Investigation on the equilibrium operation regime, its ideal magnetohydrodynamics (MHD) stability and edge localized modes (ELM) characteristics is performed for the China Fusion Engineering Test Reactor (CFETR). The CFETR operation regime study starts with a baseline scenario (R = 5.7 m, B T = 5 T) derived from multi-code integrated modeling, with key parameters β N , β T , β p varied to build a systematic database. These parameters, under profile and pedestal constraints, provide the foundation for the engineering design. The long wavelength low-n global ideal MHD stability of the CFETR baseline scenario, including the wall stabilization effect, is evaluated by GATO. It is found that the low-n core modes are stable with a wall at r/a = 1.2. An investigation of intermediate wavelength ideal MHD modes (peeling ballooning modes) is also carried out by multi-code benchmarking, including GATO, ELITE, BOUT++ and NIMROD. A good agreement is achieved in predicting edge-localized instabilities. Nonlinear behavior of ELMs for the baseline scenario is simulated using BOUT++. A mix of grassy and type I ELMs is identified. When the size and magnetic field of CFETR are increased (R = 6.6 m, B T = 6 T), collisionality correspondingly increases and the instability is expected to shift to grassy ELMs.

We have identified a robust grassy-ELM operation regime for future tokamak reactors. The regime exists within a pedestal top electron collisionality ( *) window at high global poloidal beta (βp). The existence of an upper  * limit for grassy-ELMs is consistent with results previously reported in experiments [N. Oyama et al 2010 Effects of edge collisionality on ELM characteristics in the grassy ELM regime Nucl. Fusion 50 064014], while the existence of a lower  * limit has not been reported previously. Using EPED and BOUT++, a theoretical model that quantitatively explains the physics of the grassy-ELMs within the window, which distinguishes them from the small mixed-ELMs at lower *, is presented for the first time. A peeling-ballooning stability boundary is obtained by scanning the operating density space. The change in density corresponds to a change in  * that affects the pedestal bootstrap current. High βp leads to a strong Shafranov shift, which affects the flux surface averaged pressure drive. The two effects combine to create a peeling-dominated window in intermediate  * buffered by ballooning-dominated regimes. Only the peeling-dominated regime shows a cyclic behavior in the perturbed pressure during the nonlinear simulation of an ELM crash, reminiscent of grassy-ELM dynamics. Similarly, the energy released across the separatrix is demonstrated to be significantly smaller. The quick recovery of the ELM crash is explainable by the rapid rise of a low n kink-peeling instability when the pedestal current Iped exceeds a threshold at high βp. It minimizes the excursion beyond marginal stability and is absent in the ballooning-dominated regime. Comparison with recent experiments over a range of βp and  * strongly supports the physical picture proposed by the modeling.

An integrated modeling workflow using OMFIT is constructed to evaluate the effects of tungsten (W) impurity on China Fusion Engineering Test Reactor (CFETR) performance. Self-consistent modeling of W core density profile, accounting for both turbulent and neoclassical transport contributions, is performed based on the steady-state scenario of CFETR phase I (Wan et al 2016 IAEA; Wan et al 2017 Nucl. Fusion 57 102009). It is found that the fusion performance degrades mildly with increasing W concentration. The main challenge arises in the sustainment of H-mode operation with significant W radiation. Assuming that the power threshold of H–L back transition is approximately the same as that of L–H transition, the W fraction at the plasma boundary is not allowed to exceed to stay in H-mode for CFETR phase I according to the scaling law proposed by Takizuka et al (2004 Plasma Phys. Control Fusion 46 A227–33). In addition, the tolerance of W concentration decreases with increasing pedestal density through a trade-off study of pedestal density and temperature. A future step is to connect the core simulation to W wall erosion modeling.

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.

An important task of the China Fusion Engineering Test Reactor physics design is to develop operation scenarios with high fusion power (1 GW), high bootstrap current fraction for steady-state and a plasma edge compatible with heat and particle exhaust. To achieve these goals, triangularity (δ) effects on the fusion performance of two candidate scenarios, with or without reversed magnetic shear (RS), namely conventional H-mode and RS H-mode, are evaluated using core-edge coupled integrated modeling in this paper. For fixed pedestal density, it is shown that higher δ is favorable for higher fusion performance in the conventional H-mode scenario while the fusion performance decreases with increasing δ in the RS H-mode scenario. In conventional H-mode, the higher fusion performance at high δ mainly comes from a higher pedestal temperature as predicted by EPED in combination with stiff core kinetic profiles. In the RS H-mode scenario with a local reversed shear region, the profiles are non-stiff and a strong internal transport barrier (ITB) exists at low δ. This results in higher density and temperature inside the ITB for low δ, leading to higher fusion power. If the pedestal temperature is kept fixed, in both scenarios the significant increase in pedestal density, which extends into the core, dominates at high δ and leads to much higher fusion power. For conventional H-mode, destabilization from increasing δ is partially balanced by stabilization due to increasing ν*. Since the normalized heat sources are quite similar, it results in minimal changes in the temperature profiles except for the lowest density case. For RS H-mode, destabilization from increasing δ is approximately balanced by stabilization due to increasing ν* in foot region, but a strong temperature ITB is still evident for low δ. The ability to take advantage of the high pedestal density in conventional H-mode and reversed shear scenario depends on its compatibility with edge density requirements from efficient heat and particle exhaust. Transport analysis is presented to elucidate the roles of δ, collisionality and magnetic shear in altering the profiles and the ITB, which contribute to the different behavior in the two scenarios.