DIII-D research in support of ITER

Abstract

DIII-D research is providing key information for the design and operation of ITER. Investigations of axisymmetric stability and of edge-localized mode (ELM) suppression with resonant magnetic perturbations have helped provide the physics basis for new axisymmetric and non-axisymmetric control coils in ITER. Discharges that simulate ITER operating scenarios in conventional H-mode, advanced inductive, hybrid and steady state regimes have achieved normalized performance consistent with ITER's goals for fusion performance. Stationary discharges with high βN and 90% non-inductive current that project to Q = 5 in ITER have been sustained for a current relaxation time (∼2.5 s), and high beta wall-stabilized discharges with fully non-inductive current drive have been sustained for more than one second. Detailed issues of plasma control have been addressed, including the development of a new large-bore startup scenario for ITER. DIII-D research also contributes to the basis for reliable operation in ITER, through active control of the chief performance-limiting instabilities. Simultaneous stabilization of neoclassical tearing modes (by localized current drive) and resistive wall modes (by magnetic feedback) has allowed stable operation at high beta and low rotation. In research aimed at improving the lifetime of material surfaces near the plasma, recent experiments have investigated several approaches to mitigation of disruptions, including injection of low-Z gas and low-Z pellets, and have shown the conditions that minimize core impurity accumulation during radiative divertor operation. Investigation of carbon erosion, transport and co-deposition with hydrogenic species, and methods for the removal of co-deposits, will contribute to the physics basis for initial operation of ITER with a carbon divertor. A broad research programme provides the physics basis for predicting the performance of ITER. Recent key results include the discovery that the L–H power threshold is reduced with low neutral beam torque, and the development of a successful model for prediction of the H-mode pedestal height in DIII-D. Research areas with the potential to improve ITER's performance include the demonstration of ELM-free ‘quiescent H-mode’ discharges with both co- and counter-neutral beam injection, and validation of the predicted torque generated by static, non-axisymmetric magnetic fields. New diagnostics provide detailed benchmarking of turbulent transport codes and direct measurements of the anomalous transport of fast ions by Alfvén instabilities. Successful comparison of experiment and modelling for off-axis neutral beam current drive provides the basis for more flexible current profile control in advanced scenarios.

Modelling resistive wall modes in ITER with self-consistent inclusion of drift kinetic resonances

“…Recent perturbative kinetic simulations for JET plasmas [10] and self-consistent kinetic computations for DIII-D plasmas [19] also confirm the partial stabilisation of the RWM with drift kinetic effects. However, recent experimental results in DIII-D [20] seem to suggest a complete stabilisation of the RWM in the linear regime and in the absence of error fields. The latter two conditions, meaning the absence of the RWM coupling to other modes and to the magnetic braking, are assumptions implicitly made in all the above mentioned numerical calculations.…”

Fast-ion transport in qmin>2, high-β steady-state scenarios on DIII-D

“…By adjusting the L-to-H-mode transition time and the starting time of high-power NBI and ECH, q min during the high-b N phase can be set to a value between $1 and $2.5. At the low end of this range are scenarios like the "high-l i " 17 and "steady-state hybrid" 18 that have potential applicability in next-step reactors. These tend to have good total energy confinement, i.e., H 89P !…”

Physics news on the Internet (based on electronic preprints)

“…These techniques include supersonic molecular beam injection (SMBI) [73,136,137], edge electron cyclotron heating (ECH) [73,138,139], lower hybrid heating and/or current drive (LHH, LHCD) [140,141], controlled periodic oscillations of the vertical centroid position (jogs) [73,[142][143][144][145][146], ELM pace-making via periodic MPs [147][148][149][150][151][152][153][154], modification of edge profiles and stability with lithium wall coatings [18,[155][156][157][158][159][160][161][162][163][164][165][166][167], and the use of naturally occurring small ELM regimes [23]. Each of these is described below.…”

“…The first reports of ELM destabilization from 3D fields were from JFT-2M [147] and COMPASS-D [148], and the techniques were further developed and refined in NSTX [151][152][153]175] and DIII-D [150,154].…”