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arxiv: 2606.26362 · v1 · pith:AOZRM5TNnew · submitted 2026-06-24 · ⚛️ physics.plasm-ph

Integrating Gyrokinetic Flux Predictions with Ideal MHD Stability Boundaries

J. McClenaghan , K. E. Thome , T. F. Neiser , J. Candy , F. D. Halpern , T. H. Osborne , S. Saarelma , the MAST-U team This is my paper

Pith reviewed 2026-06-26 00:39 UTC · model grok-4.3

classification ⚛️ physics.plasm-ph
keywords pedestal stabilitygyrokinetic transportMHD stabilitykinetic ballooning modetokamakSPARCELITECGYRO
0
0 comments X

The pith

The SPARC H-mode pedestal operates in an intermediate regime bounded by local KBM second stability on one side and global finite-n ballooning instability on the other.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper constructs an integrated workflow that scans equilibria, applies MHD stability codes ELITE and GATO, and computes gyrokinetic fluxes with CGYRO and QLGYRO to predict pedestal height and width. This workflow recovers the first- and second-stability regions of the kinetic ballooning mode previously seen in local gyrokinetic studies. When applied to spherical tokamaks the results match earlier work once low-n peeling modes are retained. For SPARC-like high-field parameters the same workflow shows that KBM and microtearing heat fluxes rise sharply with toroidal field, narrowing the path to second stability. Direct comparison with global ELITE finite-n calculations places the operating point between the local KBM second-stability boundary and the global ballooning limit, recovering the EPED picture once global effects are restored.

Core claim

The integrated pedestal-stability workflow reproduces the characteristic KBM first- and second-stability structure. In spherical tokamaks the workflow agrees with prior studies when low-n peeling stability is retained. For SPARC-like high toroidal field plasmas, KBM and MTM heat fluxes increase strongly with toroidal field, limiting access to the KBM second-stability region. Comparison with global ELITE finite-n analysis indicates that the H-mode pedestal lies in an intermediate regime bounded by local KBM second stability and global finite-n ballooning instability, consistent with the EPED model once global effects are included.

What carries the argument

Integrated workflow of equilibrium scans, ELITE/GATO MHD stability analysis, and CGYRO/QLGYRO gyrokinetic transport predictions that maps local KBM stability boundaries onto global finite-n constraints.

If this is right

  • Low-n peeling modes must be resolved to obtain correct pedestal stability limits in spherical tokamaks.
  • KBM and MTM heat fluxes rise rapidly with increasing toroidal field, making second-stability access harder in high-field devices.
  • The pedestal operating point in SPARC parameters is set by competition between local KBM second stability and global finite-n ballooning.
  • The EPED model remains consistent once global finite-n effects are restored to the local gyrokinetic picture.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • High-field reactor designs may need additional control techniques or profile shaping to reach the second-stability region.
  • The intermediate regime identified here could be tested by varying toroidal field in existing or near-term devices while holding other parameters fixed.
  • Global electromagnetic effects omitted from the local gyrokinetic step might further narrow or widen the allowed pedestal window.

Load-bearing premise

Local gyrokinetic flux predictions remain accurate when extrapolated to SPARC parameters without global electromagnetic or finite-n corrections that could shift the KBM second-stability boundary.

What would settle it

Measurement of the actual pedestal height and width in a high-field device such as SPARC, compared against the workflow's predicted intermediate-regime boundary, would confirm or refute the placement between local KBM second stability and global ballooning limits.

Figures

Figures reproduced from arXiv: 2606.26362 by F. D. Halpern, J. Candy, J. McClenaghan, K. E. Thome, S. Saarelma, T. F. Neiser, the MAST-U team, T. H. Osborne.

Figure 1
Figure 1. Figure 1: FIG. 1. Scanned pedestal values are plotted on typical VARYPED grid of [PITH_FULL_IMAGE:figures/full_fig_p006_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. QLGYRO predicted fluxes in [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Ratio of CGYRO growth rates [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Maximum growth rate [PITH_FULL_IMAGE:figures/full_fig_p009_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. GATO maximum growth rates [PITH_FULL_IMAGE:figures/full_fig_p010_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. GATO maximum growth rate [PITH_FULL_IMAGE:figures/full_fig_p011_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. QLGYRO predicted fluxes combined with MHD threshold [PITH_FULL_IMAGE:figures/full_fig_p011_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. QLGYRO predicted fluxes combined with MHD threshold ( [PITH_FULL_IMAGE:figures/full_fig_p013_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9. QLGYRO predicted fluxes combined with MHD threshold ( [PITH_FULL_IMAGE:figures/full_fig_p013_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10. QLGYRO predicted fluxes scanning [PITH_FULL_IMAGE:figures/full_fig_p015_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: FIG. 11. QLGYRO predicted heat flux versus [PITH_FULL_IMAGE:figures/full_fig_p016_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: FIG. 12. CGYRO predicted growth rate of the 2 T ∆ [PITH_FULL_IMAGE:figures/full_fig_p017_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: FIG. 13. QLGYRO predicted heat flux versus [PITH_FULL_IMAGE:figures/full_fig_p018_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: FIG. 14. ELITE growth rate [PITH_FULL_IMAGE:figures/full_fig_p019_14.png] view at source ↗
read the original abstract

Accurate prediction of pedestal height and width in tokamaks remains a critical issue as it strongly influences the predicted plasma performance of all future reactors. We present an integrated pedestal-stability workflow that combines equilibrium scans with magnetohydrodynamic (MHD) stability analysis using ELITE and GATO and gyrokinetic transport predictions using CGYRO/QLGYRO. The workflow reproduces the characteristic KBM first- and second-stability structure previously identified in gyrokinetic pedestal studies. Applied to spherical tokamaks (STs), the workflow shows good agreement with past studies when low-n peeling stability is included, emphasizing the importance of resolving low-n physics in ST pedestals. Extending the analysis to SPARC-like, high toroidal field plasmas, kinetic ballooning mode (KBM) and microtearing mode (MTM) heat fluxes are found to increase strongly with toroidal field, suggesting that access to the KBM second-stability region may become significantly more difficult in high toroidal field devices. Comparison with global ELITE finite-n analysis at SPARC parameters suggests that the H-mode pedestal lies in an intermediate regime bounded by local KBM second stability on one side and global finite-n ballooning instability on the other, consistent with the EPED picture once global effects are included

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 0 minor

Summary. The paper presents an integrated pedestal-stability workflow that combines equilibrium scans with MHD stability analysis using ELITE and GATO and gyrokinetic transport predictions using CGYRO/QLGYRO. The workflow reproduces the characteristic KBM first- and second-stability structure previously identified in gyrokinetic pedestal studies. Applied to spherical tokamaks, it shows good agreement with past studies when low-n peeling stability is included. Extending to SPARC-like high toroidal field plasmas, KBM and MTM heat fluxes increase strongly with toroidal field. Comparison with global ELITE finite-n analysis suggests the H-mode pedestal lies in an intermediate regime bounded by local KBM second stability on one side and global finite-n ballooning instability on the other, consistent with the EPED picture once global effects are included.

Significance. If the central claims hold, the workflow provides a practical method for combining local gyrokinetic flux predictions with global ideal MHD stability boundaries. This could help assess pedestal access in high-field devices such as SPARC by quantifying the increase in KBM/MTM fluxes with toroidal field and the resulting difficulty in reaching the second-stability region. The reproduction of known KBM structures and agreement with prior ST studies are noted strengths.

major comments (1)
  1. [Abstract] Abstract (SPARC extrapolation): The claim that the pedestal occupies an intermediate regime bounded by the local KBM second-stability boundary rests on the assumption that local CGYRO/QLGYRO predictions remain quantitatively valid at SPARC parameters. No quantitative error bars, data exclusion criteria, or assessment of how global electromagnetic or finite-n effects might shift the KBM threshold are provided; this assumption is load-bearing for the consistency with the EPED picture.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive review and recognition of the workflow's strengths. We address the single major comment below.

read point-by-point responses

  1. Referee: [Abstract] Abstract (SPARC extrapolation): The claim that the pedestal occupies an intermediate regime bounded by the local KBM second-stability boundary rests on the assumption that local CGYRO/QLGYRO predictions remain quantitatively valid at SPARC parameters. No quantitative error bars, data exclusion criteria, or assessment of how global electromagnetic or finite-n effects might shift the KBM threshold are provided; this assumption is load-bearing for the consistency with the EPED picture.

    Authors: The manuscript presents an integrated workflow that deliberately combines local gyrokinetic flux predictions with global ideal MHD boundaries, as described in the introduction and methods. We agree that the local approximation carries inherent uncertainties at SPARC parameters and that quantitative error bars or explicit global-effect assessments are not provided. We will revise the abstract and add a paragraph in the discussion section to explicitly state this limitation, reference relevant literature on global electromagnetic effects in pedestals, and note that the EPED consistency is presented as suggestive within the local-global hybrid framework. This addresses the concern without requiring new simulations. revision: partial

Circularity Check

0 steps flagged

No significant circularity detected in the integrated workflow

full rationale

The paper presents a workflow that combines outputs from independent established codes (CGYRO/QLGYRO for local gyrokinetic fluxes and ELITE/GATO for MHD stability) to reproduce known KBM first- and second-stability features and compare local vs. global boundaries at SPARC parameters. The conclusion that the pedestal occupies an intermediate regime consistent with the EPED picture is an interpretive result of this cross-code comparison, not a quantity forced by definition, parameter fitting, or self-citation reduction. No load-bearing step equates a prediction to its own input by construction, and the derivation remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Only abstract available; no explicit free parameters, axioms, or invented entities can be extracted.

pith-pipeline@v0.9.1-grok · 5791 in / 992 out tokens · 17160 ms · 2026-06-26T00:39:23.621916+00:00 · methodology

discussion (0)

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Reference graph

Works this paper leans on

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