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The EXTRAP T2R experiment is a large aspect ratio Reversed Field Pinch device. The main focus of interest for the experiment is the active feedback control of resistive wall modes. With feedback it has been possible to prolong plasma discharges in T2R from under about 10 ms to nearly 100 ms. With longer pulses plasma-surface interaction effects become increasingly important, impurity production and particle recycling, but also surface heating and erosion that pose a danger to the vacuum vessel. The device has an all metal vessel with stainless steel bellow walls and molybdenum mushroom limiters, which cover about 18% of the surface area. In particular the superthermal electron population in the edge region causes intense surface heating and the integrity of the stainless steel wall is a major concern.
This report describes a series of experiments in T2R where quantitative spectroscopy and passive collector probes have been used to study the particle flux and impurity production. Time resolved measurements of the metal fluxes (Cr and Mo) were made through visible spectroscopic techniques. Absolutely calibrated spectrometers with photo-multiplier tube detectors are utilised to measure photon intensities and hence determine particle fluxes. Discharge integrated fluxes of Cr, Fe, Ni and Mo were with collector probe measurements, where graphite and silicon probes were exposed at the wall position or inserted into the plasma edge region up to the limiter radius. Good agreement is found in the elemental composition of the metallic impurity flux. The observed high net erosion yields, in excess of 1% imply that erosion mechanisms other than physical sputtering contribute to the impurity release. The roles of sputtering, melting and arcing in impurity production are evaluated for the operations scenarios with and without feedback. Additional evidence on both impurity production and particle recycling was provided by an isotopic exchange experiment.
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