Helium exhaust in divertor–closure configuration with W-shaped divertor of JT-60U

The W-shaped divertor of JT-60U was modified from inner-leg pumping to both-leg pumping. After the modification, the pumping rate was remarkably improved with both-leg pumping in a divertor–closure configuration, which means both separatrixes close to the divertor slots. In steady state, efficient helium exhaust was realized in a divertor–closure configuration with both-leg pumping in ELMy H-mode plasmas. A global particle confinement time of τ∗He=0.4 s and τ∗He/τE=3 was achieved in attached plasmas, well within the range generally considered necessary for successful operation of a future fusion reactor, such as ITER-FEAT. The helium exhaust efficiency in the divertor–closure configuration was extended by 45% as compared to the one with inner-leg pumping. In the high X-point configuration with both-leg pumping, the helium exhaust efficiency and the pumping rate of deuterium deteriorated because of the backflow through the outer slot.