Very high field-gradient has become available by a new Magnetic Alloy (MA)-loaded cavity developed for high intensity proton synchrotrons. The available RF voltage per core is ten times larger than that of the ordinary ferrite core. The maximum voltage of 20 kV has been achieved by the High-Field Gradient Cavity(HGC) of 40 cm in length [1].Because the intrinsic Q-value of the MA core is low, acceleration without any tuning system also becomes possible. The first beam acceleration test using the HGC has been performed successfully at the HIMAC(Heavy Ion Medical Accelerator in Chiba). Furthermore, the dual harmonic RF and barrier bucket experiments have been carried out.Another advantage of the MA-loaded cavity is that it is easy to compensate the beam loading. The feed forward beam compensation was applied for both HGC on the test bench using electron beam and MA-loaded cavity installed in the AGS for the barrier bucket experiment. A new development for high-Q HGC using cut core configuration will be also reported.

The first beam acceleration test using a High Gradient Cavity (HGC) loaded with Magnetic Alloy (MA) cores has been carried out at the HIMAC (Heavy Ion Medical Accelerator in Chiba). Acceleration of heavy ions, which required a wide frequency sweep from 1 MHz to 8 MHz, was successfully carried out without resonant frequency tuning.

Multi-orbit synchrotron(MOS) with fixed field alternating gradient(FFAG) focusing is attractive for acceleration of high intensity hadron beams because acceleration cycle could be increased. The magnetic field of MOS is static, therefore, the repetition rate of acceleration could be increased more than 10 times larger than that of ordinary rapid cycling synchrotron(RCS) if an efficient high voltage RF accelerating system becomes available. Recently, a new type of high gradient RF cavity (HGC) using high permeability magnetic alloy (MA) has been developed and MOS with FFAG focusing becomes very promising. In order to clarify the feasibility of rapid cycling MOS (RCMOS) experimentally, proof-of-principle (POP) machine, which accelerates protons up to 1MeV with 1kHz repetition, is under development. We have also made several designs on high intensity proton accelerators with RCMOS for various applications such as accelerator driven system(ADS) for energy breeder, spallation neutron source and proton driver for muon collider.

A high field gradient cavity(HGC) which has been developed for high intensity proton synchrotrons is suitable to generate multi-harmonic voltage on the accelerating gap because of the broad band impedance of the cavity. In this paper, the experimental results on the dual harmonic rf, saw-tooth rf, and bunch compression by barrier bucket are described.

The PoP(Proof of Principle) FFAG accelerator, which was the world first FFAG proton synchrotron, was constructed at KEK. After the first proton beam acceleration of FFAG, we studied various aspects of the PoP FFAG. The observed beam orbit shift during the acceleration was consistent with the simulation result. It was confirmed that the beam intensity increased about five times with multi-bunch injection. The horizontal acceptance of the PoP FFAG was estimated at about 4000π mm.mrad experimentally.