Experimental results of a magnetic field modification to the radio frequency driver of a negative ion source

A magnetic field produced by a current flowing through the Plasma Grid (PG) is one of the solutions to reduce collisional loss of negative ions in a negative ion source, which reduces the electron temperature in front of the PG. However the magnetic field diffused into the driver has some influences on the plasma outflowing. In order to investigate the effect of changing this magnetic field on the outflowing of plasma from the driver a circular ring (absorber), of high permeability iron has been introduced at the driver exit, which can reduce the magnetic field around and improve plasma outflowing. With the application of absorber, the electron density is increased by about 35%, and the extraction current measured from extraction grid is increased from 1.02 A to 1.29 A. The results of extraction experiment with cesium injection show that both Extraction Grid (EG) current and H− current are increased when the absorber introduced.