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High-power TCV scenario for conventional and alternative divertor studies

This paper presents a new high-power scenario on the TCV tokamak, utilizing 2.5 MW of electron cyclotron resonance heating to achieve reactor-relevant boundary plasma conditions and record parallel heat fluxes of up to 100 MW m⁻², thereby enabling the evaluation of alternative divertor configurations under power exhaust parameters comparable to future fusion reactors like SPARC, ITER, and ARC.

Original authors: K. Lee, C. Theiler, M. Carpita, M. Zurita, P. Sintre, O. Février, F. Pastore, H. Reimerdes, K. Verhaegh, M. Winkel, D. Brida, B. Y. K. Brown, M. J. H. Cornelissen, R. Ducker, G. Durr-Legoupil-Nicoud

Published 2026-08-20

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: K. Lee, C. Theiler, M. Carpita, M. Zurita, P. Sintre, O. Février, F. Pastore, H. Reimerdes, K. Verhaegh, M. Winkel, D. Brida, B. Y. K. Brown, M. J. H. Cornelissen, R. Ducker, G. Durr-Legoupil-Nicoud, D. Hamm, R. I. Morgan, A. Perek, O. Sauter, E. Tonello, Y. Wang, the TCV Team, the EUROfusion Tokamak Exploitation Team

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

To understand the challenge tackled in this new research, one must first grasp the fundamental problem of building a fusion power plant. Fusion, the process that powers the sun, involves smashing atoms together to release immense energy. However, on Earth, this reaction creates a superheated gas called plasma that is so hot it would melt any solid container. To keep the plasma suspended, scientists use powerful magnetic fields to hold it in a donut-shaped ring. While the magnetic cage keeps the main body of the plasma away from the walls, a thin layer of gas at the very edge, known as the scrape-off layer, inevitably escapes. This escaping gas carries away the heat and must be guided safely to a specific exhaust area, called a divertor, where it can be cooled and removed. If this heat is not managed, it will damage the machine. The goal is to find a way to spread this heat out or cool it down before it hits the exhaust target, a state known as "detachment," without losing the energy needed to keep the fusion reaction going.

For decades, researchers have been testing different shapes for these magnetic exhaust systems, hoping to find a design that can handle the extreme heat expected in future power plants. Most previous experiments on smaller machines have been limited because they could not generate enough heat to truly stress-test these new designs. They were like trying to test a car's brakes by driving slowly in a parking lot, rather than at highway speeds. A team of scientists at the TCV tokamak in Switzerland has now changed this by creating a new, high-power operating mode that pushes the machine to its limits. They managed to inject a massive amount of heating energy into the plasma while keeping the density of the gas very low. This combination created a scenario where the heat hitting the exhaust target reached levels never before seen on this specific machine, bringing the conditions much closer to what a real power plant would face.

The researchers achieved this by using a specialized heating system that fires microwave beams directly into the center of the plasma. They pumped in 2.5 megawatts of power, which was almost entirely absorbed by the plasma, and maintained a strong electrical current of 300,000 amperes flowing through the gas. Crucially, they kept the amount of gas in the machine very low, far less than what is typically used in similar experiments. This low density is important because it makes the heat exhaust problem much harder to solve, forcing the magnetic shapes to work harder. Under these intense conditions, the heat flowing along the magnetic field lines to the exhaust target reached a peak of 100 megawatts per square meter. This is ten times higher than what this machine had ever produced in previous studies and enters the range of heat fluxes found in larger, more powerful fusion devices around the world.

With this new high-power setup, the team was able to test a wide variety of magnetic shapes designed to improve heat handling. They explored configurations with long, stretched-out exhaust legs, as well as designs that use extra magnetic points to spread the heat over a larger area. They found that while the machine could sustain these extreme heat loads, cooling the plasma down to a safe, detached state using gas seeding was surprisingly difficult. When they injected nitrogen gas to try and cool the exhaust, the radiation increased, but the core of the plasma also became hotter and more impure. The heat flux at the target dropped only slightly, and the plasma did not reach the fully detached state where the heat is completely removed before hitting the wall. This suggests that in this specific high-power, low-density environment, the usual methods of cooling the exhaust are less effective than they are in lower-power experiments.

The significance of this work lies in how closely the conditions now match those predicted for future reactors like ITER and SPARC. The researchers calculated that the difficulty of cooling the plasma in their new scenario is comparable to what these massive future machines will face. They measured a specific metric for detachment difficulty that reached values around 200 to 350, which is similar to the values expected for SPARC and ARC, two proposed next-generation fusion devices. While the machine itself is much smaller than a power plant, the physics of the heat exhaust it is dealing with is now on the same scale. This means that the TCV machine can now serve as a vital testbed for understanding how different magnetic shapes will perform under reactor-level stress, helping scientists refine their designs before building the full-scale power plants of the future. The study confirms that while the heat loads are now high enough to be relevant, the path to safely managing them remains a complex challenge that requires further investigation.

Technical Summary: High-Power TCV Scenario for Conventional and Alternative Divertor Studies

Problem Statement
Alternative divertor configurations (ADCs) are being developed to improve tokamak power exhaust and facilitate plasma detachment at reduced upstream densities and impurity seeding requirements. However, a significant challenge exists in validating these concepts: most ADC experiments to date have been conducted at relatively low exhaust powers (peak parallel heat fluxes $\lesssim 20$ MW m $^{-2}$ ), whereas future reactors (e.g., ITER, SPARC, ARC) are expected to face unmitigated parallel heat fluxes on the order of GW m $^{-2}$ . There is currently no intermediate, ITER-like step between present experiments and reactor conditions to bridge this extrapolation gap. Consequently, ADCs must be evaluated under boundary plasma conditions approaching reactor-level values to be considered reliable, physics-based solutions.

Methodology
This work presents a new high-power, stationary L-mode scenario on the TCV tokamak designed to maximize the power exhaust challenge across a wide variety of divertor magnetic geometries. The experimental strategy focuses on three primary parameters:

Maximized Heating: Utilizing the full 2.5 MW of Electron Cyclotron Resonance Heating (ECRH) power, fully absorbed in the core, to maximize the power entering the Scrape-Off Layer ( $P_{SOL}$ ).
High Current, Low Density: Operating at a high plasma current ( $I_P = 300$ kA, $q_{95} \approx 2.5$ ) to narrow the heat flux decay width, while simultaneously maintaining low upstream separatrix densities ( $n_{e,u} \approx 0.8 \times 10^{19}$ m $^{-3}$ , Greenwald fraction $f_G \approx 0.06$ ) to increase the exhaust difficulty.
Geometric Flexibility: Leveraging TCV's variable configuration capabilities to test long-legged divertor shapes, including conventional Single Null (SN), X-divertor, Super-X divertor, and X-Point Target (XPT) divertors, often combined with variable gas baffles.

To maintain precise control over the low-density regime, a novel density profile observer (RAPDENS-EKF) was employed, reconstructing the core density profile in real-time to avoid systematic overestimation common in standard interferometry. Nitrogen seeding was used to explore detachment access, though core radiation was minimized by avoiding high-Z impurities.

Key Contributions and Results
The study successfully generated a stationary high-power scenario with the following key results:

Record Heat Fluxes: The scenario achieved stationary parallel heat fluxes at the divertor target up to 100 MW m $^{-2}$ (measured at the inner strike point), representing an order-of-magnitude increase over previous TCV power exhaust studies and entering the parameter space of larger devices like AUG and DIII-D.
Reactor-Relevant Metrics: The derived SOL collisionality ( $\nu^*_{SOL}$ ) and the Lengyel detachment scaling metric ( $L_m$ ) were evaluated. The high-power TCV scenario yielded $L_m$ values of $\sim 200\text{--}350$ and $\nu^*_{SOL}$ values of 2–4. These values place the TCV scenario within the range of expected conditions for future reactors, specifically comparable to SPARC ( $L_m \approx 191$ ) and ARC ( $L_m \approx 297$ ).
Detachment Challenges: Despite the high power and low density, achieving full detachment via nitrogen seeding proved difficult. The CIII emission front showed limited movement from the strike point, and the total radiated power fraction only reached 40% (with 25% radiated in the SOL), significantly lower than the 60–80% typically seen in Ohmic L-mode. This suggests that the ability of nitrogen to radiate in the SOL is more limited in this high-power, low-density regime.
Transport Regime: Analysis of heat flux decay widths ( $\lambda_{q,u} \approx 3$ mm) indicated a weakly collisional SOL regime, consistent with flux-limited heat conduction rather than Spitzer-Härm conduction.

Significance
The paper claims that this new TCV scenario significantly elevates the power exhaust challenge attainable on the device, expanding the accessible SOL parameter space for both conventional and alternative divertor studies. By achieving reactor-relevant values for the Lengyel metric and SOL collisionality, the scenario provides a critical intermediate step for testing ADCs under conditions that better approximate future fusion power plants. The work demonstrates that TCV can now serve as a testbed for stationary power exhaust studies at heat flux levels previously inaccessible on the device, offering valuable data to help close the extrapolation gap between current experiments and reactor designs. The authors note that while the scenario is a significant step forward, further work is needed to fully understand detachment access in this high-power, low-density regime.

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