Impact of boronizations on impurity sources and performance in Wendelstein 7-X

Abstract

The low-Z oxygen and carbon were the main plasma impurities in the Wendelstein 7-X (W7-X) stellarator in the last experimental campaign with the passively cooled graphite divertor. To tackle this issue boronization [1] was applied, which has led to one of the main achievements of the campaign: plasma operation at high core densities of more than 10 20 m -3 in hydrogen fueled plasmas due to the reduced radiation-induced density limit. In total three boronizations were applied. After the first boronization the oxygen to hydrogen flux ratio (normalized influx of oxygen) at the divertor substantially decreased by a factor of 10 and the carbon to hydrogen flux ratio (normalized influx of carbon) decreased by a factor of 4 as obtained from spectroscopy. In the same time, boron emission appeared in the spectra. Between the boronizations oxygen and carbon normalized influxes increased but never reached the pre-boronization values. With each subsequent boronization O level decreased even more, reaching the lowest values after the third boronization which were more than a factor of 100 lower than before the first boronization. Such a decrease in low-Z impurity concentration significantly extended the operation window of W7-X in terms of line-integrated electron density (from 4‧10 19 m -2 to more than 1‧10 20 m -2 ) and diamagnetic energy (from 330 kJ up to 510 kJ). Zeff decreased from 4.5 down to values close to 1.2 as obtained from bremsstrahlung measurements. The above mentioned values are given for the two reference discharges before and after boronization.

Measurements of plasma parameters in the divertor island of Wendelstein 7-X through line-ratio spectroscopy on helium

“…To get a qualitative understanding of the link between impurity line radiation and the localization of the temperature reductions, one can analyze the line emission profiles from low ionization stages of carbon measured by the divertor spectroscopy. Carbon is considered to be the main impurity species in the divertor region after the application of boronization [29]. In Figure 9(a) and (b), the C III emission profiles from the divertor spectroscopy in radial (Figure 9(a), horizontal view) and poloidal direction (Figure 9(b), vertical view) are shown.…”

Overview of the results from divertor experiments with attached and detached plasmas at Wendelstein 7-X and their implications for steady-state operation

“…The simulations made for the plasmas discussed in figure 13 are shown in figure 14 [63]. In the attached state, plasma radiation (dominated after the boronization [37] by the carbon spectral lines) is localized within the magnetic island, with a radiation band extending from the strike line to the O-point. As the radiation fraction increases, radiation still mostly occurs in the island region, but the radiation layer shifts closer to the separatrix.…”

EMC3-EIRENE simulation of first wall recycling fluxes in W7-X with relation to H-alpha measurements

“…These regions of expected plasma-wetting from simulation were instead compared to in-vessel inspection photos after plasma operation to test the validity of the simulation results. Although these interaction zones are the accumulated result of several different magnetic field configurations which were operated over the experimental campaign, the standard magnetic field configuration was operated for the largest number of plasma seconds [15]. Therefore, the patterns seen on the heat shield should largely be an effect of this particular configuration.…”