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

Advanced tokamaks use D-shaped cross-section plasmas to optimize fusion per formance. In turn, the divertor (which handles heat and particles) must ope rate efficiently in these shaped plasmas. In this paper, we report on recen t experiments at the DIII-D National Fusion Facility that compare the advan tages/disadvantages of I) double-null (DN) versus single-null (SN) configur ations, 2) particle pumping at low and high density, and 3) open versus tig htly baffled diverters. The focus of this paper will be on the important en gineering consequences of these physics results for future tokamak designs. Accurate control over the magnetic balance is required by the plasma shapi ng coils for DN land near-DN) operation because of the strong sensitivity o f the heat flux to small changes in magnetic balance. Alternatively, additi onal 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 den sity (detached) operation. We also find that minimizing the angle between t he divertor structure and the divertor plasma legs is very useful in reduci ng the peak divertor heat flux for lower density (attached) plasmas but is of Limited benefit for higher density (detached) plasmas. Finally, the phys ics results imply that significant heating and damage at the divertor "slot " opening may occur, even if several heat flux scrape-off lengths are allow ed for clearance.