Radiation characteristics of detached divertor plasmas in W7-X

Detached divertor plasmas has been achieved by density ramp in W7-X. During the transition into detachment, the main divertor radiation region moves from the region close to the divertor targets to the region around LCFS. The typical spatial distributions of carbon and hydrogen Balmer lines in detached divertor plasma are presented. They show that the line emissions of CH produced by chemical sputtering is still strong close to the targets. The C I fills up the whole detached divertor plasma and its intensity increases when approaching the targets due to the dissociation from hydrocarbon and self-sputtering. The C II and C III are mainly located at around LCFS region. Meanwhile, the distribution of hydrogen Balmer lines is considerably flatter in detached divertor plasma. The Balmer line ratios indicate that the detached divertor plasma seems to be still in excitation or partial recombination state. This is consistent with the divertor Langmuir probe measurements which shows that the electron temperature near strike line is around 6 eV. Finally, the electron pressure at upstream and downstream position is measured and its ratio shows that the electron pressure drops significantly along the magnetic field lines. However, the pressure can not drop further due to the lack of the volume recombination in detached divertor plasma.