STRAHL modeling of impurity transport experiments with on- and off-axis heating during the first divertor campaign on Wendelstein 7-X

In the first divertor campaign of Wendelstein 7-X, iron impurity transport experiments were performed via laser blow-off injection during an on- to off-axis ECRH scan at constant power. The iron line emission was measured by the x-ray imaging spectrometer systems, HR-XIS and XICS, and the high efficiency XUV overview spectrometer, HEXOS. The spectral measurements show an increase in the global impurity transport time as ECRH power was deposited more off-axis. To understand this observed change, the 1D transport code STRAHL was employed to model the iron line radiation using anomalous diffusion and convection velocity profiles in addition to the neoclassical {\&} classical transport profiles provided by the drift kinetic equation solver, DKES. The observed line-integrated iron emissivity was matched using a chi-squared minimization by varying the anomalous diffusion and convective velocity profiles within STRAHL. Although in all cases the measured data could only be well-matched with anomalous diffusion at levels 50 times larger than neoclassical predictions, the inferred transport uncertainties were too large to make clear comparisons between the on- and off-axis cases. Therefore a sensitivity study using synthetic data was performed to better capture the systematic uncertainties.