The pedestal and boundary follow-ups are four papers published between 2024 and 2026 that take up what 📝Jerry Hughes's 2020 SPARC pedestal chapter had to assume: what sets pedestal height, what sets its density, and what the divertor receives as a result.
That chapter projected a peeling-limited 📝pedestal for 📝SPARC using 📝Phil Snyder's EPED framework, which fixes pedestal height and width from peeling–📝ballooning mode stability and kinetic ballooning mode onset. J. F. Parisi and eighteen co-authors reopened the second constraint. With a gyrokinetic threshold model they predict a bifurcation in pedestal width–height scaling arising from the first and second stability properties of kinetic ballooning modes: a wide branch and a narrow one, whose accessibility depends strongly on shaping — 📝elongation and 📝triangularity — and on 📝aspect ratio. The wide branch admits high core pressure with pedestals free of 📝edge localized modes; at negative triangularity, low aspect ratio is predicted to give the steeper pedestal. Both branches have been attained in tokamak experiments. Which one SPARC lands on has not been measured (Physics of Plasmas 31(3), 2024, 10.1063/5.0190818; 📝Alex Creely and 📝Adam Kuang are the CFS co-authors on a paper otherwise led from Princeton).
M. A. Miller and eighteen co-authors supply the measured half, on 📝Alcator C-Mod. High-resolution 📝Thomson scattering profiles across the transition between ELMy 📝H-mode and EDA H-mode were fitted twice, once tuned to the pedestal and once to the 📝separatrix and near-SOL, then compared against main-chamber neutral measurements. Pedestal density responds to neutral sources in the ELMy regime and not in the EDA regime. The Saarelma–Connor density model holds for ELMy H-modes up to 2.0 × 10²⁰ per cubic metre; adding a transport channel driven by resistive ballooning modes extends agreement for EDA H-modes to 3.0 × 10²⁰. Alcator C-Mod no longer operates, so this is a 2026 result drawn from an archived database, and it constrains the 📝operational space SPARC is designed to enter (Nuclear Fusion 66(8) 086039, 10.1088/1741-4326/ae823f; 📝Thomas Eich and 📝Thomas Body are the CFS co-authors).
Downstream of the pedestal is the exhaust. Jeremy D. Lore and eight co-authors ran SOLPS-ITER across a range of 📝scrape-off layer heat-flux widths, input powers and fuelling locations. At an upstream separatrix density of 1 × 10²⁰ per cubic metre, the most conservative heat-flux-width extrapolation — about 0.15 millimetres — produces extremely high unmitigated particle and energy fluxes to the 📝divertor, both at full field with 29 megawatts crossing the separatrix and at two-thirds field with 10 megawatts. Raising cross-field diffusivity two- to tenfold shrinks the problem without removing it: 📝impurity seeding or 📝strike-point sweeping will likely still be needed. The solutions bifurcate sharply and show hysteresis, and neon seeding drops upstream density at fixed throughput, so holding a chosen divertor state requires main-ion and impurity levels controlled together. 📝Divertor detachment here is not a switch but a state with a narrow, controlled window (10.1088/1741-4326/ad85f3, with 📝Matt Reinke among the authors).
Hughes returns as first author in 2025, and the framing has inverted. "High confinement regimes on SPARC: operational conditions for access and avoidance" combines core 📝L-mode power-balance simulation — PORTALS flux-matching, nonlinear CGYRO, quasilinear TGLF — with the best empirical 📝L-H transition and L–I threshold scalings, including a metal-wall H-mode scaling and an I-mode projection from critical edge ion heat flux, both applied to SPARC for the first time. An H-mode pedestal remains the basis of the reference 📝Q ≈ 10 discharge at 12.2 📝tesla in a machine of 1.85-metre major radius and 0.57-metre minor radius. But early operation will seek to avoid H-mode formation and accept a lower Q, and the simulations find a substantial L-mode window because net power through the edge generally sits below projected thresholds. Access at full field in 📝deuterium–📝tritium plasmas still looks likely (10.1088/1741-4326/adc3a9).
In 2020 we asked whether SPARC could reach H-mode. Five years on we are publishing the conditions under which we will deliberately avoid it — and the divertor loads that make that a design choice rather than a preference.