Ix. Boundary Physics

In this chapter, we focus on the physics basis for the expected operating conditions at the BPX divertor and inner wall limiter. The essential features of the BPX device have been fully described in previous chapters. The main reasons for choosing the divertor configuration are the desire to obtain H-mode operation with high confinement and to obtain “high-recycling” operation in which a highdensity, low-temperature plasma is formed in front of the target plates so as to minimize sputtering and impurity transport back to the core plasma. It is planned for BPX to operate primarily in the double-null (DN) configuration to maximize the contact area for heat removal. However, upper and lower single-null (SN) operation is also possible to minimize the H-mode power threshold. Inner-wall limiter configurations can also be obtained to provide a further option for increasing the power handling capabilities of the device. In terms of the plasma conditions in the scrape off layer (SOL), BPX represents a significant extrapolation from present machines because of its high power and relatively compact size. It also op erates at more than twice the toroidal field of any other operating divertor tokamak. Table 9.1 gives a comparison between BPX and other tokamaks in terms of expected heat flux across the separatrix and at the divertor targets. (In the table, A, is the plasma surface area, V, the volume, and Adi,, the actual contact area at the divertor targets.) With 17 MW of ion cyclotron resonance frequency (ICRF) + ohmic heating (OH) power, BPX could produce 420 MW of fusion power at Q = 25. Then Plea = 0.2 x Q + PICH = 100 MW should be deposited in the plasma and must be lost by radiation to the walls and conduction to the divertor plates. The peak divertor heat flux at the end of the Q = 25 burn phase (~20 MW/m2 exclusive of toroidal peaking factors) is expected to be higher than values in most present tolcamaks1-3: 2130 MW/m2 in JET, 115 MW/m2 in JT*O and DIII-D. At Q = 5 with Pfw = 100 MW and P loss = 40 MW, the peak divertor heat flux will be down to about 12 MW/m2. Active sweeping of the X-points will be used to reduce the temperature rise to manageable levels for inertially cooled divertor targets. The predicted operating conditions at the diver-