Injector System for Linac-based Infrared Free-electron Laser in Thailand

A possibility to develop a compact linac-based Infrared free-electron laser (IR FEL) facility has been studied at Chiang Mai University (CMU) in Thailand. Characteristics of the emitted FEL light and reliability in operation of the FEL system are determined by the properties of the electron injector, the undulator, and the optical cavity. The proposed injector system for the future IR FEL is based on the electron linear accelerator system at the Plasma and Beam Physics Research Facility at CMU (PBP-CMU). Numerical and experimental studies to adjust the existing system to be able to drive the IR FEL have been performed. The results of preliminary studies and the proposed parameters for the injector and the FEL system are concluded in this contribution. INTRODUCTION Linac-based free-electron lasers (FELs) have recently gain interest worldwide in the accelerator and particle beam community as the new generation light source, which can be utilized in numerous applications. Characteristics of the output FEL radiation are determined by the properties of the electron beam and the undulator, where the light is emitted. Since the FEL light sources require electron beams of high quality, the development and optimization of the injector system are important. Electromagnetic radiation in the infrared wavelength regime, especially the far-infrared (FIR) or THz radiation, is of a great interest source for applications in various fields [1-4]. A possibility to develop an infrared freeelectron laser (IR FEL) is studied at the Plasma and Beam Physics Research Facility, Chiang Mai University (PBPCMU). At this initial stage, we concentrate on the development of an FEL covering the THz radiation wavelength around 50-200 m. Study of the FEL radiation in the mid-infrared (MIR) and near-infrared (NIR) regime will be considered in the future. In order to develop the FEL system, optimization of both injector and FEL system is ongoing. In this paper, we concentrate on an overview of the project and preliminary optimization of a thermionic based electron radiofrequency (RF) injector to produce electron beams with properties yielding the requirements for an IR FEL. PROPOSED IR FEL FACILITY Generally, an IR FEL facility consists of an injector system for generating and accelerating electron beam, an undulator magnet for FEL lasing and an optical cavity for amplifying the FEL radiation output power. For the considered injector system of the proposed IR FEL at CMU, we plan to make use of the existing linac system as much as possible while maintaining its functionality as the femtosecond electron and photon pulse facility. The proposed IR FEL system shown in Fig. 1 consists of an injector system, an accelerating structure, a 180 achromat section, an undulator magnet and an optical cavity. The injector system combines a thermionic cathode RF-gun and an alpha magnet as a magnetic bunch compressor. The accelerating structure is an S-band travelling wave SLAC-type linac, which can be used to accelerate an electron beam to reach a maximum energy of about 30 MeV. The injector system, the linac structure, beam steering and focusing elements as well as beam diagnostic instruments upstream the achromat section will be modified from the existing PBP-CMU linac system [5]. The undulator magnet is a planar type with a length of 1.67 m. The optical cavity composes of two symmetric spherical mirrors with a coupling hole on one of the mirrors. Some parameters of the undulator and the optical cavity used in preliminary FEL optimization are listed in Table 1. Figure 1: Schematic layout of the possible IR/THz FEL system at Chiang Mai University, Thailand. In electron beam and FEL optimizations, we consider two scenarios. The first one is studying the FEL radiation in the case of the electron beam whose bunch length is longer than the radiation wavelength. The other one is for the case of the electron beam whose bunch length is shorter than the radiation wavelength. ____________________________________________ *[email protected] Proceedings of FEL2012, Nara, Japan WEPD32 Progress and Projects ISBN 978-3-95450-123-6 441 C op yr ig ht c ○ 20 12 by th e re sp ec tiv e au th or s Table 1: Parameters of Undulator and Optical Cavity Used in FEL Calculations