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arxiv: 2605.27549 · v1 · pith:DFKKAFNAnew · submitted 2026-05-26 · ⚛️ physics.plasm-ph

Compact Experimental Negative TriAngUlarity Reactor (CENTAUR): A design study for a compact, affordable breakeven tokamak

Pith reviewed 2026-06-29 15:00 UTC · model grok-4.3

classification ⚛️ physics.plasm-ph
keywords tokamaknegative triangularitybreakevenfusion reactorhigh-temperature superconductorREBCOplasma stabilityreactor design
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0 comments X

The pith

A compact negative-triangularity tokamak design reaches 40 MW fusion power and scientific breakeven at $1.6 billion overnight cost.

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

The paper presents a full engineering design study for CENTAUR, a small high-field tokamak that uses negative triangularity to achieve predicted fusion power of 40 MW and scientific energy gain of 1.3. Ballooning stability calculations place the pedestal in the first regime, consistent with ELM-free operation, while edge transport simulations show 13.5% radiated power that keeps heat loads below material limits. An 18-coil REBCO magnet system at 10.9 T is protected by a 12 cm B4C shield that limits heating below quench limits and supports magnet lifetime beyond 3000 pulses. Iteration between technical choices and a custom costing model produces a total overnight cost of $1.6B ±0.2B, meeting the $2B target. A sympathetic reader would care because the work supplies a concrete, cost-constrained blueprint for a breakeven-scale experiment that avoids the scale of conventional tokamak projects.

Core claim

The CENTAUR design achieves a predicted total fusion power of 40MW and scientific energy gain of 1.3, with ballooning stability in the first regime, 13.5% radiated power fraction, heat loads below limits, magnet survival over design lifetime, and overnight cost of $1.6B±0.2B using a custom costing model.

What carries the argument

Negative triangularity geometry in the divertor region, which enables high radiated power fraction and first-regime ballooning stability while supporting compact high-field operation with REBCO coils.

If this is right

  • Ballooning stability in the first regime supports expected ELM-free operation of negative-triangularity plasmas.
  • The 13.5% radiated power fraction between separatrix and walls keeps heat loads on plasma-facing components below material limits.
  • The 12 cm B4C shield keeps superconducting magnet heating below the 33 K quench limit and allows survival more than ten times the 3000-pulse design lifetime.
  • The custom costing model iterated with the technical design yields an overnight cost of $1.6B ±0.2B that meets the $2B goal.

Where Pith is reading between the lines

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

  • If the modeled negative-triangularity radiation and stability hold at scale, the same geometry could be used to shrink the size of higher-power devices.
  • Early coupling of costing models to plasma and engineering choices may allow other tokamak concepts to stay within similar budget envelopes.
  • Experimental tests of the NT divertor radiation fraction on existing machines would provide direct data to refine the heat-load predictions.
  • Success at the modeled parameters would demonstrate that high-field REBCO magnets plus negative triangularity can lower the capital threshold for breakeven experiments.

Load-bearing premise

The ballooning stability calculations, edge heat transport simulations, neutronics analysis for the 12 cm B4C shield, and custom economic costing model accurately represent real-device behavior for this negative-triangularity geometry without unmodeled losses or cost overruns.

What would settle it

Construction and 10-second 40 MW DT operation of the device to measure whether actual fusion power reaches 40 MW with energy gain 1.3, heat loads stay below limits, magnets avoid quench, and total overnight costs remain within the modeled $1.6B ±0.2B range.

Figures

Figures reproduced from arXiv: 2605.27549 by Abdullah Hyder, Alexandra Lachmann, Andreas Holm, Andrew O. Nelson, Anson Braun, Avigdor Veksler, Carlos Paz-Soldan, C. F. B. Zimmermann, Christopher J. Hansen, Daniel A. Burgess, Eliot Felske, Evan Bursch, Filippo Scotti, Frederick Sheehan, Haley Wilson, Hiro J. Farre-Kaga, Ian G. Stewart, Jacob Halpern, Jamie L. Xia, Javier Chiriboga, John Labbate, Kalen Richardson, Kian Orr, Matthew Pharr, Matthew Tobin, Melanie Russo, Mohammed Haque, Nathaniel Chen, Nils Leuthold, Orso Meneghini, Rohan Lopez, Samuel W. Freiberger, Shreyas Seethalla, Sophia Guizzo, The CENTAUR Collaboration, Tim Slendebroek.

Figure 1
Figure 1. Figure 1: Full CAD render of the CENTAUR design. The volume in pink is [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Poloidal cross section showing the primary components and the equi [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: POPCON visualization of parameter space surrounding CENTAUR’s [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Operating point indicated by stars at βN = 1.5, fGW = 0.61, and q95 = 2.58. Operational space data from supplementary data supplied with the work by Paz-Soldan et. al. [24]. Stars indicate where the CENTAUR operat￾ing plot falls within operating space of strongly shaped, negative triangularity plasmas on DIII-D. DIII-D Average Experimental NT Profile Normalized minor radius 0.92 (edge) 1 (separatrix) MHD s… view at source ↗
Figure 5
Figure 5. Figure 5: Integrated modeling flowchart showing the codes used over various [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: The normalized pressure gradient (blue curve) occupies the 1st stabil [PITH_FULL_IMAGE:figures/full_fig_p009_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Selected ASTRA profiles as a function of [PITH_FULL_IMAGE:figures/full_fig_p011_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: (a) Vertical control coil voltage needed to stabilize plasma for a given initial perturbation scale. (b) Vertical position of the plasma over time in the case of a 14.5% perturbation. (c) Control coil voltage and current over time in the case of a 14.5% perturbation. 12 [PITH_FULL_IMAGE:figures/full_fig_p012_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: The UEDGE Computational mesh, configured to be from normalized [PITH_FULL_IMAGE:figures/full_fig_p014_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Imposed electron heat, ion heat, and particle cross-field diffusivity [PITH_FULL_IMAGE:figures/full_fig_p015_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Electron heat radial falloff outside the LCFS at the midplane (left). [PITH_FULL_IMAGE:figures/full_fig_p018_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: Inner (left column) and outer (right column) divertor plate per [PITH_FULL_IMAGE:figures/full_fig_p019_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: Peak perpendicular heat flux (red) and full-width-half-max (FWHM) [PITH_FULL_IMAGE:figures/full_fig_p020_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: Power radiated in the computational domain calculated by the [PITH_FULL_IMAGE:figures/full_fig_p021_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: Simplified representation of the region between the divertor plate [PITH_FULL_IMAGE:figures/full_fig_p021_15.png] view at source ↗
Figure 16
Figure 16. Figure 16: (a) Solenoid flux swing as a function of solenoid inner radius and [PITH_FULL_IMAGE:figures/full_fig_p024_16.png] view at source ↗
Figure 17
Figure 17. Figure 17: Cross section of CENTAUR with PF coils locations where blue, [PITH_FULL_IMAGE:figures/full_fig_p026_17.png] view at source ↗
Figure 18
Figure 18. Figure 18: Magnet and structural build of one toroidal section. The CS resin [PITH_FULL_IMAGE:figures/full_fig_p027_18.png] view at source ↗
Figure 19
Figure 19. Figure 19: Von Mises stress on the toroidal field coils, central solenoid,and [PITH_FULL_IMAGE:figures/full_fig_p028_19.png] view at source ↗
Figure 20
Figure 20. Figure 20: Current quench stresses on the vacuum vessel from VDE and ver [PITH_FULL_IMAGE:figures/full_fig_p029_20.png] view at source ↗
Figure 21
Figure 21. Figure 21: CENTAUR radial build compared to SPARC[78] at the midplane of [PITH_FULL_IMAGE:figures/full_fig_p031_21.png] view at source ↗
Figure 22
Figure 22. Figure 22: Representative OpenMC Monte Carlo neutron heating simulation [PITH_FULL_IMAGE:figures/full_fig_p032_22.png] view at source ↗
Figure 23
Figure 23. Figure 23: HTS magnet neutron heating over a 10-second shot. The star rep [PITH_FULL_IMAGE:figures/full_fig_p033_23.png] view at source ↗
Figure 24
Figure 24. Figure 24: Total cost breakdown of CENTAUR by system. [PITH_FULL_IMAGE:figures/full_fig_p034_24.png] view at source ↗
read the original abstract

This work presents the compact experimental negative triangularity reactor (CENTAUR), a low overnight cost, high-field tokamak, breakeven reactor design, achieving a predicted total fusion power of 40MW and scientific energy gain of 1.3. Ballooning stability calculations confirm that the device's pedestal is within the first stability regime, which is consistent with the expected ELM-free operation associated with negative triangularity (NT) plasmas. The geometry of the NT divertor allows for high fraction of radiated power (13.5$\%$) between the separatrix and plasma facing components. Heat transport modeling based on simulations of the edge region show heat loads into plasma facing components well below material limits. The magnet system employs rare-earth barium copper oxide (REBCO) high-temperature superconductors in 18 toroidal field coils, an hourglass-shaped central solenoid, and six poloidal field coils to support high-field ($B_0=10.9$ T) plasma confinement, shaping, and current drive. Neutronics analysis shows that a 12 cm $B_4C$ shield keeps superconducting magnet heating below the 33~K quench limit during 10 s, 40 MW DT pulses. With this shielding, the modeled fluence indicates HTS components can survive more than ten times the 3000-pulse design lifetime. Iteration of economic analysis in tandem with the technical design process allows CENTAUR to achieve its overnight cost goal of $\$$2B determined using a custom costing model that predicts a total overnight cost of $1.6$B$\pm0.2$B.

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

2 major / 1 minor

Summary. The manuscript presents a design study for the CENTAUR compact high-field negative-triangularity tokamak, claiming 40 MW total fusion power and scientific Q=1.3 at B0=10.9 T using REBCO magnets. It reports first-regime ballooning stability consistent with ELM-free NT operation, 13.5% radiated power fraction, edge heat loads below material limits from transport simulations, magnet survival over >10x the 3000-pulse lifetime with a 12 cm B4C shield, and an overnight cost of $1.6B±0.2B obtained via a custom economic model iterated with the technical design to meet a $2B target.

Significance. If the simulation chain and costing model prove accurate for this NT geometry, the work would demonstrate a viable low-cost pathway to breakeven using high-field HTS and NT for ELM-free operation, advancing compact reactor concepts. The integration of stability, transport, neutronics, and economic modeling is a positive feature of the study.

major comments (2)
  1. [Abstract] Abstract: the reported overnight cost of $1.6B±0.2B is obtained by iterating the custom costing model in tandem with the technical design to achieve the $2B goal; this process makes the cost prediction dependent on the model's internal parameters and iteration choices rather than an independent forecast anchored to external device benchmarks or scaling laws for NT at B0=10.9 T.
  2. [Stability/Transport/Neutronics sections] Ballooning stability, edge transport, and neutronics sections: the central performance claims (first-regime stability, 13.5% radiation, heat loads, and shield performance for 10 s 40 MW pulses) rest on a chain of simulations whose validation details, sensitivity to free parameters (B0, shield thickness), and applicability to this specific NT geometry at high field are not shown; without these, the predictions remain untested against real-device behavior.
minor comments (1)
  1. [Title] The stylized acronym 'TriAngUlarity' in the title is unconventional; consider standardizing to 'triangularity' for clarity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the detailed and constructive review. The comments highlight important aspects of how the cost model and simulation chain are presented. We address each major comment below and have revised the manuscript to improve clarity and transparency without altering the core technical claims.

read point-by-point responses
  1. Referee: [Abstract] Abstract: the reported overnight cost of $1.6B±0.2B is obtained by iterating the custom costing model in tandem with the technical design to achieve the $2B goal; this process makes the cost prediction dependent on the model's internal parameters and iteration choices rather than an independent forecast anchored to external device benchmarks or scaling laws for NT at B0=10.9 T.

    Authors: We agree that the reported cost results from an iterative process in which the technical parameters were adjusted in tandem with the economic model to meet the $2B target. This is standard practice in conceptual reactor design studies. The underlying costing model draws on component-level estimates and scaling relations from the literature, but the final figure is indeed goal-constrained. We have revised the abstract and the economic analysis section to state explicitly that the $1.6B±0.2B value is the outcome of a target-driven iteration rather than an unconstrained prediction. revision: yes

  2. Referee: [Stability/Transport/Neutronics sections] Ballooning stability, edge transport, and neutronics sections: the central performance claims (first-regime stability, 13.5% radiation, heat loads, and shield performance for 10 s 40 MW pulses) rest on a chain of simulations whose validation details, sensitivity to free parameters (B0, shield thickness), and applicability to this specific NT geometry at high field are not shown; without these, the predictions remain untested against real-device behavior.

    Authors: The stability, transport, and neutronics results were obtained with established codes whose validation on conventional tokamaks is documented in the literature. The first-regime ballooning stability for NT is consistent with published experimental observations on devices such as TCV. Nevertheless, the manuscript does not include explicit sensitivity scans with respect to B0 or shield thickness, nor a dedicated discussion of code applicability limits for high-field NT. We have added a new subsection summarizing the validation basis for each code, reporting sensitivity to the key free parameters, and noting the extrapolation uncertainties for the CENTAUR parameter regime. revision: yes

Circularity Check

1 steps flagged

Cost 'prediction' obtained by iterating custom model to meet $2B target; physics chain uses unanchored NT-specific simulations

specific steps
  1. fitted input called prediction [Abstract]
    "Iteration of economic analysis in tandem with the technical design process allows CENTAUR to achieve its overnight cost goal of $2B determined using a custom costing model that predicts a total overnight cost of $1.6B±0.2B."

    The model and design parameters are adjusted together until the $2B target is met; the reported $1.6B figure is therefore the output of that fitting loop rather than an independent forecast from external benchmarks or first-principles costing.

full rationale

The sole load-bearing circular step is the economic costing model, which is explicitly iterated in tandem with the design to achieve the $2B goal and then presented as predicting $1.6B. This matches the fitted-input-called-prediction pattern. Ballooning stability, edge transport, and neutronics results are simulation outputs rather than reductions to their own inputs by construction; no self-citation chains or ansatzes imported from prior author work are quoted in the provided text. The derivation is therefore partially circular but not fully forced by definition.

Axiom & Free-Parameter Ledger

3 free parameters · 2 axioms · 0 invented entities

The design depends on domain assumptions about plasma stability and transport models plus a custom economic model with adjustable parameters tuned to the target cost; no new entities are postulated.

free parameters (3)
  • B0 on-axis field = 10.9 T
    Set at 10.9 T to enable compact high-performance operation
  • Custom costing model parameters
    Adjusted iteratively with design choices to reach the $2B target
  • Shield thickness = 12 cm
    Chosen as 12 cm B4C to keep magnet heating below quench limit
axioms (2)
  • domain assumption Ballooning stability calculations confirm first-regime operation consistent with ELM-free NT plasmas
    Invoked to support expected stable pedestal behavior
  • domain assumption Edge heat transport simulations accurately predict heat loads to PFCs
    Used to claim compliance with material limits

pith-pipeline@v0.9.1-grok · 5990 in / 1479 out tokens · 55743 ms · 2026-06-29T15:00:55.293741+00:00 · methodology

discussion (0)

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Forward citations

Cited by 1 Pith paper

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