Modelling of the pressure distribution in the cyclotron central region

Beam loss due to charge exchange with the residual gases plays a significant role in the cyclotrons when heavy ions or H-IDare accelerated. For this reason the gap's dimensions, the radial dimension and the pump positions should be optimised. In 1965 Ehler described the performance of his HPIG source [1]. Small internal H-IDsources based on Ehler's PIG source design have been used in many medical cyclotrons needing only 50-200 IlA of external beam. It has been noticeable that some of them use special geometrical configuration of puller. This geometrical configuration has been chosen to improve some properties of puller such as optical property, mechanical stability and heat conductivity. In this paper we investigate the pressure distribution in such puller's geometry and calculate the transmission factor due to charge exchange. A model representing this geometry has been manufactured and was compared with other configurations to investigate the transmission factor.