A compact accelerator-base source of THz Coherent Undulator Radiation (THz-CUR) at the Kyoto University has been developed with the purpose of providing intense quasimonochromatic and tunable THz-CUR at frequencies below 1 THz. The source is driven by a compact accelerator using a photocathode RF gun, which emits an electron beam with a fixed energy of 4.6 MeV and high bunch charge. The THz-CUR from our source can be generated when a compressed electron bunch passes through a planar undulator. In this study, we investigate the properties of this radiation, including the total radiation energy, spatial distribution, and power spectrum. With an electron beam of 160 pC bunch charge, the total radiation energy of THz-CUR at 0.16 THz was 1.2 µJ in the micropulse. The THz-CUR covering the frequency range from 0.16 THz to 0.65 THz could be produced by changing the magnetic field of the undulator at a 60 pC bunch charge. Due to the influence of the space charge forces causing the degradation of electron beam qualities, saturation of the radiation energy occurred, and the radiation power spectrum at a frequency of 0.65 THz could not be observed in the high charge condition (160 pC). The effects of bunch lengthening, energy spread, and emittance growth and the influence of the phase error on the generation of THz radiation are also discussed in this paper. This opens up the possibility of understanding the generation of THz-CUR and sheds further light on the enhancement of the radiation power.

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A THz coherent undulator radiation (THz-CUR) source has been developed at the Institute of Advanced Energy, Kyoto University. A photocathode Radio-Frequency (RF) gun and a bunch compressor chicane are used for generating short-bunch electron beams. When the electron beam energy is low, the space-charge effect strongly degrades the beam quality, such as the bunch length and the energy spread at the high bunch charge condition at around 160 pC, and results in the reduction of the highest frequency and the maximum radiated power of the THz-CUR. To mitigate the space charge effect, we have investigated the dependence of the electron beam quality on the laser distribution in transverse and longitudinal directions by using a numerical simulation code, General Particle Tracer GPT. The manipulation of the laser distribution has potential for improving the performance of the THz-CUR source. The electron bunch was effectively compressed with the chicane magnet when the laser transverse distribution was the truncated Gaussian profile, illuminating a cathode. Moreover, the compressed electron bunch was shortened by enlarging the laser pulse width. Consequently, an enhancement of the radiated power of the THz-CUR has been indicated.

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