The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device’s unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly non-inductive H-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power ‘starvation’ reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached L-mode phase, increasing the outer connection length reduces the in–out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variable-configuration baffles and possibly divertor pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECRH and 1 MW neutral beam injection heating will be added.

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The observation that fast ions stabilize ion-temperature-gradient-driven microturbulence has profound implications for future fusion reactors. It is also important in optimizing the performance of present-day devices. In this work, we examine in detail the phenomenology of fast ion stabilization and present a reduced model which describes this effect. This model is derived from the high-energy limit of the gyrokinetic equation and extends the existing "dilution" model to account for nontrivial fast ion kinetics. Our model provides a physically-transparent explanation for the observed stabilization and makes several key qualitative predictions. Firstly, that different classes of fast ions, depending on their radial density or temperature variation, have different stabilizing properties. Secondly, that zonal flows are an important ingredient in this effect precisely because the fast ion zonal response is negligible. Finally, that in the limit of highly-energetic fast ions, their response approaches that of the "dilution" model; in particular, alpha particles are expected to have little, if any, stabilizing effect on plasma turbulence. We support these conclusions through detailed linear and nonlinear gyrokinetic simulations. arXiv:1801.00664v3 [physics.plasm-ph] 5 Jun 2018Effect of fast ions on microturbulence 2 Fast ion stabilization of ITG microturbulenceMicroturbulence fundamentally limits the confinement time in current and future tokamak experiments [2]. The very gradients that are required to achieve high central densities and temperatures also provide a source of free energy. This free energy drives transport of particles, momentum, and energy, usually far in excess of collisional transport. The ion temperature gradient (ITG) mode, for instance, limits the core temperature of tokamaks [3,4,5]. This turbulence occurs on the scale of the thermal ion Larmor radius ρ i .Gyrokinetics is the reduction of the Fokker-Planck kinetic equation that rigorously handles electromagnetic fields whose fluctuations vary on spatial scales similar to ρ i , but on timescales much slower than the gyro-frequency Ω i [6,7,8]. A multitude of computational tools have been developed to solve the nonlinear gyrokinetic equation [9,10,11,12]. These tools take as inputs the equilibrium magnetic field, density, and temperature profiles, and predict the ensuing turbulent fluctuations and the associated transport fluxes. By using experimentally-determined profiles and comparing the computed fluxes to experimentally-inferred fluxes (usually from power-balance calculations), and through more detailed comparisons of turbulence characteristics, the validity of the gyrokinetic approach can be verified [13]. This has been widely demonstrated [14,15,16,17].However, due to the scarcity of computational resources, these matching exercises of necessity entail the use of simulations that neglect various parts of the complex physics of the experimental setup. It was discovered during one of these exercises, in which experiments on the Joint European Tor...

The tokamak à configuration variable (TCV) continues to leverage its unique shaping capabilities, flexible heating systems and modern control system to address critical issues in preparation for ITER and a fusion power plant. For the 2019–20 campaign its configurational flexibility has been enhanced with the installation of removable divertor gas baffles, its diagnostic capabilities with an extensive set of upgrades and its heating systems with new dual frequency gyrotrons. The gas baffles reduce coupling between the divertor and the main chamber and allow for detailed investigations on the role of fuelling in general and, together with upgraded boundary diagnostics, test divertor and edge models in particular. The increased heating capabilities broaden the operational regime to include T e/T i ∼ 1 and have stimulated refocussing studies from L-mode to H-mode across a range of research topics. ITER baseline parameters were reached in type-I ELMy H-modes and alternative regimes with ‘small’ (or no) ELMs explored. Most prominently, negative triangularity was investigated in detail and confirmed as an attractive scenario with H-mode level core confinement but an L-mode edge. Emphasis was also placed on control, where an increased number of observers, actuators and control solutions became available and are now integrated into a generic control framework as will be needed in future devices. The quantity and quality of results of the 2019–20 TCV campaign are a testament to its successful integration within the European research effort alongside a vibrant domestic programme and international collaborations.

A core fluctuation diagnostic based on the phase-contrast imaging (PCI) technique has been designed for the JT-60SA tokamak, with the assistance of a synthetic diagnostic coupled to a gyrokinetic code. Using a tangentially viewing geometry, this system would be able to resolve small-scale microturbulence as well as macroscopic fluctuations, with good spatial and temporal resolution, throughout the plasma cross-section and in all plasma regimes. The spatial resolution will be optimal (<5% of the minor radius) in the pedestal region and near the magnetic axis. The accessible wave-number range will cover the main ion-scale instabilities predicted to be at play, and optionally also the electron-scale ones. The new superconducting tokamak JT-60SA, which is due to begin operating in 2021, will be the largest tokamak ever built and the most significant intermediate step in magnetic-confinement fusion before the inception of ITER operations. Turbulence is the primary cause of anomalous transport, one of the primary limiting factors in controlled nuclear fusion. Understanding and possibly controlling turbulence thus remain paramount to the fusion quest, and JT-60SA offers the possibility of performing such studies for the first time in a true reactor-relevant environment, providing also a unique opportunity for code validation. This proposal is supported by a direct modeling effort, employing the gyrokinetic code gene and a PCI synthetic diagnostic. Linear and nonlinear flux-tube simulations have been performed for a typical high-performance scenario, to identify the spectral and spatial areas of interest and illustrate the potential for fruitful theory-experiment comparison. While only electrostatic simulations could be carried to full completion with the computing resources deployed for this study, the work has highlighted the importance of including electromagnetic effects for proper comparisons.

JT-60SA, the largest tokamak that will operate before ITER, has been designed and built jointly by Japan and Europe, and is due to start operation in 2020. Its main missions are to support ITER exploitation and to contribute to the demonstration fusion reactor machine and scenario design. Peculiar properties of JT-60SA are its capability to produce long-pulse, high-β, and highly shaped plasmas. The preparation of the JT-60SA Research Plan, plasma scenarios, and exploitation are producing physics results that are not only relevant to future JT-60SA experiments, but often constitute original contributions to plasma physics and fusion research. Results of this kind are presented in this paper, in particular in the areas of fast ion physics, highbeta plasma properties and control, and non-linear edge localised mode stability studies.

A large superconducting machine, JT-60SA has been constructed to provide major contributions to the ITER program and DEMO design. For the success of the ITER project and fusion reactor, understanding and development of plasma controllability in ITER and DEMO relevant higher beta regimes are essential. JT-60SA has focused the program on the plasma controllability for scenario development and risk mitigation in ITER as well as on investigating DEMO relevant regimes. This paper summarizes the high research priorities and strategy for the JT-60SA project. Recent works on simulation studies to prepare the plasma physics and control experiments are presented, such as plasma breakdown and equilibrium controls, hybrid and steady-state scenario development, and risk mitigation techniques. Contributions of JT-60SA to ITER and DEMO have been clarified through those studies.

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