G. W. Hammett

Full-f global gyrokinetic simulations of the plasma boundary have until now required heroic computational efforts and case-by-case expert intervention, precluding systematic parameter scans. Here we demonstrate a paradigm shift: hundreds of independent, concurrent, and unsupervised full-f boundary gyrokinetic simulations in a geometry inspired by the Tokamak à Configuration Variable (TCV), covering both the closed flux surface region and the open-field-line scrape-off layer (SOL) while scanning triangularity, elongation, and heating power. All simulations are evolved much longer than the turbulence relaxation time until the steady state is reached. Analysis of the steady-state profiles reveals that the impact of plasma shaping on confinement is strongly power dependent: at low power, triangularity primarily controls the SOL ion temperature, while at high power it mostly affects the edge ion temperature gradient. The low-power hot SOL observed for positive triangularity is explained by a neoclassical trapped-ion mechanism in which triangularity modifies the field-line arc length between banana turning points and the high-field-side limiter, altering the interaction with cold neutral-ionization regions. Fingerprint analysis of turbulent transport categorize the simulations in a regime dominated by ion temperature gradient (ITG) or trapped electron modes (TEMs), confirmed by dedicated local linear gyrokinetic calculations. The generated open data represents a previously unobtainable resource. It can serve both as a benchmark for boundary transport models, and as a training dataset for data-driven methods in fusion foundation and surrogate models.

Typically, fluid simulations are used for tokamak divertor design. However, fluid models are only valid if the SOL is highly collisional, an assumption that is valid in many present day experiments but is questionable in the high-power scenarios envisioned for burning plasmas and fusion pilot plants. This paper reports on comparisons between fluid and kinetic simulations of the scrape off layer (SOL) for parameters and geometry representative of the Spherical Tokamak for Energy Production (STEP) fusion pilot plant. The SOLPS-ITER (fluid) and Gkeyll (gyrokinetic) codes are operated in a two-dimensional (2D) axisymmetric mode, which replaces turbulence with ad-hoc diffusivities. In kinetic simulations, we observe that the ions in the upstream SOL experience significant mirror trapping. This substantially increases the upstream temperature and has important implications for impurity dynamics. We show that the mirror force, which is excluded in SOLPS's form of fluid equations, enhances the electrostatic potential drop along the field line in the SOL. We also show that the assumption of equal main ion and impurity temperatures, which is made in commonly used fluid codes, is invalid. The combination of these effects results in superior confinement of impurities to the divertor region in kinetic simulations, consistent with our earlier predictions. This effect can be dramatic, reducing the midplane impurity density by orders of magnitude. These results indicate that in reactor-like regimes the tolerable downstream impurity densities may be higher than would be predicted by fluid simulations, allowing for higher radiated power while avoiding unacceptable core contamination. Our results highlight the importance of kinetic simulations for divertor design and optimization for fusion pilot plants.

The gyrokinetic turbulence code GS2 was used to investigate the effects of plasma beta on linear, collisionless ion temperature gradient (ITG) modes and trapped electron modes (TEM) in National Compact Stellarator Experiment (NCSX) geometry. Plasma beta affects stability in two ways: through the equilibrium and through magnetic fluctuations. The first was studied here by comparing ITG and TEM stability in two NCSX equilibria of differing beta values, revealing that the high beta equilibrium was marginally more stable than the low beta equilibrium in the adiabatic-electron ITG mode case. However, the high beta case had a lower kinetic-electron ITG mode critical gradient. Electrostatic and electromagnetic ITG and TEM mode growth rate dependencies on temperature gradient and density gradient were qualitatively similar. The second beta effect is demonstrated via electromagnetic ITG growth rates' dependency on GS2's beta input parameter. A linear benchmark with gyrokinetic codes GENE and GKV-X is also presented.