Spectroscopic studies of fuel recycling and impurity behaviors in the divertor region of Wendelstein 7-X

The first divertor operation phase (OP1.2a) was carried out on Wendelstein 7-X in the second half of 2017. Fuel recycling and impurity behaviors in the divertor region were investigated by employing a newly built ultraviolet-visible-near infrared overview spectroscopy system. The characteristic spectral lines of the working gases (hydrogen and helium), intrinsic impurities (carbon, oxygen and iron), and seeded impurities (neon and nitrogen) were identified and analyzed. The divertor electron temperature and density were measured using He I (667.8, 706.5, and 728.1 nm) line intensity ratios. The H-alpha (656.3 nm), He I (587.6 nm), C II (514.5 nm), and O I (777.2 nm) emissions were investigated over a wide range of operating conditions. The results showed that fuel and impurity emissions in the divertor region exhibit a strong dependence on magnetic topology and plasma conditions. The levels of H-alpha, He I, C II, and O I emissions are all reduced moving from the standard configuration to the high mirror configuration, and even further reduced for the high iota configuration, which is associated with decreasing connection length in these island divertor configurations. The H/He influx ratio shows that the plasma is a mixture of helium and hydrogen. The neutral and impurity influxes from the divertor target tend to increase with increasing divertor electron temperature.