Interpretation and implementation of an ion sensitive probe as a plasma potential diagnostic on Alcator C-Mod

An ion sensitive probe (ISP) is developed as a robust diagnostic for measuring plasma potentials (ΦP) in magnetized plasmas of Alcator C-Mod. The ISP relies on the large difference between the ion and electron gyro-radii (ρi/ρe ∼ 60) to reduce electron collection at a collector recessed at a distance ∼ρi behind a separately biased wall. We develop a new ISP method based on voltage sweeping to measure ΦP that is independent of the precise position and shape of the collector: ΦP is found to be the wall voltage when net current to the collector vanishes during an increasing voltage sweep. We have designed and installed an ISP to study ΦP and radio frequency (RF) sheath rectification on Alcator C-Mod. Preliminary results show that the ISP is a viable plasma potential diagnostic in Alcator C-Mod plasmas and ΦP measurements are in good agreement with independent results obtained with an emissive probe.