Charge-Exchanged Beam Measurement by Using a Grid-Biased Faraday Cup

A method for neutral beam measurement by using a Faraday cup is proposed in this paper. The method enables us to detect neutral beams by controlling secondary electrons by using a biasing grid in front of the Faraday cup. A procedure is also proposed for in situ determination of the secondary electron emission coefficient of the Faraday cup. Experimental results show that appropriate emission coefficients are determined for helium beams with energies of 2-10 keV. The neutral flux charge-exchanged from a helium ion beam passing through a helium gas is also measured. Saturation of the neutral flux is observed above the pressure expected from the solution of a rate equation. The method is promising for neutral beam measurement, especially for small current-neutral beams.