Design, R&D and assessment of performance of the JT-60SA upper divertor

Modification of JT-60 to a fully superconducting coil tokamak (JT-60 Super Advanced) has been started last year. Divertor plasma performance for high triangularity and low aspect ratio plasma in two types of divertor geometries was studied and compared by using 2D plasma fluid (SOLDOR) and neutral Monte-Carlo (NEUT2D) codes. ‘Vertical target with deep V corner’ is effective to reduce heat flux even in low density operation without additional gas puffing, but large pumping speed is required to change particle balance. On the other hand, the particle balance in ‘W-shaped with shallow V’ can be changed with small pumping speed and larger main plasma can be obtained due to its compactness. Performance of water-cooled mono-block carbon fiber composite (CFC) divertor target has been confirmed by heat load cycle test of small mockup. Obvious performance degradation was not found during 1450 cycles at 15 MW m−2 and 600 cycles at 20 MW m−2.