Spectroscopic determination of inverse photon efficiencies of W atoms in the scrape-off layer of TEXTOR

Abstract

Optical emission spectroscopy can be applied to determine in situ tungsten particle fluxes from erosion processes at plasma-facing materials. Inverse photon efficiencies convert photon fluxes of WI and WII line transitions into W and particle fluxes, respectively, dependening on the local plasma conditions. Experiments in TEXTOR were carried out to determine effective conversion factors for different WI and WII transitions with the aid of WF6 injection into deuterium scrape-off layer plasmas in the electron temperature Te range between and . The inverse photon efficiencies or so-called effective -values have been determined for WI lines at , 429.5 nm, 488.7 nm, 498.3 nm, and 522.5 nm as well as for WII at and compared with theoretical calculations from the ADAS data base. Moreover, a multi-machine scaling for the -value in the range of Te between has been determined for the most prominent WI line at to considering experimental data from TEXTOR, ASDEX Upgrade, PSI and PISCES. Comparison with ADAS calculations for the same transition reveal a good qualitative agreement with the dependence on Te, but an underestimation of ADAS calculations of less than 25% over the full covered range of experimentally accessible Te in the multi-machine scaling. A good agreement within the experimental uncertainties is found between TEXTOR and ADAS -values for WI at and whereas an underestimation of up to a factor two of ADAS values for WI at and was measured. Potentially, reasons for the discrepancy are an overestimation of applied ionisation rate coefficients in ADAS for neutral W and a stronger electron dependence ne for these transitions.