Langmuir probe measurements in the TEXTOR Tokamak during ALT-I pump limiter experiments

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Abstract
Langmuir probes have been used to characterize the edge plasma of the TEXTOR Tokamak and measure the parameters of the plasma incident on the ALT-I pump limiter during ohmic and ICRH heating. Probes mounted directly on the ALT limiter, and a scanning probe located 90 degrees toroidally from the limiter, provide data for the evaluation of pump limiter performance and its effect on the edge plasma. The edge plasma is characterized by density and flux e-folding lengths of about 1.8 cm when ALT is the main limiter. These scrape-off lengths do not vary significantly as ALT is moved between the normal 42-46 cm minor radii, but increase to over 2.2 cm when ALT is inserted to 40 cm. The flux to probes at a fixed position in the limiter shadow varies by less than 25% for core density changes of a factor of five. This suggests that the global particle confinement time, tau p, scales as the core density. Estimates from the probes indicate that tau p is on the order of the energy confinement time, tau E. The edge electron temperature, Te, typically decreases by a factor of two when the core density is raised from 1 to 4*1013 cm3.

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