RF System for IR FEL

476 MHz Pulsed RF system for IRFEL

Infra-Red Free Electron Laser (IR FEL) facility is being developed at RRCAT. It requires an electron beam injector. This injector system requires three synchronized and stable high frequency amplifiers of 29.75 MHz (or its harmonics) for Gun, 476 MHZ for Sub Harmonic Pre-Buncher (SHPB) and 2856 MHz RF system for fundamental frequency Pre-Buncher and LINAC. For generation of these three synchronized and stable RF signals, RF distribution system has been developed. IR FEL uses a 476 MHz SHPB for proper bunching of the electron beam. To feed SHPB RF cavity a pulsed 476 MHz 10 kW RF power amplifier was designed and developed using planar triode tube, along with PXI based Digital Low Level RF system. The amplifier assembled in 19” rack, consists of output cavity assembly, input circuit assembly, RF socket for mounting the tube, output coupler, anode bias assembly, cathode bias assembly. The rectangular waveguide output cavity form the output matching, which has a tuning range of ±50MHz.Input matching and tuning network is realized using strip line based network with required frequency tuning range. The amplifier is having 8.0 kV modulator DC power supply for anode and -90 volts grid power supply. Siemens make PLC based control, interlock and monitoring system is realized for monitoring various parameters and facilitate proper start-up and shutdown of the amplifier system.

[Fig. 8: 476 MHz Pulsed RF system with Digital LLRF System for IR-FEL]

Constant gap voltage in SHPB cavity is very critical as any variation in gap voltage could lead to change in velocity modulation imparted to electron consequently affecting the bunching of the beam at the entry of LINAC. In such situation bunch does not reaches the LINAC on the ideal RF phase. To achieve highly stable RF gap voltage and phase in SHPB cavity,Digital LLRF system is used. It has been tested with RF amplifier and SHPB RF cavity with pulse width of 50µsec at 10 Hz repetition rate with measured phase and amplitudestability of 0.1° and 0.1% respectively.

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