6. Fast Beam Cut-Off Method in RF-knockout Extraction For Spot Scanning
Takuji Furukawa, Koji Noda, Shinji Shibuya,Masayuki Muramatsu, Mitsutaka Kanazawa,Eiichi Takada and Satoru Yamada
Keywords: RF-knockout slow extraction, synchrotron, heavy ion therapy, dose management, spot scanning
Beam scanning methods, such as spot and raster scanning, have been developed in order to achieve a high irradiation accuracy, even for an irregular-shaped target. Early studies indicated that to significantly reduce any unwanted dose in these irradiation methods, the time structure of an extracted beam (spill) should have a ripple of less than ±20%, and the response time to beam-off should be less than 1% of the shortest irradiation time of one spot, because the extracted particles after the cut-off signal considerably affect the dose management for irradiation.
At the HIMAC synchrotron, the RF-knockout extraction has utilized a bunched beam to reduce the beam-spill ripple. Therefore, particles near the resonance can be spilled out from the separatrices by the synchrotron oscillation as well as by a transverse RF field. From this viewpoint, a fast beam cut-off method, turning off the transverse and longitudinal RF fields at the same time, has been proposed and verified by both simulations and experiments. The delay from the beam cut-off signal to beam-off has been improved to around 60 µs from 700 µs by turning off only the transverse RF field. Unwanted dose has been considerably reduced by around a factor of 10 compared with that by the usual method.
The delay of 700 µs corresponds to one period of the synchrotron oscillation. In the synchrotron, the delay is limited by the excursion time from the position just outside the separatrix to the extraction channel. The delay of 60 µs, obtained by the new method, is in good agreement with the analytical solution of this excursion time. Therefore, our proposal is one of the best methods to minimize the unwanted dose. The proposed method will play an important role for precise dose management in spot scanning.
Publications:
Furukawa T. and Noda K.: Nucl. Instrum. Meth. A, 489, 59-67, 2002.
|