Within the single fluid resistive magneto-hydrodynamic (MHD) model, systematic toroidal modelling efforts are devoted to investigate the plasma response induced screening of the applied external 3D magnetic field perturbations in the presence of sheared toroidal flow. One particular issue of interest is addressed, when the local flow speed approaches zero at the perturbation rational surface inside the plasma. Subtle screening physics, associated with the favourable averaged toroidal curvature effect (the GGJ effect [Glasser A H et al 1975 Phys. Fluids 7 875]), is found to play an essential role at slow flow near the rational surface by enhancing the screening at reduced flow. A strong cancellation effect between different terms of the Ohm's law is discovered, leading to different screening physics in the GGJ regime, as compared to that of the conventional screening of the typical resistive-inertial regime occurring at faster flow. These modelling results may be applicable to interpret certain mode locking experiments, as well as type-I edge localized mode suppression experiments, with resonant magnetic field perturbations being applied to tokamak plasmas at low input toroidal torque.

DIII-D physics research addresses critical challenges for the operation of ITER and the next generation of fusion energy devices. This is done through a focus on innovations to provide solutions for high performance long pulse operation, coupled with fundamental plasma physics understanding and model validation, to drive scenario development by integrating high performance core and boundary plasmas. Substantial increases in off-axis current drive efficiency from an innovative top launch system for EC power, and in pressure broadening for Alfven eigenmode control from a co-/counter-I p steerable off-axis neutral beam, all improve the prospects for optimization of future long pulse/steady state high performance tokamak operation. Fundamental studies into the modes that drive the evolution of the pedestal pressure profile and electron vs ion heat flux validate predictive models of pedestal recovery after ELMs. Understanding the physics mechanisms of ELM control and density pumpout by 3D magnetic perturbation fields leads to confident predictions for ITER and future devices. Validated modeling of high-Z shattered pellet injection for disruption mitigation, runaway electron dissipation, and techniques for disruption prediction and avoidance including machine learning, give confidence in handling disruptivity for future devices. For the non-nuclear phase of ITER, two actuators are identified to lower the L–H threshold power in hydrogen plasmas. With this physics understanding and suite of capabilities, a high poloidal beta optimized-core scenario with an internal transport barrier that projects nearly to Q = 10 in ITER at ∼8 MA was coupled to a detached divertor, and a near super H-mode optimized-pedestal scenario with co-I p beam injection was coupled to a radiative divertor. The hybrid core scenario was achieved directly, without the need for anomalous current diffusion, using off-axis current drive actuators. Also, a controller to assess proximity to stability limits and regulate β N in the ITER baseline scenario, based on plasma response to probing 3D fields, was demonstrated. Finally, innovative tokamak operation using a negative triangularity shape showed many attractive features for future pilot plant operation.

A toroidal resistive magneto-hydrodynamic plasma response model, involving large magnetic islands, is proposed and numerically investigated, based on local flattening of the equilibrium pressure profile near a rational surface. It is assumed that such islands can be generated near the edge of the tokamak plasma, due to the penetration of the resonant magnetic perturbations, used for the purpose of controlling the edge localized mode. Within this model, it is found that the local flattening of the equilibrium pressure helps to mitigate the toroidal curvature induced screening effect [Glasser et al., Phys. Fluids 7, 875 (1975)]-the so called Glasser-Greene-Johnson screening, when the local toroidal flow near the mode rational surface is very slow (for example, as a result of mode locking associated with the field penetration). The saturation level of the plasma response amplitude is computed, as the plasma rotation frequency approaches zero. The local modification of the plasma resistivity inside the magnetic island is found to also affect the saturation level of the plasma response at vanishing flow. Published by AIP Publishing.[http://dx.doi.org/10.1063/1.4976987]The type-I edge localized modes (ELMs), with low bursting frequency and large amplitude, can transiently load a large amount of heat from the plasma onto the plasma facing components within each ELM crash, which can reduce the lifetime of these components, especially for ITER. 1Recent experiments in several tokamak devices have successfully achieved the mitigation/suppression of the type-I ELMs by applying resonant magnetic perturbation (RMP) fields.2-7 During the process of the RMP penetration, the radial transport of particles and thermal energy can be enhanced near the edge of the plasma, in particular, near the pedestal top. One channel of enhancing the transport is the presence of magnetic islands, and possibly their overlapping. In fact, recently there seems to be a solid experimental evidence from 9 showing the presence of magnetic islands near the pedestal top during the ELM suppression. Large magnetic islands can partially or fully flatten the pressure profile near the resonant surface associated with the perturbation, 10 which in turn can modify the field screening physics by the plasma.The screening of the RMP fields, due to the plasma response, plays an important role in the physics understanding of ELM mitigation/suppression, 11-16 especially with a slow plasma flow, which is often the case near the plasma edge. On the other hand, according to the single fluid theory, the favorable averaged toroidal curvature effect, 17 at slow plasma flow, can provide a field screening effect-the so called GGJ (Glasser-Greene-Johnson) screening effect. 18,19 The GGJ screening is rather different from the conventional resistive inertial (RI) screening. 20 In the GGJ regime, the amplitude of the total plasma response field (the sum of the vacuum field and the field produced by the perturbed plasma current) is reduced, i.e., the screening effect is enhanced, ...

Numerical simulation on the resonant magnetic perturbation penetration is carried out by the newly-updated initial value code MDC (MHD@Dalian Code). Based on a set of two-fluid four-field equations, the bootstrap current, parallel, and perpendicular transport effects are included appropriately. Taking into account the bootstrap current, a mode penetration-like phenomenon is found, which is essentially different from the classical tearing mode model. To reveal the influence of the plasma flow on the mode penetration process, E × B drift flow and diamagnetic drift flow are separately applied to compare their effects. Numerical results show that a sufficiently large diamagnetic drift flow can drive a strong stabilizing effect on the neoclassical tearing mode. Furthermore, an oscillation phenomenon of island width is discovered. By analyzing it in depth, it is found that this oscillation phenomenon is due to the negative feedback regulation of pressure on the magnetic island. This physical mechanism is verified again by key parameter scanning.

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