Influence of an electrostatic LEBT chopper on linac beam parameters at CNAO

The Centro Nazionale di Adroterapia Oncologica (CNAO) located in Pavia, Italy, will be the first Italian facility for hadrontherapy of deeply sited tumours [1]. The accelerator consists of a RFQ and of an IH-type drift tube linac, both delivered by GSI [2], followed by a synchrotron built by CNAO. In the low energy beam line (LEBT), an electrostatic chopper horizontally deflects the DC ion source beam via an electric field across two parallel plates at opposite potential. The field is generated by a dual-polarity high voltage module (max. ±5 kV). When triggered, a fast HV switch allows a beam pulse of adjustable length to pass into the RFQ. Within about 1 μs the potential across the plates drops to a few Volt and, after a defined down-time, is restored with a similar time constant. Unlike the Heidelberg facility HIT, CNAO has not foreseen a chopper behind the linac. Hence, chopper disturbances on the ~20 μs pulse can not be corrected for and might affect the multi-turn synchrotron injection. Studies of chopper effects on the beam are subject of this article.