Low-impedance RF system with a new type of feedback amplifier for a high-intensity synchrotron under heavy beam loading

In the ISIS facility at Rutherford Appleton Laboratory (RAL) in the UK, a second target station project is in progress. Four 2nd-harmonic RF cavities were installed in the ISIS synchrotron to increase the trapping efficiency, and to mitigate the space charge detuning, allowing a 50% increase in the accelerated beam intensity. A very low-impedance RF system for the 2nd-harmonic cavity was developed by a collaboration of Argonne National Laboratory (US), KEK (Japan) and RAL. The system consists of a 240 kW triode as a final-amplifier with plate-to-grid feedback. The measured output-impedance is less than 30 Ω over a frequency range of 2.6–6.2 MHz. High-power tests were performed using a swept-frequency at 50 Hz repetition rate. The operation was stable, and a 6.2 kV peak per gap was obtained. The voltage gain of the final-amplifier is 25–30, which gradually decreases with the frequency. A test with beam is planned at ISIS in the near future.