Interpretation of Ion Velocity Distributions Measured with a Grounded Retarding Field Energy Analyzer (RFEA) in an Inductively Coupled Helicon Plasma

摘要

A Retarding Field Energy Analyzer (RFEA) with a grounded surface in a plasma will be surrounded by a sheath. The ion distribution function (IDF) measured at the probe will be distorted as compared to the IDF in a plasma unaffected by the sheath. In the present study we investigate whether a practical method can relate the measured distribution function to the undisturbed IDF, thus allowing for derivation of useful plasma parameters from the measured IDFs. We study the IDF in the sheath by 3D numerical simulations of a spherical RFEA for parameters similar to the Njord helicon device at the University of Troms?. By comparison with the numerical results, we critically assess the results from simple 1D models that are often used in the analysis of the RFEA signal. With the PIC simulations we also show that the acceptance angle of the probe has a significant effect on the low energy part of the measured IDF. This result is compared with real probe measurements from the Njord device.