Elevating the Precision of Plasma Probe Diagnostics by Elimination of Bare Probe Protective Shields’ Influence

Abstract

Probe diagnostics of the low pressure inductive xenon plasma were conducted using classic cylindrical Langmuir probes with conventional protection of their circuits against radio frequency interferences by bare metal shields. The dimensions of their probetips were determined in the special experiment that provided negligible local plasma distortions. Accurate probe measurements were used to determine the spatial plasma parameter distributions in a gas discharge unit of an ion thruster model which helped to develop its ion-extracting grate. The subsequent analysis of probe measurements showed that in these experiments, the plasma electron energy distribution function (EEDF) was quite noticeably deviated from the Maxwell function depending on the main probe shield length that varied from maximum to zero. Use of an additional probe in the special position where its shield was rather long with zero shield length of the main probe showed that the additional shield lowered all plasma parameters. Comparison of both probes’ data identified the principled relationship between measurement errors and EEDF distortions caused by bare probe shield and this dependence was used to correct the initial probe measurements. Therefore plasma probe diagnostics became more precise due to the lowered influence of the bare probe protective shields. Its physical analysis based on previous authors’ works showed that this effect was caused by a shortcircuited double-probe phenomenon in the bare metal shields.