Gyrokinetic Simulations of a Low Recycling Scrape-off Layer without a Lithium Target

TL;DR

The paper addresses the feasibility of a low-recycling Scrape-Off Layer (SOL) in a tokamak by combining gyrokinetic simulations with neutrals (Gkeyll-EIRENE) and contrasting them with fluid SOLPS results in STEP-like geometry. It shows that high edge temperature $T_e$ and low edge density $n_e$ can be achieved without relying on low-recycling target materials by coating side walls with low-recycling material, while targeting robust, high-heat-capacity divertor targets. Kinetic effects improve impurity confinement to the divertor and broaden the heat-flux width via mirror trapping and ion banana orbits, reducing peak heat loads and potentially mitigating core contamination. The findings support wall-material strategies and kinetic-edge modeling as pathways toward feasible low-recycling SOL scenarios, with liquid-metal divertor concepts and helium exhaust remaining important avenues for future work.

Abstract

Low-recycling regimes are appealing because they entail a high edge temperature and low edge density which are good for core confinement. However, due to considerably enhanced heat flux, the exhaust problems become severe. In addition, in the low-recycling regime, the conventional fluid simulations may not capture the physics of the Scrape-Off Layer (SOL) plasma that lies in the long mean free path regime; kinetic calculations become necessary. In this paper, by performing both Kinetic and fluid simulations, we explore the feasibility of a low-recycling regime in the magnetic geometry of the Spherical Tokamak for Energy Production (STEP); kinetic effects come out to be crucial determinants of the SOL dynamics. The simulation results indicate that a high SOL temperature and low SOL density could be achieved even when the divertor target is not made of a low recycling material. This can be done by using a low recycling material as a wall material. This is an important step towards demonstrating the feasibility of a low-recycling scenario. Lithium, a commonly used low recycling material, tends to evaporate at high heat fluxes which counteracts the desired high temperature, low density regime, and materials that can handle high heat fluxes are generally high recycling. Comparisons of gyrokinetic and fluid simulation results indicate that one can take advantage of kinetic effects to address some of the issues associated with a low-recycling SOL. Specifically, kinetic simulations show better confinement of impurities to the divertor region and greater broadening of the heat flux width due to drifts when compared with fluid simulations. Impurity confinement would help prevent core contamination from sputtering, and a broader heat flux width would help reduce the peak heat load at the target in the absence of detachment.

Figures (6)

• Figure 1: Simulation domain used for the coupled Gkeyll-EIRENE simulation. The colored grid lines show the Gkeyll simulation domain and the thick green boundary enclosing the Gkeyll domain indicates the machine boundary which is the EIRENE simulation domain.
• Figure 2: Simulation results from the coupled Gkeyll-EIRENE simulation of STEP in a low redycling regime. The electron density, neutral deuterium density, electron temperature, and deuterium ion temperature are plotted in the poloidal plane. The upstream plasma temperature remains at $\sim$9 keV and the downstream density remains low at 1.5$\times$10$^{16}$$m^{-3}$.
• Figure 3: Electron temperature, ion temperature, and total heat flux at the lower inboard and upper outboard divertor plates.
• Figure 4: Total charged argon density (summed over charge states) plotted along separatrix in the outboard SOL in SOLPS and Gkeyll. The upstream impurity density in Gkeyll is a factor of 100 lower than in SOLPS Shukla25.
• Figure 5: Ion distribution at the OMP. The red line indicates the trapped-passing boundary including the effect of the potential and the green line is a contour of a Maxwellian with the same temperature as the distribution function Shukla25.