Development of the Flat-top Acceleration System for the Rcnp Avf Cyclotron

A new flat-top acceleration system has been developed for an RCNP K140 AVF cyclotron to reduce the energy spread of a beam and to improve beam transmission to an RCNP K400 ring cyclotron. A flat-top cavity with a length of 700 mm and an outer diameter of 170 mm is capacitively coupled to a main resonator. A maximum peak harmonic voltage of 5 kV can be produced on a dee electrode by feeding power from a 10 kW transistor amplifier. A wide range of harmonic frequencies from 50 to 80 MHz is covered by the flat-top system. Three kinds of harmonic modes, fifth, seventh and ninth harmonics, are available to produce a flat-top voltage waveform in the whole fundamental frequency range between 6 and 18 MHz. The existing main resonator of the AVF cyclotron has been modified to avoid the interference by the parasitic resonance mode generated in the transversal direction of the dee electrode. Development of the flattop accelerated beam is now in progress. The beam intensity of a high quality 300 MeV proton beam has been remarkably increased by a factor of four. The energy spread of the 300 MeV proton beam, transferred to the Grand Raiden in achromatic mode, has been improved to ΔE/E = 1 × 10.