Impurity Transport Studies at Wendelstein 7-X by Means of X-ray Imaging Spectrometer Measurements

This paper reports on the effect of on- and off-axis heating power deposition on the impurity confinement in purely electron cyclotron resonance heated He plasmas on the stellarator Wendelstein 7-X. Therefore, impurity transport times tau(I) have been determined after Fe impurity injections by laser ablations and monitoring the temporal impurity emissivities by the x-ray imaging spectrometer HR-XIS. A significant increase of tau(I) has been observed when changing the power deposition from on- to off-axis heating with energy confinement times tau(E) being mainly unaffected. In addition, the scaling of impurity transport properties with respect to a variation of heating power P-ECRH and electron density n(e) has been investigated by keeping the heating power deposition on-axis. The observed tau(I) scaling compares well to known tau(I) scaling laws observed in other machines. A comparison of tau(I) and tau(E) yields an averaged ratio of tau(E)/tau(I) = 1.3 and transport times in the range of tau(I)= 40-130 ms and tau(E) = 40-190 ms. Comparing those absolute values to neoclassical predictions supports the recently observed nature of anomalous transport in Wendelstein 7-X, given within the up to now investigated operational parameters.