OBSERVATIONS OF RISING TUNE DURING THE INJECTION INSTABILITY OF THE IPNS RCS PROTON BUNCH *

In the Rapid Cycling Synchrotron (RCS), a single proton bunch (h=1) is accelerated from 50 MeV to 450 MeV in approximately 14.2 ms. The bunch experiences an instability shortly after injection (<1 ms). During the first millisecond, the beam is bunched but little acceleration takes place; therefore this period is similar to a storage ring mode of operation. Natural vertical oscillations (assumed to be tune lines) show the vertical tune to be rising toward the bare tune value, suggesting neutralization of space charge and a reduction of its detuning effects. Neutralization time near injection ranges from 250 μs- 500 μs, depending on the background gas pressure. Oscillations move from the LSB to the USB before disappearing. Tun