Improvement of the Kei2 source for a new carbon therapy facility

A compact electron cyclotron resonance ion source with all permanent magnets has been developed for a new carbon therapy facility at NIRS. Initial tests with the Kei2 source have given a maximum intensity of 530𝑒𝜇A for C4+ at an extraction voltage of 40 kV and beam stability of better than 6% for 90 h at 280𝑒𝜇A at 30 kV. In these beam tests, there was continuously unwanted discharge around the puller at high-voltage extraction (above 40 kV). The start of discharge is caused by the short distance of 5 mm between the end of the plasma chamber and puller. At high-voltage extraction, this discharge will give rise to unwanted vapor from the copper puller. Then, the vacuum pressure in the extraction region will become worse. In order to suppress the discharge, the plasma chamber and puller were modified. Finally, a beam intensity of 380𝑒𝜇A was obtained with good stability at an extraction voltage of 30 kV. This article describes the modification of the puller for stable beam extraction, measurements of beam emittance, and improvement of C4+ intensity