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 in the FUS of RRCAT with the ultimate aim of building a high power FEL capable of producing short pulsed (femtosecond) radiation at short wavelengths ranging from ultraviolet (UV) to vacuum ultraviolet (VUV) in stage-3. The development of such an FEL requires the mastering of several technologies for the development of sub-systems 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 ongoing FEL program are aimed at addressing some of these issues, while simultaneously developing an IR-FEL 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. This ‘green-field’ technology demonstrator FEL was built with the aim of learning about the different science and technology issues related to building of an FEL, viz. Radio frequency (RF) accelerating structures, 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 signature of build-up of coherence in the CUTE-FEL setup was observed in 2012.

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, which is presently in an advanced stage of commissioning, has been built to serve as a ‘user facility’ for experiments that have initially been planned in-house in the area of Condensed Matter Physics. 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 has been observed in the setup in November 2016 [Current Science, Vol. 114, No. 2, p. 367], and experiments are presently underway on fine-tuning of individual sub-systems of the IR-FEL to achieve saturation.

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 setup installed inside its
Figure 1: A picture of the IR-FEL setup installed inside its

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