Alessandro Balestri

Real-time estimation and control of the electron plasma density profile is fundamental for monitoring and regulating particle confinement, fusion power, heating efficiency, exhaust performance, impurity concentration, and proximity to the density limit (Greenwald et al Nucl. Fusion 28 2199–207) in present and future tokamaks. This work presents the experimental applications of a multi-rate electron density observer (Pastore Fusion Eng. Des. 192 113615) based on RAPDENS (Blanken Fusion Eng. Des. 126 87–103), integrated into the TCV plasma control system, to control the density profile in a range of plasma scenarios relevant to future fusion power plants. Three main experimental contributions are reported. First, the observer enables control of the line-averaged electron density within the last-closed flux surface ( NEL LCFS ) in ohmic, nitrogen-seeded plasmas with alternative divertor geometries, decoupling upstream density from scrape-off layer pickup in the interferometer signal—a limitation of the traditional control scheme. Second, local control of the central electron density below the ECH cutoff is demonstrated in L-mode plasmas with mixed ECH and neutral beam injection heating, where auxiliary-heating-induced profile peaking is treated as a disturbance to the control task. Real-time estimation and adjustment of the electron pinch velocity-to-diffusivity ratio ν / D improves the spatial accuracy of the profile reconstruction, especially in the core region. The underlying turbulent transport in this scenario is characterised via linear and non-linear gyrokinetic simulations with GENE, confirming quantitatively a TEM-dominated pumpout regime during ECH, which affects the amount of gas flux needed to sustain the target density reference. Last, simultaneous control of the edge-normalised density fraction and toroidal beta is demonstrated in high-performance H-mode plasmas ( β N ≈ 2.15 , f GW ≈ 0.80 ), yielding reproducible scenarios with a device-agnostic density metric and robust handling of interferometer fringe jumps.

This paper summarizes experimental and modelling work carried out across the last 5 y on negative triangularity (NT) scenarios in view of future DTT operations. The shapes foreseen for DTT have been tested experimentally on TCV and ASDEX upgrade (AUG). Integrated modelling using ASTRA/TGLF and local gyrokinetic simulations using GENE have been performed on the experimental results and to predict DTT scenarios. Experiments and modelling find a stronger beneficial effect of NT in TCV than in AUG. DTT predictions are more in line with AUG results. Various mechanisms to explain this difference have been examined and are discussed. In all devices, the NT scenarios provide a viable ELM-free alternative to positive triangularity ELMy H-modes, with central values of pressure comparable or even higher.

Negative triangularity (NT) is a promising edge-localized mode-free candidate for future fusion reactors. In this paper, the impact of increased divertor closure on density-ramp induced detachment in NT is assessed in the Tokamak á configuration variable (TCV). Previous experiments in TCV have shown that at similar line-averaged density and divertor shape, NT shaping makes detachment access in L-mode more difficult than in positive triangularity (PT) and results in a lower divertor neutral pressure. In this paper we show that increasing the divertor neutral pressure in NT with the help of TCV’s divertor gas baffles cools the outer target and the outer leg compared to unbaffled discharges. However, baffled NT discharges, at a divertor neutral pressure comparable to unbaffled PT, at similar separatrix density and similar divertor geometry, still show higher target electron temperature and no ion flux rollover, suggesting lower level of divertor detachment. Langmuir probe measurements within the divertor volume, using a reciprocating divertor probe array, show that the scrape-off layer broadening along the outer leg, typical in PT configurations, is not observed in NT, indicating a lower level of divertor cross-field transport. For the divertor geometry used in this study, it is further found that having the ion ∇ B -drift oriented from the core towards the X -point is beneficial for outer target cooling and detachment onset, contrary to the general picture of particle redistribution via divertor drifts. However, even in this case, in NT, no ion flux rollover at the target and only partial cooling of the divertor volume are obtained. When core density is further increased, a collapse in separatrix density is observed.

Experiments have shown that negative triangularity (NT) configurations in tokamak plasmas exhibit good confinement properties associated to reduced turbulent transport. Trapped Electron Modes (TEMs) have proven to be particularly sensitive to this shaping-driven stabilization. A reduced semi-analytical model derived in [Garbet et al. 2024 Nucl. Fusion 64 106055] unravels the origin of the linear stabilization of TEMs in NT plasmas. This model only includes the trapped electrons curvature dynamics. Here, we en- rich the model introducing the adiabatic response of electrons and a simplified term for ions dynamics. This enables a satisfactory benchmark with the linear predictions obtained using the GENE gyro-kinetic code. The ballooning character of TEM turbulence is confirmed to play a central role in the stabilizing process, extending the results of [Garbet et al. 2024 Nucl. Fusion 64 106055] to a broader parameter space. Here, the key physical mechanism is the large weight given by ballooning to deeply trapped electrons, whose preces- sion frequency is lower in NT than in PT – conversely to barely trapped ones. The stabilizing role of deeply trapped electrons is further supported by the analysis of the energy transfer resolved in velocity phase space between electrons and electrostatic modes, using both the reduced model and the global gyro-kinetic code GYSELA.

Strong E × B plasma flow shear is beneficial for reducing turbulent transport. However, traditional methods of driving flow shear do not scale well to large devices such as future fusion power plants. In this paper, we use a large number of nonlinear gyrokinetic simulations to study a novel approach to increase flow shear: decreasing the momentum diffusivity to make the plasma ‘easier to push’. We first use an idealized circular geometry and find that one can obtain low momentum diffusivity (LMD) at tight aspect ratio, low safety factor, high magnetic shear and low temperature gradient. This is the so-called LMD regime. To drive intrinsic momentum flux, we then tilt the flux surface, making it up–down asymmetric. In the LMD regime, this intrinsic momentum flux drives strong flow shear that can significantly reduce the heat flux and increase the critical temperature gradient. We also consider the actual experimental geometry of the Mega Ampere Spherical Tokamak (MAST) to illustrate that this strategy can be practical and create experimentally significant flow shear. Lastly, a preliminary prediction for the SMall Aspect Ratio Tokamak (SMART) is made.

We present the first edge E _r measurements in negative triangularity (NT) plasmas in the Tokamak à Configuration Variable (TCV). The Doppler backscattering measurements of v ⊥ ≈ E r / B reveal a significant impact of triangularity on the E _r well: in Ohmic, neutral beam injection, and electron cyclotron resonance heated discharges, the E _r well and associated E r × B shear are stronger in NT-shaped plasmas compared to their positive triangularity (PT) counterpart. This suggests a connection to the concomitant NT performance gain relative to PT L-mode.

In this work, we study the interplay between triangularity and micro-tearing turbulence using linear and nonlinear flux tube GENE simulations. We consider scenarios with negative and positive triangularity (PT) plasma shaping taken from existing tokamaks (TCV, DIII-D, MAST-U and SMART) and EU-DEMO. The study of all these tokamaks reveals a coherent picture. Negative triangularity (NT) geometry is more susceptible to micro-tearing modes (MTMs), which, when present, make transport much worse than in PT. At sufficiently large β (the ratio of plasma pressure over magnetic pressure), magnetic shear and ratio of electron to ion temperature gradient, all the scenarios with NT are dominated by MTM turbulence. In contrast, the corresponding scenarios with PT remain dominated by electrostatic turbulence and MTMs are subdominant or stable. We observe that conventional tokamaks usually operate in a parameter space far away from the onset of this MTM-dominated regime in NT, thus preserving the beneficial effect of NT on turbulent transport. In contrast, spherical tokamaks (ST) operate close to this regime and may ultimately exhibit worse transport at NT than PT. We find that lowering the magnetic shear in STs can preserve the beneficial effect of NT on electrostatic turbulence and prevent strong MTM transport. Finally, linear and nonlinear simulations reveal the reason for stronger MTMs: the magnetic drifts are faster in the NT geometry.

Negative triangularity (NT) plasmas achieve enhanced confinement without edge localized modes nor the need to exceed the L-H heating power threshold. Removing this H-mode accessibility constraint opens a new region of parameter space: scenarios with low external heating power. Using simple zero dimensional power balance and standard empirical scaling laws for confinement, we analyze the impact of external heating on several different reactor-relevant devices (i.e. SPARC, MANTA, ITER and DEMO). We determine the ideal amount of external heating to maximize performance and test whether it is possible to reach satisfactory performance with no external heating at all. For devices with sufficiently high magnetic field and/or fusion gain, Ohmically heated NT scenarios achieve better performance than positive triangularity H-mode scenarios with external heating. We conclude that NT enables attractive low input power scenarios that merit further investigations with more comprehensive models. Such low power scenarios appear particularly well suited for attaining high fusion gain Q (e.g. a burning plasma experiment).

In this work, we explore the effects of collisions on turbulent transport, focusing on two different TCV magnetic equilibria featuring positive and negative triangularity. The investigation is conducted using gyrokinetic modeling. Both gradient-driven and flux-driven approaches are employed using the global code ORB5, and a comparison with GENE flux-tube modeling is also carried out. Linear and nonlinear simulations show that negative triangularity retains its beneficial effect on turbulent transport in both collisionless and collisional regimes. However, flux tube and global nonlinear simulations show contrasting trends on the impact of collisions on ion transport. Flux-driven simulations confirm that edge stiffness is significantly reduced in negative compared to positive triangularity, and a spontaneous pedestal-like logarithmic gradient develops for negative triangularity in the collisionless setup. However, without proper realistic profiles and physical boundary conditions, it is not possible to fully replicate the experimental differences observed in the temperature profiles between positive and negative triangularity. A collisionality scan reveals that the ion and electron transport coefficients do not change monotonically with collisionality, stressing how deep the interplay is between turbulent features and collisions. The general picture of collisions stabilizing TEM and damping zonal flows is confirmed, but it is shown that the effects on transport cannot be predicted without a numerical assessment.