Plasma-edge studies using carbon resistance probes

The plasma edge in the PLT and PDX Tokamaks was studied using a technique which involves measuring the change in resistance of a thin carbon film due to bombardment by energetic particles escaping the plasma. The probes were calibrated by measuring the resistance change caused by implantation of various ions at different energies. A model was developed which can be used to determine the flux and energy of the incident particles from the measured resistance changes. For probes exposed in PDX and PLT, near the wall resistance changes were observed due to charge exchange neutrals. Larger changes were observed in the ion scrape-off region closer to the plasma. In PLT the effect of ions at the plasma edge begins to dominate the neutral flux near the radius of the ring limiter. The energy of ions at the plasma edge was estimated to be low ( or 100 eV) in PDX during neutral beam heated discharges, but higher ( 300 eV) in PLT during ion cyclotron resonance heating.