arXiv subjects

Thomas Eich

Publications and source records attributed to Thomas Eich.

5 recordsLinked to original sources

arXiv2025

A simple, accurate model for detachment access

In next-step fusion tokamaks such as SPARC and ITER, achieving high levels of scrape-off-layer power dissipation will be essential to protect the divertor while maintaining good core plasma performance. The Lengyel model for power dissipation is easy to interpret and fast enough to incorporate into plasma control and scoping tools, but it systematically overestimates the impurity concentration required to reach detachment by a factor of ~5 relative to experiments and higher-fidelity simulations. In this work, we extend the Lengyel model to match the semi-empirical Kallenbach scaling, which successfully describes detachment access on several operating tokamaks. We found that we can reproduce the experimental scaling by accounting for cross-field transport in the divertor, power and momentum loss due to neutral ionization close to the divertor target and turbulent broadening of the upstream heat flux channel. These corrections cause the impurity concentration required for detachment to decrease faster than $n_{e,u}^2$, reproducing the $c_z\propto 1/n_{e,u}^{2.7-3.2}$ scalings found in experiment. The model also quantitatively reproduces the impurity concentration needed to reach detachment in experiment, demonstrating that the extended Lengyel model can be used as a simple, accurate model for detachment access.

physics.plasm-ph↗

arXiv2024

The separatrix operational space of next-step fusion experiments: From ASDEX Upgrade data to SPARC scenarios

Fusion power plants require ELM-free, detached operation to prevent divertor damage and erosion. The separatrix operational space (SepOS) is proposed as a tool for identifying access to the type-I ELM-free quasi-continuous exhaust regime. In this work, we recast the SepOS framework using simple parameters and present dedicated ASDEX Upgrade discharges to demonstrate how to interpret its results. Analyzing an extended ASDEX Upgrade database consisting of 6688 individual measurements, we show that SepOS accurately describes how the H-mode boundary varies with plasma current and magnetic field strength. We then introduce a normalized SepOS framework and LH minimum scaling and show that normalized boundaries across multiple machines are nearly identical, suggesting that the normalized SepOS can be used to translate results between different machines. The LH minimum density predicted by SepOS is found to closely match an experimentally determined multi-machine scaling, which provides a further indirect validation of SepOS across multiple devices. Finally, we demonstrate how SepOS can be used predictively, identifying a viable QCE operational point for SPARC, at a separatrix density of 4e20/m3, a separatrix temperature of 156eV and an alpha-t of 0.7 - a value solidly within the QCE operational space on ASDEX Upgrade. This demonstrates how SepOS provides a concise, intuitive method for scoping ELM-free operation on next-step devices.

physics.plasm-ph↗

arXiv2024

Detachment scalings derived from 1D scrape-off-layer simulations

Fusion power plants will require detachment to mitigate sputtering and keep divertor heat fluxes at tolerable levels. Controlling detachment on these devices may require the use of real-time scrape-off-layer modeling to complement the limited set of available diagnostics. In this work, we use the configurable Hermes-3 edge modeling framework to perform time-dependent, fixed-fraction-impurity 1D detachment simulations. Although currently far from real-time, these simulations are used to investigate time-dependent effects and the minimum physics set required for control-relevant modeling. We show that these simulations reproduce the expected rollover of the target ion flux - a typical characteristic of detachment onset. We also perform scans of the input heat flux and impurity concentration and show that the steady-state results closely match the scalings predicted by the 0D time-independent Lengyel-Goedheer model. This allows us to indirectly compare to SOLPS simulations, which find a similar scaling but a lower value for the impurity concentration required for detachment for given upstream conditions. We use this result to suggest a series of improvements for the Hermes simulations, and finally show simulations demonstrating the impact of time-dependence.

physics.plasm-ph↗

arXiv2017

Divertor Heat Load in ASDEX Upgrade L-Mode in Presence of External Magnetic Perturbation

Power exhaust is one of the major challenges for a future fusion device. Applying a non-axisymmetric external magnetic perturbation is one technique that is studied in order to mitigate or suppress large edge localized modes which accompany the high confinement regime in tokamaks. The external magnetic perturbation brakes the axisymmetry of a tokamak and leads to a 2D heat flux pattern on the divertor target. The 2D heat flux pattern at the outer divertor target is studied on ASDEX Upgrade in stationary L-Mode discharges. The amplitude of the 2D characteristic of the heat flux depends on the alignment between the field lines at the edge and the vacuum response of the applied magnetic perturbation spectrum. The 2D characteristic reduces with increasing density. The increasing divertor broadening $S$ with increasing density is proposed as the main actuator. This is supported by a generic model using field line tracing and the vacuum field approach that is in quantitative agreement with the measured heat flux. The perturbed heat flux, averaged over a full toroidal rotation of the magnetic perturbation, is identical to the non-perturbed heat flux without magnetic perturbation. The transport qualifiers, power fall-off length $λ_q$ and divertor broadening $S$, are the same within the uncertainty compared to the unperturbed reference. No additional cross field transport is observed.

physics.plasm-ph↗

arXiv2016

Analytic 1D Approximation of the Divertor Broadening S in the Divertor Region for Conductive Heat Transport

Topic is the divertor broadening $S$, being a result of perpendicular transport in the scrape-off layer and resulting in a better distribution of the power load onto the divertor target. Recent studies show a scaling of the divertor broadening with an inverse power law to the target temperature $T_t$, promising its reduction to be a way of distributing the power entering the divertor volume onto a large surface area. It is shown that for pure conductive transport in the divertor region the suggested inverse power law scaling to $T_t$ is only valid for high target electron temperatures. For decreasing target temperatures ($T_t < 20\,$eV) the increase of $S$ stagnates and the conductive model results in a finite value of $S$ even for zero target temperature. It is concluded that the target temperature is no valid parameter for a power law scaling, as it is not representative for the entire divertor volume. This is shown in simulations solving the 2D heat diffusion equation, which is used as reference for an analytic 1D model describing the divertor broadening along a field line solely by the ratio of the perpendicular to the parallel diffusivity. By assuming the temperature dependence of these two quantities an integral form of $S$ is derived, relying only on the temperature distribution along the separatrix between X-point and target. Integration along the separatrix results in an approximation for $S$, being in agreement with the 2D simulations. This model is also applicable to scenarios including heat losses, e.g. due to radiation. Convective transport can not be neglected for high recycling conditions, hence the derived expression for $S$ is not expected to hold. However, based on the non-vanishing parallel transport, respective the finite parallel transport time, the divertor broadening is expected to reach a finite value.

physics.plasm-ph↗