10 kW pulsed RF system with Digital LLRF for IR FEL

Infra-Red Free Electron Laser (IR FEL) facility being developed at RRCAT requires electron beam injector. This injector system requires three synchronized and stable high frequency channels 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. IRFEL 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 with Low Level RF (LLRF) system has been developed. The RF power amplifier at 476MHz is developed using planar triode tube having 6.5 kV modulator DC power supply for anode and -200 volts grid power supply. 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. To achieve highly stable RF gap voltage (0.1%) and phase (0.1 °) in SHPB cavity a PXI based Pulsed Digital LLRF system at 476 MHz has been developed. Adaptive feed forward algorithm has been developed and tested with RF amplifier along with SHPB RF cavity in pulse mode with pulse width of 50µsec at 10 Hz rep rate. Experiments with beam were carried out and parameters of adaptive feed forward algorithm were optimized to achieve Phase and amplitude stability of 0.1° and 0.1 %. 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 amplitude stability of 0.1° and 0.1% respectively.

Fig. 9 : 476 MHz RF system with DLLRF System