X. Impurity Control and Materials Physics

The design for the BPX power and particle handling system consists of a double-null (DN) poloidal divertor with an inner-wall bumper limiter. In addition, a substantial portion of the remainder of the inside of the torus is covered with graphite tiles to withstand high heat flux. In divertor operating modes, the diverted plasma is “swept” across the divertor plates to reduce the time-average heat loads on the plates. The poloidal divertor is designed both to provide access to H-mode operation and to ensure high recycling in the divertor region, which minimizes impurity production and consequent contamination of the main plasma. The device is capable of both single-null (SN) and DN divertor operation. In addition, an inner bumper limiter is designed to provide an alternative to divertor operation. Additional toroidally and/or poloidally localized limiters are provided on the outboard wall for plasma startup and for protection of the radiofrequency (RF) antennas. All of the plasma-facing components must withstand the high heat flux anticipated in BPX (ranging from levels comparable to present devices at Pfus = 100 MW at Q = 5, up to -2.5 times that of present devices at Pfus = 500 MW). Pyrolytic graphite has been chosen for the divertor plates because very high thermal conductivity can be obtained. All the plasmafacing components must be able to be removed and reinstalled using the BPX remote handling systems. The peak heat loads on the divertor during disruptions are also an important design consideration. These transient heat loads are much larger than those during normal operation. Another reason for choosing graphite as the plasma-facing material is that under extremely high heat loads, it sublimates rather than melting like most metals. In addition, the plasma energy losses due to impurity radiation are much lower with carbon than with higher 2 metals. However, graphite has a number of special properties, such as high retention for water and hydrogen, that must be considered in the design and operation of the experiment. The distribution of plasma power on the first-wall components is discussed in Chap. IX. Power handling and the choice of material for the divertor plates and limiters is discussed in Sec. X.B. A discussion of surface temperature limits and erosion mechanisms is in Sec. X.C. Wall conditioning and bakeout requirements form Sec. X.D. The effect of disruptions on the plasma-facing components follows in Sec. X.E. Finally, tritium retention and particle pumping issues related to the choice of graphite as the first-wall material are discussed.