Impurity peaking of SPARC H-modes: a sensitivity study on physics and engineering assumptions

TL;DR

This work addresses impurity accumulation and fusion performance in SPARC H-modes under pedestal-edge transport uncertainties using an integrated model that couples ASTRA+STRAHL (with FACIT and TGLF-SAT2) to predict core transport and impurity dynamics, aided by a neural-network pedestal predictor trained on EPED. The study conducts extensive benchmarks across three H-mode scenarios and sensitivity scans for Ar/W pedestal concentrations, rotation, and DT fuel mix, finding turbulence-dominated impurity transport with robust fusion performance even when boundary conditions vary. A key finding is that the maximum fusion power occurs at a 55–45% DT mix due to isotope effects on ion temperature and impurity transport, while Ar and W influence Z_eff and radiative losses to nontrivial but compensating degrees. Overall, the results suggest low tungsten accumulation in SPARC’s low-collisionality regime and validate fixed-concentration approaches for large-scale impurity databases, with important implications for impurity seeding strategies and DT fueling in next-generation devices.

Abstract

In this paper, an overview of the impurity transport for three H-mode plasmas in the upcoming SPARC tokamak has been provided. The simulations have been performed within the ASTRA+STRAHL framework, using FACIT and TGLF-SAT2 to predict, respectively, neoclassical and turbulent core transport, while a neural network trained on EPED simulations has been employed to calculate the pedestal height and width self-consistently. A benchmark with previous simulations at constant impurity fraction has been provided for three H-modes, spanning different plasma current and magnetic field values. For a scenario, additional simulations have been performed to account for uncertainties in the modeling assumptions. The predictions are nearly insensitive to changes in the top of pedestal W concentrations. Varying the Ar pedestal concentration has shown a small effect on the impurity peaking and nearly constant fusion gain values, due to multiple effects on pedestal pressure, main ion dilution and density peaking. The inclusion of rotation in ASTRA simulations has shown minimal impact on confinement and impurity transport predictions. An exploratory study has been provided with a first set of simulations treating D and T separately, experiencing a maximum fusion power at 55-45% DT fuel composition, and an asymmetric distribution with respect to the D concentration. All the results, including sensitivity scans of toroidal velocity and ion temperature and density gradients, highlighted that turbulent impurity transport prevails on the neoclassical component, aligning with previous ITER predictions, and suggesting that next generation devices like SPARC, operating at low collisionality, will experience low W accumulation.

Figures (23)

• Figure 1: On the left and right are shown respectively fusion gain and $f_{LH}$ values for an ICRH power (top plots, green) and density (bottom plots, red) scan of the PRD scenario. The squares indicate simulations with impurity transport, while the crosses are simulated keeping fixed radial impurity concentrations.
• Figure 2: PRD profiles. Top left: turbulent (red) and neoclassical (blue) W diffusivity; top right: turbulent (red) and neoclassical (blue) W convection; bottom left: impurity concentrations; bottom right: $Z_{eff}$. In the bottom plots, the dashed lines indicate the simulations with fixed concentrations. The colored shades indicate the maximum and minimum values computed across the scans. The gray area is from top of pedestal to the separatrix.
• Figure 3: On the left and right are shown respectively fusion gain and $f_{LH}$ values for a density scan of the 8T H-mode scenario, with $P_{ICRH}=25MW$. The squares indicate simulations with impurity transport, while the crosses are simulated keeping fixed radial impurity concentrations.
• Figure 4: Integrated radial profiles of the power fluxes for a density scan of the SPARC H8 scenario, using STRAHL (blue) and fixed impurity concentrations (red). Top left: total ion power; top right: ohmic power; center left: collisional exchange; center right: radiative power; bottom left: fusion power to electrons; bottom right: fusion power to ions. The solid/dashed lines indicate the nominal simulations, while the shaded area show the maximum and minimum values of the profiles across the density scan. The gray region indicates from top of pedestal to the separatrix.
• Figure 5: Temperature profiles at the lowest density of the SPARC H8 scenario, using STRAHL (solid) and fixed impurity concentrations (dashed). Red (blue) indicates the electron (ion) temperature. The gray region indicates from top of pedestal to the separatrix.