Development of a compact electromagnetic undulator for linac-based coherent THz radiation source in Thailand

Abstract

A linac-based accelerator system at the Plasma and Beam Physics Research Facility of Chiang Mai University in Thailand can produce relativistic femtosecond electron bunches for generating coherent THz radiation. Originally, this accelerator system was built to produce high peak power THz/FIR radiation via transition radiation technique. To extend the system’s capability in production of higher average power and tunable coherent radiation, a project to add an undulator magnet as a source of coherent THz radiation was started. Construction of a compact 40-period electromagnetic undulator with short period length is in progress. Low electricity consumption, inexpensive materials and uncomplicated fabrication process are aimed. An undulator prototype was designed and built to investigate the magnetic field properties and to estimate the possibility on manufacturing of a future undulator magnet. A computer program RADIA was used to model a three-dimensional undulator prototype based on the fabricated magnet material and dimensions. This prototype has 4.5 periods consisting of 8 main-poles and 2 end-poles for magnetic field correction. The simulated peak magnetic field was 0.1858 T or equivalents to an undulator parameter of 0.98. This led to a calculated undulator radiation wavelength at the first harmonic of 109 μm for electron beam with energy of 10 MeV. This radiation is in the forward direction. After manufacturing, the magnetic field of the undulator prototype was measured and analyzed. The measurement result at an optimal excitation current of 5 A showed that an average period length of 54.8 mm and an average peak magnetic field of 0.1855 T were achieved. The measured undulator parameter was 0.97, which resulted in the undulator radiation wavelength of about 107 μm. A future electromagnetic undulator with 40 periods was designed and will be built based on the study results of this undulator prototype. The estimated angular flux density of the radiation emitted from 40 period undulator magnet is dominated at the first harmonic radiation wavelength of 109 μm, which is equivalent to the radiation frequency of 2.8 THz.