JT-60U W-shaped divertor with in-out divertor pumping slots

In JT-60U, outer divertor pumping slot was added to W-shaped divertor with inner pumping slot during maintenance period, November to December 1998, in order to improve plasma performance especially in detached state with no or inverse in-out particle flux asymmetry. This modification was already planned at the beginning of W-shaped divertor design activity. Hence, the previous W-shaped divertor had adaptable structure to the in-out pumping slots. The modified components, especially armor tiles, may be exposed to high heat load and high electromagnetic load in extreme plasma configuration. Thermal analyses showed that peak surface temperatures exceeded 2000 degrees C. Maximum induced thermal stress, however, was below acceptable level. Furthermore, each armor tile size and position of the tile attachment were optimized to avoid fracture due to halo current. Therefore, high power operations are possible without tile fracture in the extreme condition. With the modified W-shaped divertor, it was observed that efficiency of helium pumping was improved, and that L-H transition threshold power was reduced. Integrated plasma performance has also been improved. These results indicate efficient divertor pumping of the modified W-shaped divertor.