Development of the phase probe for the NIRS small cyclotron HM-18

The small cyclotron HM-18 of the National Institute of Radiological Sciences (NIRS) has been operated for use in RI production since 1994. The HM-18 allows us to accelerate protons and deuterons at fixed energies of 18 and 9 MeV, respectively. It has four trim coils for generation of the isochronous fields. Until recently, currents of the four trim coils had been adjusted only by monitoring the output beam intensity. In order to exactly produce the isochronous fields, a new phase probe has been installed in the HM-18. The phase probe has a simple structure in which four copper electrode plates of 60 mm x 70 mm in area are glued to a copper base plate with a polyimide insulator between them. The thicknesses of the copper plates and the polyimide are 0.1 mm. This structure has an advantage that it can be easily installed in the cyclotron; only one part of a pair of upper and lower electrodes, which is usually adopted, is simply attached on the surface of the (lower) sector pole. The development of the phase probe and some results of a preliminary beam test using it are reported. (author)