FEL & Utilization Section

Design & development of an infrared Free Electron Laser (IR-FEL)

A long-term, three-stage program on Free Electron Lasers (FELs) is being pursued by the FUS at RRCAT, with the possible future directions being a high average power FEL at infrared wavelength, or high power, short pulsed (femtosecond), short wavelength FEL in the ultraviolet (UV) to vacuum ultraviolet (VUV) wavelength region. The development of such an FEL requires the several high-end technologies like stable low-level and high power RF systems, high energy linear accelerators, diagnostics for short pulsed electron and photon beams, long undulator sections, seeding schemes, etc. The first two stages of the FEL program at RRCAT are aimed at addressing some of these issues, while simultaneously developing an IR-FEL to serve a user facility.

FEL & Utilization Section

In the first stage, a far infra-red FEL [called the ‘Compact Ultrafast Terahertz FEL’ (CUTE-FEL)] was developed as a technology demonstrator. This ‘green-field’ FEL was built with the aim of learning about the different science and technology issues related to building of an FEL, viz. FEL Physics, Radio frequency (RF) accelerating structures and injector systems, undulators, electron beam transport line and diagnostics, etc. The injector system of this FEL was based on an 8-cell Plane Wave Transformer (PWT) linac structure developed in-house, and a 2.5m long, pure permanent magnet undulator for the FEL was also built in-house. Both these important sub-systems were qualified through offline characterization before installation in the CUTE-FFEL setup. The first radiation from the CUTE-FEL setup was observed in 2012, and the setup has since been de-commissioned.

The second stage, which is presently underway, envisages the development of an infra-red FEL (IR-FEL) that is tunable in the wavelength range of 15 – 50 μm. This FEL radiation will be beamed to a ‘user facility’ that has been setup for experiments initially planned in the area of Condensed Matter Physics. More details of the user facility are given in the next section on ‘Development of facilities for Terahertz - Infrared (THz-IR) spectroscopy’. Table 1 summarizes some important design parameters of the IR-FEL, and Fig. 1 shows the IR-FEL setup installed inside its radiation shielded area. The first signature of lasing was observed in the setup in November 2016 [Current Science, Vol. 114, No. 2, p. 367] employing an injector linac system developed in-house. Subsequently, a major upgrade of the injector system was undertaken in 2018-19, which has resulted in a very significant improvement (almost doubling) of the peak current with improved electron beam stability and repeatability. Recent experiments on the IR-FEL setup with the upgraded injector system have successfully demonstrated saturation of the IR-FEL with ~ 7 mW CW average out-coupled power at 28 micron wavelength [Fig.2] for 2 Hz operation. This corresponds to a peak out-coupled power greater than the design goal of 2 MW in 10 ps micro-pulses. The transverse optical mode profile has been measured using a pyrocam [Fig. 3]. Experiments are presently underway on fine-tuning of the operation parameters, alignment of the optical cavity, and on the characterization of the optical (IR) radiation from the FEL. This optical radiation will subsequently be transported to the dedicated user facility to facilitate the planned experiments in condensed matter physics.


Table 1: Important design parameters of the IR-FEL

Design wavelength 15 –50 μm
Design electron beam energy 15 – 25 MeV
Peak current > 30 A
Undulator period /length 5 cm / 2.5m
RMS Undulator parameter 1.2 at 27mm gap
Peak /Average out-coupled power 2 MW /15 - 30 mW @ 10 Hz


Figure 1: A picture of the IR-FEL injector system (Top) and the transport line and optical cavity (Bottom).
 
Figure 1: A picture of the IR-FEL injector system (Top) and the transport line and optical cavity (Bottom).
Figure 1: A picture of the IR-FEL injector system (Top) and the transport line and optical cavity (Bottom).
 
Figure 1: A picture of the IR-FEL injector system (Top) and the transport line and optical cavity (Bottom).
Figure 1: A picture of the IR-FEL injector system (Top) and the transport line and optical cavity (Bottom).
Fig. 2: Laser power meter display showing the measured CW average power > 7 mW.
Fig. 3: Transverse optical mode profile of the FEL output.


Best viewed in 1024x768 resolution