Measurements of the parallel ion velocity distribution at the plasma–sheath interface

Abstract The ion velocity distribution parallel to magnetic field lines, f is extracted from retarding field analyzer (RFA) measurements in the ASDEX Upgrade scrape-off layer. The RFA ion current–voltage characteristic is transformed into a system of linear equations, from which f is unfolded using a standard regularization method. The algorithm is validated on numerically generated data. The experimentally measured f compares favorably with the ion velocity distributions calculated from a quasi-neutral kinetic simulation of the plasma pre-sheath. The technique can be used to measure f by RFAs in plasma processing, high-energy particle accelerators or by satellite-born RFAs.