Short-time change of heavy-ion microbeams with different mass to charge ratios by scaling method for the JAEA AVF cyclotron

The JAEA AVF cyclotron provides heavy-ion beams covering a wide range of linear-energy-transfers for microbeam formation. Two types of microbeam formation systems, one using a micro-aperture and the other focusing lenses, are installed on two vertical beam lines of the cyclotron. The average beam time for an experiment using the former system is usually less than 3 h, that is comparable to the time for cyclotron tuning. The time ratio between experiment and tuning determines the usage efficiency of the facility. In order to reduce the tuning time, a scaling method has been introduced to change the ion species with various mass to charge ratios (M/Q) in a shorter total time. The principle of the scaling method is to keep the magnetic rigidity of ion beams constant. This requirement is easily achieved by adjusting the extraction voltage of an ion source proportionally to the M/Q in the beam injection line. Although some cyclotron adjustments, other than the magnetic field strength at the extraction radius, are required, the tuning can be completed within 20 min, and no change is basically required in the beam transport line downstream of the cyclotron. Using the scaling method, 255 MeV 20Ne7+, 335 MeV 20Ne8+, and 440 MeV 40Ar13+ beams were extracted from the cyclotron in sequence after the usual tuning of a 220 MeV 12C5+. As a result, we have succeeded in changing the ion species of the heavy-ion microbeam within a total of 30 min.