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

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 (Winteret al1989J. Nucl. Mater.162-4712-23) 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 4c10(19)m(-2)to more than 1c10(20)m(-2)) and diamagnetic energy (from 330 kJ up to 510 kJ). Z(eff)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.