Christian Theiler

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.

Radiated power information is crucial to diagnose and optimize the performance of fusion plasmas. Traditionally, at the TCV tokamak, radiated power analysis has only ever been possible following plasma discharge termination. However, recently, TCV bolometer data have become available in real-time. This offers the opportunity of integrating the radiated power information into the TCV plasma control system. In this work, we propose a novel real-time tomography-based Bayesian technique allowing estimation of the power radiated from user-defined regions of interest in the plasma. The real-time estimates are obtained as computationally cheap linear combinations of bolometer measurements, using pre-computed coefficients that are optimized for the specific discharge planned. This method is not, thus, trained on a set of synthetic or tomographically reconstructed emissivity profiles. We detail the derivation of the technique and show its equivalence to traditional tomographic estimates under suitable conditions. We then demonstrate that this technique enables accurate real-time estimation of the total, core, divertor and main chamber radiated power, by its application to a representative and heterogeneous set of TCV discharges. Finally, we discuss the robustness of the technique to faulty detectors, showing that simple precautions allow safe handling of many common issues. The computational routines implementing the described technique are provided as open-source code.

The first high-power ( ⩾ 3 MW) H-mode experiments using a double-null X -point-target (XPT) divertor configuration have been performed on MAST-U. The XPT geometry is obtained by combining a large strike point radius, similar to the Super- X divertor (SXD), with an additional X -point near the separatrix in the baffled outer divertor chambers and leads to additional exhaust benefits over the SXD. The broader electron density profile near the secondary X -point leads to additional plasma-neutral interactions, evidenced by a broader hydrogenic emission profile, and resulting in larger power and ion sinks. The increase in plasma-neutral interactions also leads to lower target electron temperatures and heat fluxes. These benefits appear to extend to transients, and preliminary evidence of improved Edge Localized Mode buffering in the XPT is presented. These results showcase how multiple alternative divertor configuration strategies can be combined to improve momentum, power, and particle losses, which may be required for the challenging exhaust conditions of future reactors.

We present a systematic experimental study of triangularity effects on power sharing, Scrape-Off Layer (SOL) transport, and boundary turbulence in Double-Null (DN) L-mode plasmas in TCV. We show that triangularity (δ) strongly influences inner/outer power sharing between divertor targets. In high-δ DN plasmas (δ = 0.58), only 13% of the exhaust power reaches the inner targets, while in low-δ DN (δ = 0.17) this fraction rises to 28%. In Lower Single-Null plasmas, the inner target power fraction increases from 33% (δ = 0.58) to 48% (δ = 0.17). Comparisons with analytic models show that the observed power-sharing trends are qualitatively reproduced, and further suggest not only a reduction of poloidally ballooned radial transport, but also enhanced cross-field transport through the high-field side. While Thomson scattering measurements show no clear δ-dependence of midplane decay lengths, embedded Langmuir probes in the lower divertor shows a ∼ 59% broadening of λq∥ between δ = 0.17 and δ = 0.58. Finally, edge turbulence exhibit a 30% reduction in fluctuation levels at low δ. On a smaller scale, blob structures are 35% smaller in radial extent and with 43% lower amplitude with regard to the background values in low-δ DN compared to high-δ, in agreement with recent GBS simulations and supporting the correlation between plasma shaping, turbulence characteristics, and power sharing dynamics.

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.

The TCV tokamak contributes to the development of nuclear fusion energy with proof-of-principle experiments and by challenging models that are used to predict reactor performance. In the next upgrade of the TCV divertor it is planned to test a tightly baffled, long-legged divertor (TBLLD), a novel concept designed to enhance power exhaust handling with minimal modification to the magnetic configuration. The project is guided by simulations using the SOLPS-ITER code that indicate that a TBLLD can improve TCV’s power exhaust capability by up to an order of magnitude over its unbaffled configuration. Tight baffling sustains a high poloidal neutral density gradient along the divertor leg, thereby increasing the neutral density in front of the divertor target, which enhances volumetric power dissipation. The simulations informed the design of a proof-of-principle TBLLD for the outer TCV divertor, while ensuring compatibility with high-power plasma scenarios. The chosen straight, vertical design incorporates graphite baffle tiles, accommodates TCV’s reciprocating divertor probe array (RDPA) and maintains engineering simplicity. While the resulting divertor is more restrictive, it provides sufficient diagnostic access to assess the TBLLD concept. Poloidally distributed wall-mounted Langmuir probes, surface thermocouples and neutral pressure gauges, and horizontal spectrometric lines of sight at several heights will enable a comprehensive characterisation of the divertor state. A dedicated experimental campaign is planned for 2026.

Far scrape-off layer fluctuations, also known as blobs/filaments, are compared for the first time across six tokamaks: Alcator C-Mod, DIII-D, TCV, KSTAR, MAST, and MAST-U, under the context of stochastic modeling in Ohmic and low confinement mode scenarios. In this model, which well describes these fluctuations, measurements from Langmuir probes are modeled as a superposition of uncorrelated, two-sided exponential temporal pulses. The key parameters of this model are the mean waiting times between consecutive pulses, the pulse duration times and the mean of the pulse amplitudes. The study examines the influence of line-averaged density, plasma current, magnetic field and aspect ratio. The mean amplitudes and pulse waiting times increase with the line-averaged density across devices, while the pulse duration times remain constant. Whereas the variation in waiting times is gradual across the density scan, the mean amplitudes increase abruptly as the density profile broadens and flattens for Alcator C-Mod. The blob waiting times decrease with increasing plasma current and the toroidal magnetic field, while they increase with aspect ratio for conventional tokamaks. The duration times follow the same trends as the waiting times in these parameters but vary more slowly, resulting in the trends in the waiting times dominating the intermittency level of the fluctuations.

A direct quantitative comparison of scrape-off layer (SOL) filament properties obtained from fluid turbulence simulations with the GBS code and from experiments on the Tokamak à configuration variable (TCV) tokamak is performed within the TCV-X21 validation case. This comparison is made possible by extending the open TCV-X21 dataset with 2D turbulence measurements obtained with gas puff imaging (GPI), providing critical information on the size, velocity, and other key characteristics of turbulent filaments at the outboard midplane and in the divertor region. For the comparison, GBS simulations of TCV-X21 are analyzed using a dedicated synthetic GPI diagnostic that models the neutral helium-plasma interaction and emission processes and accounts for line-integration effects. Poloidal and radial filament velocities are found to be in good agreement between simulations and experiments, while the simulations overestimate the filament radial and poloidal sizes. The simulations further indicate that filaments in the SOL are predominantly represented by density perturbations rather than temperature perturbations, consistent with previous assumptions in experimental analyses of cross-field turbulent transport from GPI data. The direction of the poloidal velocity of filaments in this scenario is found to agree with the time-averaged E × B direction at the outboard midplane and X -point region, but not in the divertor leg. Possible explanations are proposed and discussed, highlighting the influence of the instantaneous E × B velocity components in both poloidal and radial directions. This study provides new insights into turbulent filament behavior and contributes to guiding future efforts to improve first-principles simulations of the boundary plasma.

The research programme performed on the Mega Amp Spherical Tokamak (MAST) Upgrade experiment has made significant advances in developing the physics understanding of low aspect ratio tokamaks in support of the operation of ITER and design of fusion powerplants. High performance plasma scenarios have been developed to facilitate a broad programme of experiments, in which confinement is constrained by the presence of m / n = 2/1 modes that cause substantial losses of fast ions. The onset of these modes coincides with the q = 2 surface residing in a local minimum in the toroidal current density profile. The maximum electron temperature at the pedestal top, T _e,ped is limited with gas fuelling to ∼350 eV to maintain regular ELMs; higher T _e,ped results in a transition to a non-stationary ELM-free regime. The operational space of spherical tokamaks has been expanded into small and ELM-free regimes. Strong shaping of the last closed flux surface can induce a transition from large to small ELMs, and ELM suppression with resonant magnetic perturbations has been observed for the first time in a low aspect ratio tokamak. Negative triangularity shaping has induced a transition from ELMy H-mode to a high-performance L-mode regime for the first time in a low aspect ratio tokamak. In studies of fast ion confinement, losses of fast particles due to Global Alfvén Eigenmodes have been identified. Interactions between fast ions generated by off-axis neutral beam injection and thermal neutrals can result in significant losses of fast ions. Experiments with on- and off-axis neutral beam injection exhibit a flux pumping mechanism, where the central safety factor is held to ∼1 in the absence of sawteeth. In studies of pedestal physics, it has been found that elevated main chamber neutral pressures result in an increase in the electron density and reduction in the temperature at the pedestal top. Advances in understanding plasma exhaust include the integration of a high-performance plasma core with detached outer divertors in the X -point target configuration. A newly commissioned lower divertor cryopump reduces the lower divertor neutral pressure by up to 50%, with minimal effect on the main chamber or upper divertor. New measurements and SOLPS-ITER simulations emphasise the importance of plasma–neutral interactions on divertor detachment in the conditions accessible in experiments. Real-time control of the ionisation front location in both divertor chambers independently has been demonstrated in double null experiments, enabled by the tightly baffled divertor chambers.

Edge plasma behaviour plays a central role in determining the overall confinement and operational limits of tokamak plasmas. In particular, turbulence at the plasma boundary strongly influences particle and heat transport and remains a key target for performance optimization. Among the shaping strategies currently explored, negative triangularity (NT) operation has emerged as a viable alternative to conventional positive triangularity (PT) plasmas, having demonstrated improved confinement, reduced turbulence-driven transport, and the absence of edge-localized modes (ELMs), thereby alleviating plasma–wall interaction constraints. In this paper, experimental observations from the upgraded Thermal Helium Beam (THB) diagnostic on the TCV tokamak are reported. The analysis is based on a large database of discharges, selected using plasma triangularity as the sole discriminant. These measurements are used to investigate how plasma geometry affects edge temperature and density profiles, and fluctuation properties. A comparative analysis between PT and NT plasmas is carried out, with emphasis on the modification of edge turbulence characteristics. The results reveal systematic differences between NT and PT plasmas, with NT discharges generally exhibiting lower turbulence levels in the scrape-off-layer (SOL), indicative of a modified edge transport regime. These findings are enabled by the ability of the THB diagnostic to access both kinetic quantities and fluctuation amplitudes from the plasma edge to the SOL, providing new insight into turbulence suppression mechanisms associated with NT shaping.