arXiv subjects

O. Février

Publications and source records attributed to O. Février.

12 recordsLinked to original sources

arXiv2026

High-power TCV scenario for conventional and alternative divertor studies

Alternative divertor configurations (ADCs) must be evaluated under boundary plasma conditions approaching reactor-level values to be considered a reliable, physics-based solution for tokamak power exhaust. Most ADC experiments performed to date were at relatively low exhaust power. This work presents a high-power scenario on the TCV tokamak enabling the study of a wide variety of divertor magnetic shapes under an expanded SOL and power exhaust parameter space. The scenario is characterized by high power levels of electron cyclotron resonance heating ($2.5\,\text{MW}$ fully absorbed in a $\sim1\,\text{m}^{3}$ plasma) at high plasma current (edge safety factor $q_{95}\approx 2.5$), and low upstream separatrix densities ($n_{e,\text{u}}\approx1\times10^{19}\,\text{m}^{-3}$, Greenwald fraction $f_{\text{G}}\approx 0.1$). Stationary parallel heat fluxes up to $100\,\text{MW m}^{-2}$ are measured at the divertor target, an order of magnitude above previous TCV power exhaust studies. The obtained SOL collisionality and Lengyel detachment scaling metric lie within range of values expected in future reactors (SPARC, ITER, ARC).

physics.plasm-ph↗

arXiv2026

Detachment dynamics and disturbance rejection in the TCV X-Point Target divertor

The X-Point Target divertor is an alternative divertor configuration with a secondary X-point in its divertor volume. In this work, we investigate the dynamic response and disturbance rejection capacity of the XPT configuration on the TCV tokamak, comparing it to a single null (SN) divertor. We employ a system identification approach using multi-sine perturbations to measure the dynamic response of the detached state in both Ohmic and auxiliary-heated L-mode scenarios upon D$_2$ fuelling, N$_2$ seeding and Electron Resonance Cyclotron Heating (ECRH) power modulations. We demonstrate an inherent disturbance rejection capacity of the XPT at its secondary X-point compared to a SN configuration for all perturbation scenarios. Upstream of its secondary X-point, the dynamic response of the detached state between the XPT and SN appears similar. The disturbance rejection capacity of the XPT could be highly beneficial for passively buffering disturbances that cannot be effectively managed by power exhaust controllers. At the same time, it presents a challenge for monitoring the detached state close to the secondary x-point.

physics.plasm-ph↗

arXiv2026

Kinetic Equilibrium Prediction at TCV using RAPTOR and FBT

We present results from a new Kinetic-Equilibrium Prediction (KEP) workflow and shot preparation for full TCV discharges, by coupling predict-first RAPTOR transport simulations with FBT inverse equilibrium calculations. RAPTOR is a 1.5D transport code which has been extensively used for plasma shot optimization and real-time modeling. We show that rapid pre-shot simulations can be performed directly using information from the pulse schedule across a wide range of plasma shapes and scenarios, given an estimate of the confinement quality factor H98(y,2) and line-averaged density. The resulting p' and TT' profiles are then provided to the pre-shot equilibrium computation performed by FBT - a static free-boundary solver routinely used at TCV - achieving convergence between the two codes in a few iterations. Finally, we show that this coupling, when integrated into the TCV shot preparation, improves the evaluation of the coil currents needed to match the target plasma shape; in particular providing an accurate estimate of critical quantities such as the internal inductance $l_i$ and normalized pressure $β_N$, giving more realistic information to tokamak operators about the expected pulse behavior and enabling them to adjust the plan correspondingly.

physics.plasm-ph↗

arXiv2025

SOLPS-ITER simulation of an X-point radiator in TCV

SOLPS-ITER simulation is performed to reproduce the X-point radiator recently observed in nitrogen-seeded TCV experiments, which is a scenario that may be favorable to solve the power exhaust problems in future fusion devices. The simulations reveal the transition from the detached regime without XPR to the XPR regime, when increasing the nitrogen seeding rate. A cold X-point core surrounded by ionizing and radiative mentals is formed inside the separatrix and slightly above the X-point, where more than 90% of the total input power is dissipated. The cold X-point core exhibits a temperature of approximately 1eV and features high recombination rate to host the convective fluxes from the ionizing mental. Increasing nitrogen seeding rate also moves the nitrogen ionization front away from the target faster than the nitrogen stagnation point, which enhances the divertor nitrogen leakage to the main chamber and benefits the XPR region cooling. Carbon radiation decreases as the nitrogen seeding increases, and carbon radiation contributes to above 5% of the core impurity radiation before entering the XPR, which decreases to 2.8% when reaching the XPR. Both baffled and unbaffled divertor geometries are simulated and compared, showing that baffles facilitate the access to XPR by increasing the X-point neutral density, but requires higher seeding rate to enter the XPR regime.

physics.plasm-ph↗

arXiv2025

Divertor shaping with neutral baffling as a solution to the tokamak power exhaust challenge

Exhausting power from the hot fusion core to the plasma-facing components is one of the biggest challenges in fusion energy. The MAST Upgrade tokamak uniquely integrates strong containment of neutrals within the exhaust area (divertor) with extreme divertor shaping capability. By systematically altering the divertor shape, this study shows the strongest evidence to date that long-legged divertors with a high magnetic field gradient (total flux expansion) deliver key power exhaust benefits without adversely impacting the hot fusion core. These benefits are already achieved with relatively modest geometry adjustments that are more feasible to integrate in reactor designs. Benefits include reduced target heat loads and improved access to, and stability of, a neutral gas buffer that 'shields' the target and enhances power exhaust (detachment). Analysis and model comparisons shows these benefits are obtained by combining multiple shaping aspects: long-legged divertors have expanded plasma-neutral interaction volume that drive reductions in particle and power loads, while total flux expansion enhances detachment access and stability. Containing the neutrals in the exhaust area with physical structures further augments these shaping benefits. These results demonstrate strategic variation in the divertor geometry and magnetic topology is a potential solution to one of fusion's biggest challenges: power exhaust.

physics.plasm-ph↗

arXiv2024

Predictive power-sharing scaling law in double-null L-mode plasmas

The physical mechanisms regulating the power sharing at the outer targets of L-mode double-null (DN) configurations are investigated using nonlinear, flux-driven, three-dimensional two-fluid simulations. Scans of parameters that regulate the turbulent level, such as the plasma resistivity and the magnetic imbalance, reveal that the power asymmetry in DN configurations is determined by the combined effects of diamagnetic drift, turbulence, and geometrical factor. Leveraging these observations, an analytical theory-based scaling law for the power-sharing asymmetry is derived and compared with nonlinear simulations. These comparisons indicate that the scaling law effectively captures the trends observed in simulations. Validation with experimental data from TCV DN discharges demonstrates agreement of the scaling law with the experimental results.

physics.plasm-ph↗

arXiv2024

Parallel flows as a key component to interpret Super-X divertor experiments

The Super-X Divertor (SXD) is an alternative divertor configuration leveraging total flux expansion at the Outer Strike Point (OSP). While the extended 2-Point Model (2PM) predicts facilitated detachment access and control in the SXD configuration, these attractive features are not always retrieved experimentally. These discrepancies are at least partially explained by the effect of parallel flows which, when self-consistently included in the 2PM, reveal the role of total flux expansion on the pressure balance and weaken the total flux expansion effect on detachment access and control, compared to the original predictions. This new model can partially explain the discrepancies between the 2PM and experiments performed on tokamak à configuration variable (TCV), in ohmic L-mode scenarios, which are particularly apparent when scanning the OSP major radius Rt. In core density ramps in lower Single-Null (SN) configuration, the impact of Rt on the CIII emission front movement in the divertor outer leg - used as a proxy for the plasma temperature in the divertor - is substantially weaker than 2PM predictions. Furthermore, in OSP radial sweeps in lower and upper SN configurations, in ohmic L-mode scenarios with a constant core density, the peak parallel particle flux density at the OSP is almost independent of Rt, while the 2PM predicts a linear dependence. Finally, analytical and numerical modeling of parallel flows in the divertor is presented. It is shown that an increase in total flux expansion can favour supersonic flows at the OSP. Parallel flows are also shown to be relevant by analysing SOLPS-ITER simulations of TCV.

physics.plasm-ph↗

arXiv2024

SPLEND1D, a reduced one-dimensional model to investigate the physics of plasma detachment

Studying the process of divertor detachment and the associated complex interplay of plasma dynamics and atomic physics processes is of utmost importance for future fusion reactors. Whilst simplified analytical models exist to interpret the general features of detachment, they are limited in their predictive power, and complex 2D or even 3D codes are generally required to provide a self-consistent picture of the divertor. As an intermediate step, 1D models of the Scrape-Off Layer (SOL) can be particularly insightful as the dynamics are greatly simplified, while still self-consistently including various source and sink terms at play, as well as additional important effects such as flows. These codes can be used to shed light on the physics at play, to perform fast parameter scans, or to interpret experiments. In this paper, we introduce the SPLEND1D (Simulator of PLasma ENabling Detachment in 1D) code: a fast and versatile 1D SOL model. We present in detail the model that is implemented in SPLEND1D. We then employ the code to explore various elements of detachment physics for parameters typical of the Tokamak à Configuration Variable (TCV), including the atomic physics and other processes behind power and momentum losses, and explore the various hypotheses and free parameters of the model.

physics.plasm-ph↗

arXiv2024

Comparison of detachment in Ohmic plasmas with positive and negative triangularity

In recent years, negative triangularity (NT) has emerged as a potential high-confinement L-mode reactor solution. In this work, detachment is investigated using core density ramps in lower single null Ohmic L-mode plasmas across a wide range of upper, lower, and average triangularity (the mean of upper and lower triangularity: $δ$) in the TCV tokamak. It is universally found that detachment is more difficult to access for NT shaping. The outer divertor leg of discharges with $δ\approx -0.3$ could not be cooled to below $5~\mathrm{eV}$ through core density ramps alone. The behavior of the upstream plasma and geometrical divertor effects (e.g. a reduced connection length with negative lower triangularity) do not fully explain the challenges in detaching NT plasmas. Langmuir probe measurements of the target heat flux widths ($λ_q$) were constant to within 30% across an upper triangularity scan, while the spreading factor $S$ was lower by up to 50% for NT, indicating a generally lower integral Scrape-Off Layer width, $λ_{int}$. The line-averaged core density was typically higher for NT discharges for a given fuelling rate, possibly linked to higher particle confinement in NT. Conversely, the divertor neutral pressure and integrated particle fluxes to the targets were typically lower for the same line-averaged density, indicating that NT configurations may be closer to the sheath-limited regime than their PT counterparts, which may explain why NT is more challenging to detach.

physics.plasm-ph↗

arXiv2024

Modelling of power exhaust in TCV positive and negative triangularity L-mode plasmas

L-mode negative triangularity (NT) operation is a promising alternative to the positive triangularity (PT) H-mode as a high-confinement ELM-free operational regime. In this work, two TCV L-mode lower single null Ohmic discharges with opposite triangularity $δ\simeq \pm 0.3$ are investigated using SOLPS-ITER modelling. The main focus is the exploration of the reasons behind the experimentally observed feature of NT plasmas being more difficult to detach than similar PT experiments. SOLPS-ITER simulations are performed assuming the same anomalous diffusivity for particles $D_n^{AN}$ and energy $κ_{e/i}^{AN}$ in PT and NT. Nonetheless, the results clearly show dissimilar transport and accumulation of neutral particles in the scrape-off layer (SOL) of the two configurations, which consequently gives rise to different ionization sources for the plasma and produces different poloidal and cross-field fluxes. Simulations also recover the experimental feature of the outer target being hotter in the NT scenario (with $T_{e, NT} \gtrsim 5 \, \mathrm{eV}$) than in the PT counterpart.

physics.plasm-ph↗

arXiv2023

Performance assessment of a tightly baffled, long-legged divertor configuration in TCV with SOLPS-ITER

Numerical simulations explore the possibility to test the tightly baffled, long-legged divertor (TBLLD) concept in a future upgrade of the Tokamak à configuration variable (TCV). The SOLPS-ITER code package is used to compare the exhaust performance of several TBLLD configurations with existing unbaffled and baffled TCV configurations. The TBLLDs feature a range of radial gaps between the separatrix and the outer leg side walls. All considered TBLLDs are predicted to lead to a denser and colder plasma in front of the targets and improve the power handling by factors of 2-3 compared to the present, baffled divertor and by up to a factor of 12 compared to the original, unbaffled configuration. The improved TBLLD performance is mainly due to a better neutral confinement with improved plasma-neutral interactions in the divertor region. Both power handling capability and neutral confinement increases when reducing the radial gap. The core compatibility of TBLLDs with nitrogen seeding is also evaluated and the detachment window with acceptable core pollution for the proposed TBLLDs is explored, showing a reduction of required upstream impurity concentration up to 18% to achieve the detachment with thinner radial gap.

physics.plasm-ph↗

arXiv2022

New insights on divertor parallel flows, ExB drifts, and fluctuations from in situ, two-dimensional probe measurement in the Tokamak à Configuration Variable

In-situ, two-dimensional (2D) Langmuir probe measurements across a large part of the TCV divertor are reported in L-mode discharges with and without divertor baffles. This provides detailed insights into time averaged profiles, particle fluxes, and fluctuations behavior in different divertor regimes. The presence of the baffles is shown to substantially increase the divertor neutral pressure for a given upstream density and to facilitate the access to detachment, an effect that increases with plasma current. The detailed, 2D probe measurements allow for a divertor particle balance, including ion flux contributions from parallel flows and ExB drifts. The poloidal flux contribution from the latter is often comparable or even larger than the former, such that the divertor parallel flow direction reverses in some conditions, pointing away from the target. In most conditions, the integrated particle flux at the outer target can be predominantly ascribed to ionization along the outer divertor leg, consistent with a closed-box approximation of the divertor. The exception is a strongly detached divertor, achieved here only with baffles, where the total poloidal ion flux even decreases towards the outer target, indicative of significant plasma recombination. The most striking observation from relative density fluctuation measurements along the outer divertor leg is the transition from poloidally uniform fluctuation levels in attached conditions to fluctuations strongly peaking near the X-point when approaching detachment.

physics.plasm-ph↗