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
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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).