Median plane and beam diagnosis in the central region for compact superconducting cyclotron

The magnetic median plane in the cyclotron does not generally coincide with the geometrical mid-plane of magnetic system. This results in radial field imperfection in the acceleration region around the mid-plane causing severe beam loss as observed in the central region of compact K500 superconducting cyclotron at VECC, Kolkata. This is primarily due to small focusing force (axial) and vertical acceptance in the central region of the cyclotron. The radial field being difficult to measure under the backdrop of large main (axial) field, a search coil based system is developed to locate the median plane. Measurement is carried out by introducing median plane perturbations in the central region by sending varying currents in the upper and lower part of the circular trim coil in an effort to diagnose the error. This causes change in median plane position which allows to find the vertical mis-alignment of iron-parts and current element in the central region. The radial field distribution is calculated analytically and then the mis-alignments of magnetic and current elements thus obtained. The present paper describes the methodology used to estimate radial field and its application for different beam analysis in the central region. The results are compared with different beam diagnostic measurements to identify the causes of beam loss. The scheme is used successfully for median plane measurement and correction in the central region of the cyclotron.