M. Kryjak | Searcharxiv

arXiv2026

Harnessing Toroidal Neutral Flows to Enhance Divertor Particle Exhaust

In 1991 Reiter et al. (1991 Plasma Phys. Control. Fusion 33 1579) considered the onerous exhaust requirements of ITER, and wrote: "The vacuum pumping problem of a fusion reactor will probably require some novel solution". Here we show that a toroidally oriented pump inlet can passively exploit intrinsic neutral flows to reduce back-flow, raise duct pressure, and ultimately improve particle-exhaust performance. Drawing on previous experimental observations and SOLPS-ITER edge-plasma simulations, we consolidate the evidence for a plasma-imprinted, multi-species toroidal neutral "wind" in detached tokamak divertors. We isolate the underlying mechanism in a prototypical divertor private-flux region using a database of two-dimensional direct simulation Monte Carlo (DSMC) calculations. The ordered neutral motion is recovered with a strong toroidal alignment, kilometre-per-second velocities, and persistence up to several centimetres across slip-to-transitional rarefied regimes (Kn=0.02$-$2). We then assess the consequences of capturing this ordered motion using a second database of idealised proof-of-principle DSMC simulations. Compared to the traditional poloidal arrangement, a toroidally oriented pump inlet reduces back-flow by up to 20% for deuterium and up to 33% for helium at 10% concentration. Partial pressures in the toroidal exhaust path are enhanced across the database, nominally by a factor of 1.78$\pm$0.04 for deuterium and 2.00$\pm$0.05 for helium. For fixed throughput, this implies a reduction in the required effective pumping speed and corresponding hardware. More broadly, these results motivate explicit retention of toroidal neutral momentum in divertor and sub-divertor modelling, and dedicated studies of neutral aerodynamics, including in stellarators, where an analogous directional imprinting is expected to occur.

physics.plasm-ph↗

arXiv2025

Multi-Machine Scaling Laws for Fuel and Impurity Puffing Rates Sufficient for Detachment Access: a Systematic Review of Magnetic Confinement Fusion Devices

An open-source database of 457 experimental and numerical entries representing 32 machines$-$including tokamaks, stellarators, and linear plasma devices$-$is assembled. From this dataset, we derive multi-machine scaling laws that predict the fuel and impurity puffing rates sufficient to edge plasma detachment$-$the leading reactor-relevant solution to the challenge of plasma-wall interaction. Validation against up to 40 L- and H-mode plasmas shows agreement within a factor of 1.5 in about 50\% of cases, and within a factor of 2 on average. Divertor volume alone is found to strongly correlate with the fuelling rate. Inclusion of plasma opaqueness leads to $\Gamma_{\text{D}} \propto [n_{\text{sep}}\, a\, (S_{\text{div}}/V_{\text{div}})^{-1.5}]^{1.05}$, valid across all toroidal devices. Its H-mode simplification, $\Gamma_{\text{D}}^{\text{HDL}} \propto 0.43\, a^{1.58}\, \lambda_q^{-0.89}$, avoids explicit dependence on $n_{\text{sep}}$ and carries intrinsic physical meaning through the H/L density limit and the power fall-off length. The impurity seeding rate is captured by a general non-linear law, from which the Greenwald-Eich-Scarabosio simplification, $\Gamma_{\text{Z}}^{\text{GES}} \propto a^{1.51}\, \lambda_q^{-0.27}$, is obtained. Similar relationships are defined for stellarators, consistent with tokamak trends but still awaiting validation$-$an opportunity for further study. These results have immediate relevance for reactor fuel-cycle design and edge plasma modelling. More broadly, they demonstrate that physics-based 0D laws can reliably link detachment access to engineering actuators, offering practical tools for reactor design. Our laws represent macroscopic patterns across machines rather than microscopic variations within an individual device$-$providing the basis for our forthcoming studies aimed at extending this framework to machine-specific behaviour.

physics.plasm-ph↗