Experimental Studies on the Roles of Space Charge Neutralization in a Ring Cusp Ion Source

Abstract

Experimental studies in a ring cusp ion source are performed for investigating the role of space charge neutralization by measuring ion beam profiles, fluxes of charge exchange ions, and distribution of ion beam neutralizing electrons. The measurements are carried out using the argon ion beam of current 30–60 mA and beam energy 1200 eV. A Faraday cup array (FCA) equipped with filtering electrodes is developed and utilized for measuring the ion beam profiles by filtering out the neutralizing electrons and slow-moving charge exchange ions from the ion beam. The radial flux of charge exchange ions is measured using a flat Faraday probe mounted at a location outside the ion beam. The flat Faraday probe is shielded from the energetic beam ions. The flux of charge exchange ions is studied to assess the effect of space charge neutralization inside the ion beam. The reduction in floating potential observed with improved space charge neutralization is found to correlate with a reduced radial flow of charge exchange ions. The FCA is also utilized to measure the distribution profile of neutralizing electrons in the ion beam by biasing its filtering electrodes and Faraday cups (FCs) at appropriate potentials. The radial profiles of neutralizing electrons are found Gaussian as similar to that of the beam ions. It is suggested that the neutralizing electrons are dispersed as similar to the fast ion distribution in order to accomplish an evenly distributed space charge neutralization.