Experimental study of a half-integer resonance with space-charge effects in a synchrotron

Beam losses due to half-integer resonance have been observed in the Heavy Ion Medical Accelerator in Chiba synchrotron, along with exciting the harmonic component of gradient field errors. During operation, while varying the defocusing quadrupole to cross a half-integer tune in the vertical space, the region of bare tunes which causes the half-integer resonance was evaluated. When the initial beam intensity was high, the bare tune where the beam loss occurred became higher. The beam loss occurred rapidly when the half-integer tune was crossed upward, but gradually when it was crossed downward. Those results mean that the half-integer resonance is affected by space-charge-induced tune shifts. This fact was verified experimentally for the first time. The results from a one-dimensional multiparticle simulation agreed with those characteristics. Finally, the beam-size growth and the change in distribution were studied by a simulation.