LOW OUTPUT-IMPEDANCE RF SYSTEM FOR 2ND HARMONIC CAVITY IN THE ISIS SYNCHROTRON

In the ISIS facility at Rutherford Appleton Laboratory (RAL) in the UK, a second target station project is under construction. Four 2nd harmonic RF cavities will be installed in the ISIS synchrotron to increase the trapping efficiency, and to mitigate the space charge detuning, allowing a 50 % increase in accelerated beam intensity. A very low output-impedance RF system for the 2nd harmonic cavity has been developed by a collaboration of RAL, Argonne National Laboratory (US) and KEK (Japan). 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 ohms over the frequency range of 2.7 - 6.2 MHz, which agrees well with calculations. High power tests were performed using swept-frequency at 50 Hz repetition rate. The operation is almost stable, and more than 12 kV peak-to-peak was obtained. The voltage gain of the final amplifier is 25 – 30. This decreases gradually with frequency due to the decreasing input-impedance of the triode. A test with beam is planned at ISIS in the near future.