Numerical studies of a second harmonic RF cavity for the IPNS RCS

The Intense Pulsed Neutron Source (IPNS) Rapid Cycling Synchrotron (RCS) delivers 450-MeV protons in 70 ns pulses at 30 Hz to a heavy-metal target producing spallation neutrons for material science research. The average current extracted from the RCS is 15 μA with a peak intensity of 10 amps. The large circulating currents in the RCS generate oscillations in the bunch which are presently controlled by modulating the phase of the two RF cavities. By adding second harmonic (SH) rf, the bunch length can be increased reducing the peak current. Simulations suggest that a 20-40 percent increase in extracted current should be achievable. The simulation program allows for phasing between fundamental and SH RF voltages. Initial studies to optimize phase indicate the need to maximize bucket area early in the acceleration cycle, whereas the bunching factor is more significant later in the cycle