The role of shaping in achieving high performance in DIII-D

Experiments to study the effect of plasma shape on performance show that the high performance levels characteristic of VH-mode plasmas are only obtained for high triangularity (δ). A characteristic difference in central heating efficiency is observed when high and low δ plasmas are compared. The authors find this difference to correlate well with the attainment of Mercier stability. The core plasma for the high δ cases achieves Mercier stability when q0 rises above a critical value which depends on δ, providing direct access to the second-stable regime. Other attributes of the VH-mode commence after a delay. The rise in q0 is attributable to the radial gradients in the neutral beam and bootstrap current sources. Simulation indicates that such plasmas would persist for many seconds if the rise in density could be controlled, maintaining a centrally peaked neutral beam deposition profile