Research Toward Resolving Key Issues for ITER and Steady-State Tokamaks

D.N. Hill, (E. Schuster), et al. (Collaboration Paper)

IAEA Fusion Energy Conference

San Diego, California, USA, October 8-13, 2012

Abstract

The DIII-D research program is addressing key ITER challenges and developing the physics basis for future steady-state tokamaks. Pellet pacing edge localized mode (ELM) control in the ITER configuration shows energy loss 􏰀proportional to 1/fpellet at frequencies up to 12x the natural rate, and complete ELM suppression with resonant magnetic perturbations (RMP) is now obtained at the q95 expected for ITER baseline scenario discharges. Long-duration ELM-free QH-mode discharges have been produced with ITER-relevant co-current neutral beam injection (NBI) using external n=3 coils to generate sufficient counter-IP torque. ITER baseline discharges at beta_􏰁N=2 and scaled NBI torque have been maintained in stationary conditions for more than 4 resistive times using electron cyclotron current drive (ECCD) for tearing mode (NTM) suppression and disruption avoidance; active tracking with steerable launchers and feedback control catch modes early and reduce the ECCD energy requirements. Disruption experiments with massive gas injection reveal runaway electron dissipation rates ~10x faster than expected and demonstrate the possibility of benign dissipation in ITER. Other ITER-related experiments show measured intrinsic plasma torque in good agreement with a physics-based model over a wide range of conditions, while first-time main-ion rotation measurements show it to be lower than expected from neoclassical theory. Core turbulence measurements show increased temperature fluctuations correlated with sharply enhanced electron transport when 􏰂\Nabla Te/Te exceeds 2.5 m-1. Near the separatrix in H-mode, data show the pedestal height and width growing between ELMs with 􏰂\Nabla P at the computed kinetic-ballooning limit, in agreement with the EPED model. Successful modification of a neutral beam line to provide 5 MW of adjustable off-axis injection has enabled sustained operation at beta_􏰁N~3 with minimum safety factors well above 2 accompanied by broader current and pressure profiles than previously observed. Initial experiments aimed at developing integrated core and boundary solutions demonstrated heat flux reduction using radiative edges and innovative divertor geometries (e.g., snowflake configuration).