Reduction of uncontrollable spilled beam in RF-knockout slow extraction

A 3D-scanning irradiation method gated with the patient's respiration has been developed for the new Heavy Ion Medical Accelerator treatment facility in Chiba. The RF-knockout slow-extraction method has been used for fast on/off switching of the beam from the synchrotron during scanning irradiation. However, a small amount of beam that remained just inside the separatrix spilled out even without the transverse RF field, and limits the controllable low beam rate. We proposed to apply another transverse RF field with a frequency component matched with the betatron frequency of particles near the stopband in addition to the original transverse RF field. The particles just inside the separatrix can be selectively extracted during the irradiation, and the uncontrollable spilled beam can be suppressed. The spilled beam was measured with the ion chamber, and was reduced to 0.4% compared to the original method.