Effect of source tuning parameters on the plasma potential of heavy ions and its influence on the longitudinal optics of the high current injector

Plasma potentials for various heavy ions have been measured using the retarding field technique in the 18 GHz High Temperature Superconducting ECR ion source, PKDELIS. The influence of various source parameters viz., RF power, gas pressure, magnetic field, negative DC bias and gas mixing on the plasma potential is studied. It is observed that the plasma potentials are decreasing for increasing charge states and a mass effect is clearly observed for the ions with similar operating gas pressures. In the case of gas mixing, it is observed that the plasma potential minimises at an optimum value of the gas pressure of the mixing gas and the mean charge state maximises at this value. The energy spread arising from the plasma potential influences the longitudinal optics of the high current injector in terms of increased phase spread which deteriorates the transmission through the RFQ. Details of the measurements carried out as a function of various source parameters and its impact on the longitudinal optics are presented.