Frequent corrections of the magnetic field of the JAERI AVF cyclotron were required for keeping a beam current constant during long time operation. We observed correlation between the magnetic field and the temperature of the cyclotron magnet yoke by measuring the magnetic field with an NMR probe and the temperature with platinum resistance thermometers As a result, this instability of a cyclotron beam was induced by temperature-change of the magnet yoke caused mainly by thermal conduction from the main coil. To restrain the thermal conduction to the yoke, we have inserted temperature-controlled copper plates between the yoke and the main coil. In addition, a temperature control system of the cooling water of the trim coils has been installed, which is independent of the total cooling system for controlling the pole tip temperature. An optimum for controlling the pole tip temperature. An optimum condition of the temperature control systems for stabilizing the magnetic field has been investigated.
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