STRAHL modeling of iron impurity transport 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 observed iron spectral lines show an increase in the impurity transport time as a larger fraction of ECRH power was deposited off-axis and as the ECRH power was decreased. Although the purely on-axis power scan demonstrated a similar core Te flattening as the 4.9 MW off-axis scan, the resulting transport time enhancement was substantially larger for the on-axis power scan. To characterize these observed changes a least squares minimization was performed to infer the anomalous transport profiles that most accurately produces the measured iron line emission using the 1D transport code STRAHL. In all cases the observed iron line emission could only be well-matched when the anomalous diffusion channel was included and at levels ~50 times larger than neoclassical predictions. A sensitivity study using synthetic data was performed capturing the systematic uncertainties in the inferred diffusion profiles. Although these profiles are consistent with an increase in the transport times across the on-to off-axis dataset, the profiles are indistinguishable when these total uncertainties are considered.