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

In JT-60U, an outer divertor pumping slot was added to the W-shaped divertor with an inner pumping slot during the maintenance period, November to December 1998, in order to improve plasma performance especially in a 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 an extreme plasma configuration. Thermal analyses showed that peak surface temperatures exceeded 2000/spl deg/C. The maximum induced thermal stress, however, was below an 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 an extreme condition. With the modified W-shaped divertor, it was observed that the efficiency of He pumping was improved, and that the L-H transition threshold power was reduced. The integrated plasma performance has also been improved. These results indicate efficient divertor pumping of the modified W-shaped divertor.