Feasibility of Negative Triangularity Equilibria in the SPARC Tokamak

TL;DR

This study assesses the feasibility of negative triangularity plasma configurations in the SPARC tokamak, demonstrating potential operational parameters and stability, and highlighting design trade-offs compared to positive triangularity configurations.

Contribution

The paper systematically explores negative triangularity equilibria in SPARC, showing their feasibility and implications for device design and operation, which was previously unexamined in this context.

Findings

NT plasmas achievable at 8 T with 2.1 MA current

NT operation requires 43% volume reduction

All equilibria meet MHD stability criteria

Abstract

We investigate the feasibility of negative triangularity (NT) plasma configurations in the SPARC tokamak, a compact, high-field device (up to 12.2 T) designed for positive triangularity (PT) operation. Using the FreeGS free-boundary equilibrium solver, we systematically explore the parameter space through 600+ equilibrium calculations spanning triangularity (-0.7-0.7) and elongation (1.0-2.1) while respecting SPARC's coil current limits and first-wall constraints and selecting a reduced-field 8 T operating point. We find that moderate NT double-null plasmas (delta = -0.35, kappa = 1.68) are achievable at 8 T with plasma currents of 2.1 MA. However, NT operation requires 43% plasma volume reduction (from 20.0 to 11.4 m$^3$) compared to the baseline PT design due to vessel walls optimized for PT conformity, and connection lengths are reduced by 40% due to geometric mismatch with the PT-optimized divertor. Central solenoid current requirements decrease by 54% in NT configurations, though specific shaping coils (PF3) require a factor of 5.5 higher currents. All equilibria satisfy fundamental MHD stability criteria with comfortable margins. Thus, while performance is degraded relative to baseline design, these results demonstrate SPARC's potential as a high-field experimental bridge between contemporary NT experiments and proposed NT reactor concepts, capable of testing whether NT's operational advantages (ELM-free operation, favorable impurity transport) persist under reactor-relevant conditions provided by SPARC.