Magnetic field stabilization by temperature control of an azimuthally varying field cyclotron magnet.

A magnetic field drift, gradual decrease of the order of 10^-4 in several tens of hours, was observed with the beam intensity decrease in an operation of an azimuthally varying field (AVF) cyclotron. From our experimental results, we show that the temperature increase of the magnet iron by the heat transfer from the excitation coils can induce such change of the magnetic field as to deteriorate the beam quality. The temperature control of the magnet iron was realized by thermal isolation between the main coil and the yoke and by precise control of the cooling water temperature of the trim coils attached to the pole surfaces in order to prevent temperature change of the magnet iron. The magnetic field stability of ±5×10^-6 and the beam intensity stability of ±2% have been achieved by this temperature control. [ABSTRACT FROM AUTHOR]