Design, construction and test of an optimized Faraday cup for beam current determination of a helicon ion source

Faraday cups are widely used to characterize ion beam current. A deep canonical Faraday cup for determining ion beam currents from a helicon ion source has been designed, built and tested with beam. In this paper, CST Studio code is used to design the Faraday cup. The precise operation of a Faraday cup relies on its capability to recapture the ejected electrons when energetic particles collide with interior cup. The interior geometry of the Faraday cup is designed to minimize the loss of particles. In addition, a biased ring is embedded in front of the cup collector at voltages up to -300 V with respect to the collector. The collector is made of copper while the suppressor ring and the Faraday cup body are made of stainless steel. The measurements are utilized at RF powers and extraction voltages up to 1300 W and 20 kV, respectively. The measured ion beam current is an indirect way to demonstrate mode transition from inductively coupling plasma (ICP) to helicon mode of operation in the constructed helicon ion source. We have detected a sharp increase in the measured ion beam as the mode transition is reached. In addition, Optical emission spectroscopy (OES) method was also employed for demonstrating the mode transition.