Deposition Diagnostics for Next-step Devices
Authors: C.H. Skinner, A.L. Roquemore, the NSTX team, A. Bader, and W.R. Wampler
Date of PPPL Report: June 2004
Presented at: the 15th Topical Conference on High-temperature Plasma Diagnostics, 19-22 April 2004, San Diego, CA; proceedings to be published in the Review of Scientific Instruments.
The scale-up of deposition in next-step devices such as ITER will pose new diagnostic challenges. Codeposition of hydrogen with carbon needs to be characterized and understood in the initial hydrogen phase in order to mitigate tritium retention and qualify carbon plasma-facing components for deuterim-tritium (D-T) operations. Plasma-facing diagnostic mirrors will experience deposition that is expected to rapidly degrade their reflectivity, posing a new challenge to diagnostic design. Some eroded particles will collect as dust on interior surfaces and the quantity of dust will be strictly regulated for safety reasons -- however diagnostics of in-vessel dust are lacking. We report results from two diagnostics that relate to these issues.
Measurements of deposition on the National Spherical Torus Experiment (NSTX) with 4 Hz time resolution have been made using a quartz microbalance in a configuration that mimics that of a typical diagnostic mirror. Often deposition was observed immediately following the discharge suggesting that diagnostic shutters should be closed as soon as possible after the time period of interest. Material loss was observed following a few discharges. A novel diagnostic to detect surface particles on remote surfaces was commissioned on NSTX.