Measurements of beam characteristics on a 70 MeV H- cyclotron system for ISOL beam production

A 70 MeV H- cyclotron system was commissioned in order to measure the beam characteristics for the isotope separation on line (ISOL) operation in the energies range between 40 and 70 MeV. Using internal beams, the cyclotron magnet was precisely isochronized by applying the Smith-Garren method, achieving a main-coil current margin of 0.2 A for the beam stability. In the center region, beam profiles were measured by a differential radial probe to verify the specified dee voltage of 50 kV, which allows well-defined turn separation. Extracted beams were then used to check the beam line alignment by monitoring beam losses on segmented collimators and measuring variations on beam profiles. We measured transverse emittances of the cyclotron beam at a current of 25 µA by observing the beam profiles with upstream quadrupole strengths varied, which was the first time for this kind of 70 MeV cyclotron. Transmission efficiency along the beam line was measured as above 98% at a current of 100 µA. A temporary beam line was installed in the ISOL area, and we measured beam current distributions using a beam profile monitor shaped by wobbling at 60 Hz at a target location for beam diameters of 2 and 5 cm. A specific current distribution is generally required to reduce the maximum thermal stresses induced on the target. Finally, a maximum beam power of 50 kW was successfully tested at 70 MeV for 6 h.