Headway on a Compact THz FEL at KAERI
At the end of the 90s the first compact THz FEL at KAERI put into operation. Syne a large number of experiments on this FEL were held. Also its specifications were improved during this time. A number of user stations were designed for this FEL. It operates now, but its lifetime is almost terminated and needs frequent repairs. In this connection, a development of a new machine seems reasonable. Its basic scheme was chosen similar to the old one. It consists of a microtron, an electron beamline, and a FEL structure.
A microtron is an RF resonance electron accelerator with a constant frequency and leading magnetic field, and a variable harmonics number. RF power is supplied by a magnetron in our project. A solid state switches modulator for it is based on storage capacities, and a high-voltage transformer. This project seems to be the cheapest alternative of a comparably high quality electron beam of the energy of several MeV.
The electron beam comes further through a beamline to the undulator. This beamline is intended to obtain the optimal beam parameters in the FEL structure at all expected entrance ones and the whole range of the undulator strengths. We selected a hybrid electromagnetic scheme for the undulator. It contains magnetically soft poles and both permanent magnets and conductive busbars. The magnetic field is controlled by current in the busbars. The main advantage of this design over the conventional hybrid one (without coils) is the absence of moving parts.
The peak electron beam current in our case is comparably small and the FEL emission wavelength is long, so it is a problem to get lasing in an open resonator. So a waveguide scheme for our FEL was selected to reduce the mode size. The main problem in this case is wave absorption in a waveguide. A lenticular shape and dielectric coating are used to solve it.
Injection of an electron beam into and extraction from a waveguide, and emission outcoupling in this case are interrelated and sophisticated problems. We consider combining a blind mirror of the FEL waveguide cavity and a beam dump, so the extraction is not necessary. A mesh mirror is used for injection and outcoupling. Another tilted mirror with a hole separates an electron beam and THz emission.
The expected basic parameters of the FEL are the following:
- Wavelength tuning range 0.4…0.6 mm;
- Spectral width 1%;
- Peak power 5 kW;
- Macropulse average power 600 W;
- Macropulse duration 5 μs;
- Repetition rate up to 100 Hz.
Currently, the scheme and the composition of equipment for the FEL were determined. Each part has been simulated and optimized. Thus, both the conceptual and technical designs are ready. The microtron has been commissioned, and is being optimized now. The undulator has been manufactured and is being commissioned. All the magnets for the beamline has been manufactured and tested. The following parts are to be mechanically designed and manufactured: the optical resonator, a vacuum chamber for the beamline, and supports. After that the whole machine can be assembled and commissioned.