Three-dimensional structure of radiative cooling in impurity seeded plasmas in the Large Helical Device

Abstract

• Three-dimensionally localization of radiative cooling due to nitrogen seeding for divertor detachment was detected experimentally. • Principal component analysis was applied to two-dimensional radiation images. • A radiation feature in nitrogen seeding was extracted as a principal component. • EMC3-EIRENE code suggested the ionization enhancement at one of the divertor legs. • The extracted feature along the magnetic field lines which pass through the divertor leg. Three-dimensionally localization of radiative cooling due to nitrogen (N 2 ) seeding for divertor detachment was detected experimentally. Since the localization along some magnetic field lines induces toroidal asymmetry of heat load reduction on divertor plates, it should be avoided for fusion reactors. The three-dimensionally localized structure was extracted using Principal Component Analysis (PCA) from two-dimensional radiation images measured with an InfraRed imaging Video Bolometer (IRVB). By applying PCA to 34 images each in N 2 seeded plasmas with toroidally-asymmetric heat load reduction and in neon (Ne) seeded plasmas with toroidally-symmetric heat load reduction, a radiation feature in N 2 seeded plasmas was found as one of the principal components (PC). The three-dimensional transport code EMC3-EIRENE indicated that the ionization in one of the divertor legs is enhanced in nitrogen seeding compared with Ne seeding due to the difference in the first ionization energy. The magnetic field lines from the divertor leg were along the extracted radiation structure and were terminated by the divertor where the heat load decreased due to the N 2 seeding. These results indicate that three-dimensionally localized structure of radiative cooling was detected experimentally.