity and usefulness.