Instruments for Plasma Diagnostics

Some diagnostic instruments that are set-up and run by the division for Stellarator Edge and Divertor Physics

Langmuir probes

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.

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