Validation of ion temperature measurement using an ion sensitive probe technique in finite boundary RF plasma

Abstract

Validation of ion sensitive probe (ISP) measurements in radio-frequency plasma was investigated using Doppler spectroscopy. Ion temperature Ti obtained by an ISP was consistent with that obtained by Doppler spectroscopy when the recessed distance between two electrodes h satisfied 10 ρ e < h < 10 ( ρ e + λ D ). However, ISP yielded a larger ion temperature than that obtained by Doppler spectroscopy when the above relation was not fulfilled. The effective ion temperature calculated based on the ion kinetic motion matched well with the ISP measurement result, which indicates that the ISP could overestimate Ti owing to the ion Larmor motion when h is not optimized. The availability of the ISP technique for measuring the radial Ti profile was also investigated. Ti showed a rapid increase in the edge region of a cylindrical plasma and finally became larger than the electron temperature even though there was no direct ion heating power source. Calculations considering the ion kinetic effect well reproduced both the magnitude and the radial trend of ion temperature and ion density near the plasma edge. This result postulates that Ti evaluation could also be inadequate due to the ion kinetic effect when an ISP is placed at the edge and outside a finite boundary plasma.