High-Performance Regimes in DIII-D

Research in DIII-D places a major emphasis on developing a scientific basis for high-performance steady-state operation for use in burning plasma tokamaks. This work has resulted in a long history of studies of high-performance regimes. Several of these regimes are described. H-mode, the first high-performance regime, is characterized by the formation of a transport barrier in the boundary region. The VH- and QH-modes, both variations of the H-mode, were both first identified through pioneering work on DIII-D. Although internal transport barriers (ITBs) had been observed previously, advanced diagnostics implemented on DIII-D and elsewhere allowed the physics of these phenomena to be elucidated. This work led to the combination of a VH-mode edge and an ITB core, which exhibits the highest fusion performance obtained in DIII-D. ITBs can also be combined with the QH-mode edge to produce the quiescent double barrier regime, characterized by nearly stationary high-performance plasmas. Like the ITB, high-l_i plasmas also exhibit performance improvements deeper in the core, in this case due to increased poloidal magnetic field. Although many of these regimes exhibit high-fusion performance only transiently, they provide important platforms for developing an understanding of the physics of transport and magnetohydrodynamic stability and provide the basis for extending to longer duration and evaluating compatibility with steady state.