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 τ _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 τ _I has been observed when changing the power deposition from on- to off-axis heating with energy confinement times τ _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 τ _I scaling compares well to known τ _I scaling laws observed in other machines. A comparison of τ _I and τ _E yields an averaged ratio of τ _E/τ _I = 1.3 and transport times in the range of τ _I = 40-130 ms and τ _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.