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 (Winter et al 1989 J. Nucl. Mater. 162–4 712–23) was applied, which has led to one of the main achievements of the campaign: plasma operation at high core densities of more than 1020 m−3 in hydrogen fueled plasmas due to the reduced radiation-induced density limit.