Implications of recent DIII-D experiments on plasma shaping for future tokamak designs

Advanced tokamaks use D-shaped cross-section plasmas to optimize fusion performance. In turn, the divertor (which handles heat and particles) must operate efficiently in these shaped plasmas. In this paper, we report on recent experiments at the DIII-D National Fusion Facility that compare the advantages/disadvantages of I) double-null (DN) versus single-null (SN) configurations, 2) particle pumping at low and high density, and 3) open versus tightly baffled diverters. The focus of this paper will be on the important engineering consequences of these physics results for future tokamak designs. Accurate control over the magnetic balance is required by the plasma shaping coils for DN land near-DN) operation because of the strong sensitivity of the heat flux to small changes in magnetic balance. Alternatively, additional protective armor may be needed for each divertor. We show that precise control over the strike point location by the coil system is important for lower density (attached) plasma operation, but much less so for higher density (detached) operation. We also find that minimizing the angle between the divertor structure and the divertor plasma legs is very useful in reducing the peak divertor heat flux for lower density (attached) plasmas but is of Limited benefit for higher density (detached) plasmas. Finally, the physics results imply that significant heating and damage at the divertor "slot" opening may occur, even if several heat flux scrape-off lengths are allowed for clearance.