Properties of THz coherent undulator radiation generated from a compact accelerator source at Kyoto University

Siriwan Krainara,Toshiteru Kii,Shuya Chatani,Hideaki Ohgaki,Heishun Zen

doi:10.1063/1.5110342

Abstract

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.

Highlights

THz Coherent Undulator Radiation (THz-Coherent Undulator Radiation (CUR)) from compressed electron bunch passing through a planar undulator is generated using a compact accelerator-based THz-CUR source developed at the Kyoto University
We identified that the total radiation energy saturates in the higher charge region, and the power spectrum of THz-CUR at 0.65 THz frequency cannot be observed under the condition of 160 pC bunch charge
The phase error obtained from the magnetic field error of undulator has no influence on the generation of THz-CUR at the fundamental frequency

Summary

Introduction

The terahertz region of the electromagnetic spectrum (0.1–10 THz) is situated between microwave and infrared radiation and is called the “THz frequency gap.” This radiation can meet the requirements of applications in the fields of medical diagnostics, noninvasive imaging, material research, telecommunications, and security. In material sciences, its application in molecular and lattice vibration has drawn attention due to its use in material identification. Several types of reliable and efficient THz sources to support the mentioned applications have been developed over the last 30 years, for example, Gunn Diodes, Quantum-cascade lasers (QCL), backward wave oscillators, and free-electron lasers (FELs). FELs can produce high-power coherent radiation with a tunable wavelength, a short pulse, and high intensity. These features make FELs attractive as sources of coherent THz radiation. Nowadays, there are many techniques developed to emit the high power THz radiation from electron beams, e.g., the Smith-Purcell FEL, the Cerenkov FEL, the Coherent Transition Radiation (CTR), and the Coherent Undulator Radiation (CUR). According to CUR source, a high-power narrowband THz radiation providing sharp resonance at a certain frequency can be produced with continuously tunable frequencies. FELs can produce high-power coherent radiation with a tunable wavelength, a short pulse, and high intensity.. FELs can produce high-power coherent radiation with a tunable wavelength, a short pulse, and high intensity.9 These features make FELs attractive as sources of coherent THz radiation.. There are many techniques developed to emit the high power THz radiation from electron beams, e.g., the Smith-Purcell FEL, the Cerenkov FEL, the Coherent Transition Radiation (CTR), and the Coherent Undulator Radiation (CUR).. According to CUR source, a high-power narrowband THz radiation providing sharp resonance at a certain frequency can be produced with continuously tunable frequencies. The specific characteristic of narrow pulse width is crucial for the investigation of nonlinear phenomena, the identification of the matter, and other applications such as THz irradiation and pump-probe spectroscopy.

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