Instruments for Plasma Diagnostics
The Langmuir probes are instruments for measuring the electron density and temperature at the plasma edge, by measuring a current from the plasma at varying voltages (U=+/-200 V, I=+/-1A).
Each Langmuir probe consists of a cylindrical graphite tip of diameter approx. 0.9 mm. A thin ceramics tube insulates the tip against the outer metal shield. After assemply of the graphite tiles, some probes end flat at a tile surface, others stick out of the tile by about 1 mm (see lower picture).
After three months of W7-X operation, slight changes in the tile color are observed at areas that were preferentially hit by the hot plasma. Besides that, the tile and the Langmuir probes are undamaged.