FEL & Utilization Section

An Infra-red free electron laser (IR-FEL) tunable in the wavelength range 12.5 – 50 μm is presently in an advanced stage of commissioning at RRCAT. Infrared (IR) radiation from the FEL will be beamed to a ‘user facility’ that has been setup for experiments initially planned in the area of Condensed Matter Physics. Details of the user facility are given in the next section on ‘Development of facilities for Terahertz - Infrared (THz-IR) spectroscopy’.

A schematic of the IR-FEL is shown below in Fig.1, and Fig. 2 shows a picture of the IR-FEL installed inside a 60 m [L] x 5 m [W] radiation shielded area. The injector section has a 90 keV pulsed electron source, a bunching section and a travelling wave linear accelerator (linac) that accelerates the electron beam to 15 – 25 MeV energy. An electron beam transport line transports the electron beam from the linac exit up to the undulator entry, while manipulating it to have the desired transverse profile and energy spread for FEL operation. The undulator is of the pure permanent magnet type with 5 cm period, 50 periods and the RMS undulator parameter Ku variable from 0.5 – 1.25. The optical radiation from the FEL is out-coupled through a hole in the downstream mirror with ~ 10 % power out-coupling

Figure 1: Schematic of the IR- FEL at RRCAT Indore, India.

Figure 2: Shows the IR-FEL setup installed inside the radiation shielded area [60 m L x 5 m W x 3.5 m H]

First commissioning of the IR-FEL was accomplished in 2016 by employing a simple injector system comprising a pulsed thermionic electron gun, one 476 MHz sub-harmonic pre-buncher and two cascaded 12-cell Plane Wave Transformer (PWT) linac structures. Figure 3 shoes a schematic of this injector system. These RF accelerating structures were developed in-house, and the commissioning experiments achieved an electron beam of ~ 25 A peak current at a maximum beam energy of 18.6 MeV. The first signature of lasing at 32 μm wavelength was observed in the setup in November 2016 [Current Science, Vol. 114, No. 2, p. 367] with a measured out-coupled power estimated to be ~ 10⁴ – 10⁵ times higher than the spontaneous radiation power expected for the electron beam parameters employed in the experiment.

Figure 3: Indigenous Injector system for first lasing in 2016.

Figure 4: Upgraded Injector system for saturated lasing from 2020 onwards.

An upgrade of the injector system was undertaken in 2018-19 with a new electron gun, and with the addition of more RF accelerating structures as per the injector system design for the IR-FEL shown in the schematic in Fig.4 above. The new electron gun is capable of delivering 1-1.5 nC charge in 0.75 – 1 ns FWHM pulses at 29.75 MHz, 59.5 MHz, or 119 MHz (user settable) for a macro-pulse duration variable from 1 – 10 μs at a pulse repetition rate (PRR) up to 25 Hz. The bunching system comprises a 476 MHz sub-harmonic pre-buncher, an S-band pre-buncher and an S-band accelerating buncher, which is followed by a 3 m long, S-band travelling wave linac structure to accelerate electron beam to the desired energy. The injector system is immersed in a magnetic field generated using 20 pancake type air-core solenoid magnets. Figure 4 shows a schematic of the upgraded injector system.

The first commissioning experiments with the upgraded injector system in 2019 achieved saturation of the IR-FEL with > 7 mW Continuous Wave (CW) average out-coupled power at 28 μm wavelength for 2 Hz operation [NIM-A 1003 (2021) 165321]. This corresponds to a peak out-coupled power of 4.1 MW in 10 ps FWHM pulses, which is greater than the design goal of 2 MW in 10 ps pulses. The transverse optical mode profile measured using an IR camera (Pyrocam) is close to Gaussian [Fig. 5].

Figure 5: Transverse optical mode profile of the FEL output.

Optimization of the IR-FEL operation in 2021 has led to an increase in the CW average out-coupled power from the device up to 19 mW at 24 μm wavelength for 2 Hz operation, with a measured power variation within +/- 5 % over an hour. A typical variation of the out-coupled power with time is shown in Fig.6. The peak out-coupled power in these experiments is estimated to be 4.4 MW in 10 ps FWHM pulses. A peak electron beam current ~ 75 A has been achieved in these experiments at ~ 25 MeV beam energy. The CW average electron beam power during these experiments was 6 W. Tunability of the IR-FEL has been established from 12.5 μm to 40 μm by varying the electron beam energy and the undulator parameter. Further experiments are underway to increase the out-coupled power to > 6 mW at wavelengths beyond 30 μm, and to extend the wavelength up to the design value of 50 μm. The IR radiation from the FEL will subsequently be transported to the user facility over a distance > 50 m to facilitate the planned experiments in condensed matter physics

Figure 6: Variation of the measured IR-FEL power with time.