In-Situ Electron Cyclotron Resonance (ECR) Plasma Potential Determination Using an Emissive Probe

Plasma potential measurements by use of a Langmuir probe (LP) and an emissive probe (EP) were compared in the ORNL CAPRICE electron cyclotron resonance (ECR) ion source. It is shown that, for normal ECR ion source operating conditions, the large population of hot electrons may make the emissive floating point method fail, and may cause the values deduced using the LP method to be in error by more than 10%. In addition, the gas mixing effect was studied by comparison of in-situ probe measurements and measurements of the extracted ion beam charge state distribution (CSD). An explanation of the effect in terms of a change in plasma potential and hot electron temperature is proposed.