Single-turn extraction from a K110 AVF cyclotron by flat-top acceleration

Kurashima, Satoshi; Miyawaki, Nobumasa; Okumura, Susumu; Ishibori, Ikuo; Nara, Takayuki; Agematsu, Takashi; Yoshida, Kenichi; Yokota, Wataru; Nakamura, Yoshiteru*; Arakawa, Kazuo; Fukuda, Mitsuhiro*

Single-turn extraction from the JAEA AVF cyclotron with a K number of 110 using a flat-top (FT) acceleration system has been achieved to reduce the energy spread of an ion beam for microbeam formation with energy up to hundreds of MeV and to increase extraction efficiency from the cyclotron. In order to generate an FT waveform voltage using the fifth-harmonic frequency on a dee electrode, an FT resonator was designed using MAFIA code to achieve downsizing and low power consumption. The FT resonator, coupled to the main resonator through a coupling capacitor, covered the full range of the fifth harmonic frequency from 55 to 110 MHz. Various ion beams, accelerated using different acceleration harmonic mode of h = 1 and 2, such as 220 MeV C (h = 2), 260 MeV Ne (h = 2), and 45 MeV H (h = 1) were developed by FT acceleration. A clear turn separation of the beam bunches was successfully observed at the extraction region of the large-scale AVF cyclotron with number of revolutions greater than 200. As a result, high extraction efficiency (over 95%) from the cyclotron was achieved. Single-turn extraction was confirmed by counting the number of beam bunches out of the cyclotron for an injected beam pulsed by a beam chopping system in the injection line. The energy spread of the 260 MeV Ne beam was measured using an analyzing magnet and we verified a reduction of the energy spread from E/E = 0.1% to 0.05% by single-turn extraction.