Measurement of the plasma potential in the edge plasma of the ORNL CAPRICE ECR ion source

Abstract

Measurements of the plasma potential by using a Langmuir probe (LP) and an emissive probe (EP) were compared. In the ORNL CAPRICE electron cyclotron resonance (ECR) ion source, we show that, for normal ECR ion source operating conditions, the large population of hot electrons may cause the emissive floating point method to fail and may cause the values deduced using the LP method to be uncertain by more than 30%. Having in this manner determined the magnitude of the possible uncertainties of the deduced potentials, we studied the chamber surface condition effect and the gas mixing effect by comparing in-situ probe measurements with measurements of the extracted ion beam charge state distribution (CSD). The plasma potential values are found to be extremely sensitive to the surface condition of the source chamber walls; a contaminated surface gave plasma potential values that were larger by 10 V. Also, the plasma potential was found to be significantly decreased during gas mixing; the corresponding increase in ion confinement time may be the dominant mechanism responsible for the gas mixing effect.