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REVIEW 3 major objections 4 minor 39 references

Recovery of hydrogen plasma at the sub-nanosecond timescale in a plasma-wakefield accelerator

T0 review · 3 major / 4 minor · reviewed 2026-08-06 · deepseek-v4-flash

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Pith's one-line read This paper reports that a hydrogen plasma recovers in under a nanosecond after a pump electron beam when the beam density stays below the plasma density, whereas an overdense beam leaves a dense ion channel lasting more than 13 ns.

desk verdict Useful proof-of-principle with a credible sub-ns recovery bound, but the headline claim overreaches slightly because 0.7 ns is the shortest delay, not a resolved timescale. read the letter →

arxiv 2507.11219 v1 pith:WAPQQBCH submitted 2025-07-15 physics.acc-ph

R. Pompili , M.P. Anania , A. Biagioni , M. Carillo , E. Chiadroni , A. Cianchi , G. Costa , L. Crincoli
show 16 more authors
A. Del Dotto M. Del Giorno F. Demurtas M. Ferrario M. Galletti A. Giribono J.K. Jones V. Lollo T. Pacey G. Parise G. Di Pirro S. Romeo G.J. Silvi V. Shpakov F. Villa A. Zigler
This is my paper · ORCID
classification physics.acc-ph
keywords plasmawakefieldaccelerationrecoveryionmotionhydrogenpump-probeexperimenthighrepetitionratedischargecapillary
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

Plasma wakefield accelerators offer gigavolt-per-centimeter accelerating fields, but their repetition rate is limited by how fast the plasma returns to its unperturbed state after each beam passes. The authors report a pump-and-probe experiment in a hydrogen-filled capillary showing that when the beam density $n_b$ stays below the plasma density $n_p$ (ratio $\alpha=n_b/n_p<1$), the plasma ions recover in less than 0.7 ns. At the opposite regime ($\alpha>1$), the pump beam creates a denser plasma channel on axis that persists for at least 13 ns and visibly changes the energy of a delayed probe beam. The result is supported by a damped-oscillator model of ion motion and by simulations of the wakefield acting on the probe. If correct, the recovery time no longer limits hydrogen plasma accelerators to low repetition rates, provided the bunch density is kept below the plasma density.

What carries the argument

The central object is the ratio $\alpha = n_b/n_p$ together with a pump-and-probe configuration in which two electron bunches from the same photo-injector are sent through a 3 cm hydrogen-filled discharge capillary with a controllable delay $\Delta t$ from 0.7 to 13 ns. The probe bunch acts as a density gauge: its mean-energy change $\Delta E$ relative to the pump-off case is proportional to the plasma density it samples, so a vanishing $\Delta E$ signals that the plasma has reverted to its initial state. The argument is carried by a simplified model of ion motion that treats the blowout region as a damped oscillator for electrons and ions, with collisional friction and pressure forces, and produces the ion trajectories and density maps.

What would settle it

A decisive test would be to measure the probe energy change at delays below 0.7 ns (for example 0.1 to 0.5 ns) using finer time steps; if the energy difference stays at zero for all sub-nanosecond delays at $\alpha<1$, the recovery claim is supported, while a rise in $\Delta E$ as the delay approaches zero would reveal that recovery is not yet complete. An independent check would be a time-resolved shadowgraph or interferometric measurement of the on-axis ion density profile at $\Delta t=0.7$ ns to verify it matches the initial flat profile.

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Extended reading notes

Core claim

The central discovery is that the recovery time of a hydrogen plasma after a single ultra-short electron bunch has two distinct regimes controlled by the density ratio $\alpha = n_b/n_p$. For $\alpha < 1$ ($n_p \gtrsim 2\times 10^{15}\,\mathrm{cm}^{-3}$ in this experiment), the probe beam's average energy loss is the same whether or not the pump beam fired, and this holds at every delay from 0.7 ns to 13 ns; the authors conclude the ions have returned to their initial positions in under a nanosecond. For $\alpha > 1$ ($n_p \lesssim 10^{15}\,\mathrm{cm}^{-3}$), the pump leaves a dense on-axis ion channel whose local density rises within about 0.2 ns to roughly 20 times the background value and remains for at least 13 ns, producing a stronger deceleration of the probe. The channel formation is attributed to ion pinching by the beam's radial electric field, and the same model reproduces the slower recovery observed in argon and lithium plasmas.

Load-bearing premise

The claim of sub-nanosecond recovery rests on assuming that a zero change in the probe beam's average energy means the ions have truly returned to their starting positions, with no residual wakefield or beam-loading effect masking an incomplete recovery.

Editorial extensions

If this is right

  • If recovery is truly sub-nanosecond for $\alpha<1$, hydrogen plasma cells could in principle be reused at repetition rates above the megahertz level demonstrated with an argon discharge, since a 0.7 ns recovery corresponds to gigahertz-class bunch spacing.
  • To sustain high accelerating gradients while keeping $\alpha<1$, both the bunch density and the plasma density must be raised together, so high-repetition-rate operation is compatible with high fields only if the plasma source can support higher $n_p$.
  • For $\alpha>1$, the persistent dense channel acts as a strong focusing and decelerating structure for subsequent bunches for tens of nanoseconds, so beam-plasma density matching is a design constraint for multi-bunch or high-repetition-rate operation.
  • The same ion-motion model reproduces the qualitatively different behavior seen in argon (outward ion expansion, roughly 60 ns recovery) and lithium (outward ion motion) plasmas, suggesting that the density ratio and ion mass, not just the gas type, set the recovery time.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Beyond the paper: if sub-nanosecond recovery is confirmed, one could push the pump-probe delay below 0.7 ns (the current minimum set by the RF bucket spacing) to map the full recovery curve and test whether the ions snap back or relax through a damped oscillation.
  • Beyond the paper: the persistent overdense channel for $\alpha>1$ could be exploited deliberately, for example as a plasma-based lens or as a density ramp to control the wake phase for a trailing bunch, rather than treated only as a liability.
  • Beyond the paper: the result suggests that repetition-rate limits for hydrogen plasma accelerators are more likely set by capillary wall heating and gas refill than by ion recovery, since the plasma itself resets in under a nanosecond.
  • Beyond the paper: a stronger test would be to use an independent density diagnostic, such as time-resolved shadowgraphy or spectral line broadening, synchronized to the sub-nanosecond delay, to verify that the on-axis density profile has truly returned to its initial flat shape at $\Delta t=0.7$ ns.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 4 minor

Summary. The paper reports a pump-probe experiment at the SPARC_LAB facility in which two electron bunches (pump and probe, separated by delays from 0.7 ns to 13 ns) traverse a hydrogen-filled discharge capillary. By measuring the mean-energy change of the probe as a function of plasma density (np ≈ 10^13–10^16 cm^-3), the authors infer the perturbation left by the pump. They find that for nb/np < 1 the probe energy is nearly unchanged at the shortest delay, which they interpret as sub-nanosecond recovery of the plasma; for nb/np > 1 they observe a persistent enhanced deceleration that they attribute to an on-axis ion-density peak that lasts at least 13 ns. A simplified ion-motion model is used to support both regimes and is also checked against published results for hydrogen, argon, and lithium plasmas.

Significance. If the central claim is correct, the result is significant for the design of high-repetition-rate plasma accelerators: it would indicate that a hydrogen discharge plasma can be reused on sub-nanosecond timescales when the beam density does not exceed the plasma density. The paper also usefully provides a wide parametric scan and a model that qualitatively reproduces several earlier ion-motion observations. The main value, however, hinges on whether the near-zero energy difference at Δt = 0.7 ns is actually a calibrated null measurement of ion recovery rather than an insensitive observable. The authors are explicit that their model is a cross-check rather than a fit, and they compare against independent experiments, which is a strength, but the missing detection-threshold analysis weakens the headline claim.

major comments (3)
  1. [Results, Fig. 5, and Conclusions] The central claim that for nb/np < 1 the plasma 'rapidly recovers in less than a nanosecond' is inferred from ΔE ≈ 0 at the smallest probe delay, Δt = 0.7 ns. Since no data are taken below 0.7 ns and no sensitivity analysis is provided, the experiment supports only the weaker statement that no perturbation is measurable by the probe mean-energy change at or after 0.7 ns. This is not the same as demonstrating that ions have returned to their initial positions. To make the claim load-bearing, the authors should provide a quantitative detection threshold: for example, a simulation or analytic estimate of the ΔE that would result from a residual on-axis ion-density perturbation of tens of percent at np ≈ 10^15–10^16 cm^-3, compared with the shot-to-shot scatter quoted at those densities. Without such a calibration, the null result cannot exclude a partially recovered plasma.
  2. [Methods and Discussion, Eqs. (4)–(9) and Fig. 7] The predicted sub-nanosecond recovery time for the α < 1 case depends on the damping coefficients βe and βi, which in turn depend on the assumed plasma temperature T = 1 eV and neutral density nn = 5 × 10^17 cm^-3. No sensitivity study is given for these inputs, yet the collision frequencies (especially fen and fin) vary strongly with T and nn, and the authors themselves note that neutral collisions become important during recombination. I ask the authors to show how the ion-trajectory time scale and the recovery time in Fig. 7 change over a plausible range of T and nn; otherwise the agreement between the simulation and the null measurement is not enough to establish that the real plasma has recovered rather than that the probe is insensitive to residual ion displacement.
  3. [Discussion, Fig. 6, and Fig. 4(a)] The factor-20 estimate for the ion-density increase in the α > 1 case is obtained by comparing the perturbed probe deceleration at np ≈ 10^14 cm^-3 with the unperturbed probe deceleration at np ≈ 2 × 10^15 cm^-3. This comparison implicitly assumes that the probe mean-energy loss is a linear and unambiguous function of the local plasma density sampled by the beam. At the relevant parameter values this may be approximately true, but the manuscript does not justify the assumption, and the wakefield amplitude is in general nonlinear in local density. The authors state that the simulation gives an average density np ≈ 3.5 × 10^15 cm^-3 'in good agreement with the factor 20,' but no quantitative uncertainty is given for either number. I recommend adding a direct comparison of measured and simulated probe spectra (rather than only the mean energy) and a statement of what uncertainty attaches to the factor-20 claim.
minor comments (4)
  1. [Abstract and Conclusions] The phrase 'recovery of a Hydrogen plasma at the sub-nanosecond timescale' and 'recovers in less than a nanosecond' should be qualified as 'recovery within the shortest accessible delay of 0.7 ns' unless a shorter-time measurement is available; as written, the wording implies a time resolution that the experiment does not have.
  2. [Methods, Eqs. (4)–(9)] In the paragraph after Eq. (9), the notation 'fei ≈ fee ≈ 10^1−3 GHz' is ambiguous; it should be written as 10^1–10^3 GHz or with explicit powers, and the same for the ion-frequency range, to avoid confusion between '10 to 1 GHz' and other readings.
  3. [Fig. 7 caption] The caption of Fig. 7 says '(b) Trajectories ...' but the figure appears to contain a single panel; this looks like a leftover label from an earlier version and should be corrected.
  4. [Acknowledgments] The Acknowledgments contain a typo: 'This work has has received funding' should read 'has received funding.'

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the sub-nanosecond recovery claim is an experimental inference from probe deceleration, and the supporting model is cross-checked against external data.

full rationale

The paper's central claim is based on direct pump-probe measurements of the probe beam's mean energy change, not on a fitted parameter or a self-citation chain. The key inference—that ΔE≈0 at Δt=0.7 ns for α<1 indicates ion recovery—is an experimental observation interpreted through the known relationship between plasma density and beam deceleration; it is not derived from an equation that contains the conclusion by definition. The numerical model used to interpret the dynamics is described with explicit physical inputs (collision frequencies, temperature, neutral density) and is cross-checked against three external experiments (Hydrogen, Argon, Lithium) using their reported beam and plasma parameters, so its use is not a self-referential validation. Self-citations in the manuscript concern the experimental apparatus (SPARC LAB, photo-injector, diagnostics) and do not carry the load of the scientific claim. While the sensitivity of the probe to small residual ion displacements is a legitimate experimental concern, it is a measurement-validity issue rather than a circularity. No equation, fitted parameter, or cited result is shown to be equivalent by construction to the stated conclusion.

Assumptions & free parameters 3 free parameters · 4 assumptions · 0 invented entities

The experimental pump-on versus pump-off comparison is self-contained and does not require the model assumptions. However, the interpretation of measured probe deceleration in terms of local ion density, and the claim that the ion-density peak builds in about 0.2 ns, depend on the model's assumed temperature, neutral density, collision frequencies, and the exponential density calibration fit.

free parameters (3)
  • Plasma temperature T = 1 eV (assumed, not measured)
    Used in collision frequency formulas (4)-(6) and in the pressure term p=n_i k_B T. Affects damping and ion dynamics in the supporting simulations.
  • Neutral density nn = 5e17 cm^-3 (assumed, not measured)
    Sets the electron-neutral and ion-neutral collision frequencies, which the authors note become important during recombination at low plasma densities.
  • Plasma density decay time tau_R = 0.7 microseconds (exponential fit to Fig. 3 data)
    The calibration curve np proportional to exp(-tau_D/tau_R) maps each discharge delay to a plasma density; this mapping is central to the parametric scan.
assumptions (4)
  • domain assumption Inside the blowout radius the plasma is free of electrons and the beam is ultra-relativistic, so the dynamics evolve only in the transverse plane.
    Methods, Eq. (1) and surrounding text: 'we assume it is free of plasma electrons as in the blowout regime'.
  • domain assumption Cylindrical symmetry holds: ions and the blowout radius evolve only radially, and the wakefield is null outside the blowout region.
    Methods: 'The model assumes cylindrical symmetry... The wakefield is null outside of the blowout region'.
  • domain assumption Collision frequencies are additive sums of electron, ion, and neutral contributions with literature cross-sections, with T and nn held constant.
    Methods, Eqs. (4)-(9), relying on Refs. [36,38,39] and on the assumed T and nn values.
  • ad hoc to paper The discharge plasma density follows an exponential decay np proportional to exp(-tau_D/tau_R) with tau_R about 0.7 microseconds.
    Fig. 3 fit used to convert discharge delay tau_D into plasma density for all measurements; this is a facility-specific calibration rather than a general physical law.

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Cite this review

Pith. "Pith review of Recovery of hydrogen plasma at the sub-nanosecond timescale in a plasma-wakefield accelerator." pith.science (2026). https://pith.science/paper/WAPQQBCH

@misc{pith2026250711219,
  author       = {Pith},
  title        = {Pith review of: Recovery of hydrogen plasma at the sub-nanosecond timescale in a plasma-wakefield accelerator},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/WAPQQBCH}},
  note         = {Machine review of arXiv:2507.11219}
}
read the original abstract

Plasma wakefield acceleration revolutionized the field of particle accelerators by generating gigavolt-per-centimeter fields. To compete with conventional radio-frequency (RF) accelerators, plasma technology must demonstrate operation at high repetition rates, with a recent research showing feasibility at megahertz levels using an Argon source that recovered after about 60 ns. Here we report about a proof-of-principle experiment that demonstrates the recovery of a Hydrogen plasma at the sub-nanosecond timescale. The result is obtained with a pump-and-probe setup and has been characterized for a wide range of plasma densities. We observed that large plasma densities reestablish their initial state soon after the injection of the pump beam (< 0.7 ns). Conversely, at lower densities we observe the formation of a local dense plasma channel affecting the probe beam dynamics even at long delay times (> 13 ns). The results are supported with numerical simulations and represent a step forward for the next-generation of compact high-repetition rate accelerators.

Figures

Figures reproduced from arXiv: 2507.11219 by the authors.

Figure 1
Figure 1. Sketch of the experiment. The pump electron beam [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Experimental measurements with pump-probe delay ∆ [PITH_FULL_IMAGE:figures/full_fig_p002_2.png] view at source ↗
Figure 3
Figure 3. Estimation of plasma density. The average plasma [PITH_FULL_IMAGE:figures/full_fig_p002_3.png] view at source ↗
Figures from the paper (6 more)
Figure 4
Figure 4. Figure 4: Data with pump-probe delay ∆t = 0.7 ns. (a) Mean energies for the spectra containing both the pump and probe (blue) and the two alone (red and yellow). The dashed green line shows the mean energies of the perturbed probe, obtained by subtracting to the pump and probe s…
Figure 5
Figure 5. Figure 5: Scan of plasma density np and pump-probe delay ∆t. The x axis shows the plasma densities (corresponding to different discharge delays τD) while y axis reports the en￾ergy variation ∆E of the perturbed probe (pump on) with respect to the unperturbed one (pump off) for s…
Figure 6
Figure 6. Figure 6: Plasma wakefield simulations. (a) Ion density map [PITH_FULL_IMAGE:figures/full_fig_p005_6.png]
Figure 7
Figure 7. Figure 7: Simulation of the plasma response for np ≳ nb. The initial density was set to np = 2 × 1016 cm−3 and the passage of the pump beam is at t = 0. (b) Trajectories (red) of the ions after the initial attraction operated by the electron beam. The evolution and damping of th…
Figure 9
Figure 9. Figure 9: Simulation of the plasma results obtained with Ar [PITH_FULL_IMAGE:figures/full_fig_p007_9.png]
Figure 10
Figure 10. Figure 10: Simulation of the plasma results obtained with [PITH_FULL_IMAGE:figures/full_fig_p008_10.png]

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Works this paper leans on

39 extracted references · 38 canonical work pages

  1. [1]

    Yoneda, Y

    H. Yoneda, Y. Inubushi, K. Nagamine, Y. Michine, H. Ohashi, H. Yumoto, K. Yamauchi, H. Mimura, H. Ki- tamura, T. Katayama, et al. , Nature 524, 446 (2015)

    work page 2015

  2. [2]

    P. K. Maroju, C. Grazioli, M. Di Fraia, M. Moioli, D. Er- tel, H. Ahmadi, O. Plekan, P. Finetti, E. Allaria, L. Gi- annessi, et al. , Nature 578, 386 (2020)

    work page 2020

  3. [3]

    Argyropoulos et al., Physical Review Accelerators and Beams 21, 061001 (2018)

    T. Argyropoulos et al., Physical Review Accelerators and Beams 21, 061001 (2018)

    work page 2018

  4. [4]

    Tajima and J

    T. Tajima and J. M. Dawson, Physical Review Letters 43, 267 (1979)

    1979

  5. [5]

    Malka et al

    V. Malka et al. , Science 298, 1596 (2002)

    work page 2002

  6. [6]

    Blumenfeld et al

    I. Blumenfeld et al. , Nature 445, 741 (2007)

    work page 2007

  7. [7]

    S. P. Mangles et al. , Nature 431, 535 (2004)

    work page 2004

  8. [8]

    Faure, Y

    J. Faure, Y. Glinec, A. Pukhov, S. Kiselev, et al., Nature 431, 541 (2004)

    work page 2004

Show all 39 references
  1. [9]

    Litos et al

    M. Litos et al. , Nature 515, 92 (2014)

    2014

  2. [10]

    Adli et al

    A. Adli et al. , Nature 561, 363 (2018)

    2018

  3. [11]

    Gonsalves et al

    A. Gonsalves et al. , Physical review letters 122, 084801 (2019)

    2019

  4. [12]

    Deng et al

    A. Deng et al. , Nature Physics 15, 1156 (2019)

    2019

  5. [13]

    Pompili et al

    R. Pompili et al. , Nature Physics 17, 499 (2021)

    2021

  6. [14]

    Pompili et al

    R. Pompili et al. , Nature 605, 659 (2022)

    2022

  7. [15]

    Gorbunov, P

    L. Gorbunov, P. Mora, and A. Solodov, Physical Review Letters 86, 3332 (2001)

    2001

  8. [16]

    J. B. Rosenzweig, A. Cook, A. Scott, M. Thompson, and R. Yoder, Physical review letters 95, 195002 (2005)

    2005

  9. [17]

    D’Arcy et al

    R. D’Arcy et al. , Nature 603, 58 (2022)

    2022

  10. [18]

    A. A. Geraci and D. H. Whittum, Physics of Plasmas 7, 3431 (2000)

    2000

  11. [19]

    Kumar, A

    N. Kumar, A. Pukhov, and K. Lotov, Physical review letters 104, 255003 (2010)

    2010

  12. [20]

    Ferrario et al

    M. Ferrario et al. , Nuclear Instruments and Methods B 309, 183 (2013)

    2013

  13. [21]

    Ferrario et al

    M. Ferrario et al. , Nuclear Instruments and Methods in Physics Research A 637, 43 (2011)

    2011

  14. [22]

    Pompili et al

    R. Pompili et al. , Optics Letters 46, 2844 (2021)

    2021

  15. [23]

    Pompili et al

    R. Pompili et al. , Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 829, 17 (2016)

    2016

  16. [24]

    Pompili et al

    R. Pompili et al. , Review of Scientific Instruments 89, 033302 (2018)

    2018

  17. [25]

    Romeo et al

    S. Romeo et al. , AIP Advances 11, 065217 (2021)

    2021

  18. [26]

    C. L. O’Connell, C. Barnes, F.-J. Decker, M. J. Hogan, R. Iverson, P. Krejcik, R. Siemann, D. Walz, C. E. Clay- ton, C. Huang, et al. , Physical Review Special Topics- Accelerators and Beams 9, 101301 (2006)

    2006

  19. [27]

    Vieira, R

    J. Vieira, R. Fonseca, W. B. Mori, and L. Silva, Physical review letters 109, 145005 (2012)

    2012

  20. [28]

    Vieira, R

    J. Vieira, R. Fonseca, W. B. Mori, and L. Silva, Physics of Plasmas 21, 056705 (2014)

    2014

  21. [29]

    Khudiakov, K

    V. Khudiakov, K. Lotov, and M. Downer, Plasma Physics and Controlled Fusion 64, 045003 (2022)

    2022

  22. [30]

    Gilljohann et al., Physical Review X 9, 011046 (2019)

    M. Gilljohann et al., Physical Review X 9, 011046 (2019)

    2019

  23. [31]

    Zgadzaj et al

    R. Zgadzaj et al. , Nature Communications 11, 4753 (2020)

    2020

  24. [32]

    W. Lu, C. Huang, M. Zhou, W. Mori, and T. Katsouleas, Physical review letters 96, 165002 (2006)

    2006

  25. [33]

    Ferrario et al

    M. Ferrario et al. , Physical review letters 104, 054801 (2010)

    2010

  26. [34]

    Pompili et al

    R. Pompili et al. , Applied Physics Letters 110, 104101 (2017)

    2017

  27. [35]

    Biagioni et al., Plasma Physics and Controlled Fusion 63, 115013 (2021)

    A. Biagioni et al., Plasma Physics and Controlled Fusion 63, 115013 (2021)

    2021

  28. [36]

    Khodachenko, T

    M. Khodachenko, T. Arber, H. O. Rucker, and A. Hanslmeier, Astronomy & Astrophysics 422, 1073 (2004)

    2004

  29. [37]

    Ariniello et al

    R. Ariniello et al. , Physical Review Accelerators and Beams 22, 041304 (2019)

    2019

  30. [38]

    Eliezer, The interaction of high-power lasers with plas- mas (CRC Press, 2002)

    S. Eliezer, The interaction of high-power lasers with plas- mas (CRC Press, 2002). 9

    2002

  31. [39]

    Zhdanov, Plasma Physics and Controlled Fusion 44, 2283 (2002)

    V. Zhdanov, Plasma Physics and Controlled Fusion 44, 2283 (2002). DA T A A V AILABILITY The data that support the findings of this study are available from the corresponding author on reasonable request. ACKNOWLEDGMENTS This work has has received funding from the Euro- pean Un...

    2002

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Reviewed August 6, 2026 · model on record in the stance chip above.