Recent Publications

The FAST tool for synthetic SXR calculations and determination of impurity density and effective charge (Z eff )

Tullio Barbui, Oulfa Chellai, Luis F. Delgado-Aparicio, Alexandre Fil, Beatriz García, Kenneth W Hill, Shinichiro Kojima, Novimir Antoniuk Pablant, Brent C Stratton, D Vezinet, et al.

A new soft x-ray (SXR) synthetic tool has been developed: it is called FAST for FLYCHK-based Atomic physics and SXR Tomography. It is based on FLYCHK SXR emissivity spectra calculated for a variety of elements, from deuterium to tungsten. FAST creates 2D plasma emissivity for arbitrary plasma compositions and plasma profiles. It computes line-integrated SXR profiles based on a specific detector geometry and response. This tool is used to reproduce signals from an absolutely calibrated multi-energy SXR (ME-SXR) camera at WEST. Thanks to its energy-resolved measurement capability, the ME-SXR camera is able to separate spectral regions dominated by impurity line emission from others containing only continuum emission. At WEST, FAST and the ME-SXR camera are employed to determine impurity density profiles for tungsten and nitrogen (used as a proxy for low-Z impurities), as well as the plasma effective charge (Zeff), through SXR measurement reproduction at different energies, from 2 to 14 keV. Tungsten density is determined in two plasma scenarios at WEST: lower hybrid heating in the X-point radiator regime and lower hybrid heating with modulations of ion cyclotron resonance heating. In the first scenario the tungsten profile is found to be moderately peaked in the core with a concentration of 0.8‒1×10-4. This concentration is found to increase up to 1.4×10-4 during ion cyclotron resonance heating. Tungsten peaking factor in steady-state conditions is also retrieved and compared to theoretical predictions. It is found to be slightly lower than the combined neoclassical and turbulent transport prediction.

Off-normal simulations of radiative collapse events in Alcator C-Mod

Alex Ryan Saperstein, Jonathan Citrin, Aaron Ho, Ryan Sweeney, Conor Perks, Mark D Boyer, Gregorio Luigi Trevisan, Yumou Wei, Cristina Rea

In preparation for ARC, supplementary offline tools are being developed alongside a real-time warning system in order to reduce its disruptivity. This paper introduces Off-Normal SIMulations (ONSIMs) to the MOSAIC pulse-simulating framework with that intent in mind. These modules probe simulated equilibria with a library of distinct perturbations, and can characterize disruptivity based on the scenario's resiliency to them. This paper addresses the development of a module for simulating radiative collapses, with a specific focus on validating for the first time the power balance dynamics characteristic of these events. The simulations are performed using the TORAX transport simulator and are benchmarked against radiative collapse precursors on Alcator C-Mod. It is found that transport models that include sufficient turbulence effects, like a surrogate of the QuaLiKiz transport model, are capable of reproducing the relevant radiative collapse dynamics for an L-mode discharge. It is also found that these dynamics are relatively insensitive to the shape of the collapse-inciting impurity profile, which bodes well for the robustness when probing various equilibria. The benchmarking reported here is part of a larger ongoing validation effort, which lays the groundwork for radiative collapse ONSIMs. Future work will be done to translate validations like this to a general implementation of ONSIMs to SPARC and ARC scenarios.

Core-edge integrated modeling of ARC: on the effect of impurity transport and detachment conditions.

Marco Muraca, Pablo Rodriguez-Fernandez, Nathan T Howard, Jo Hall, Giovanni Tardini, Davide Silvagni, Thomas A Body, Jon Hillesheim

Integrated modeling of the flat-top phase of ARC H-modes has been conducted to assess the feasibility of high-performance scenarios compatible with divertor detachment. The analysis incorporates self-consistent evolution of impurity radiation and density profiles, demonstrating that fusion power levels approaching a GW can be achieved while maintaining divertor temperatures below 2 eV with Ar seeding. Sensitivity studies reveal a strong dependence of fusion power on the separatrix density, with performance spanning 750–1000 MW, and a weaker dependence on enrichment factor and pedestal density. Alternative seeding strategies using Neon have also been explored. Plasmas with Argon seeding consistently access H-mode, providing the highest fusion power and detached divertor operation, whereas Neon seeding leads to lower performance (600–850 MW) and less robust H-mode access, due to excessive core impurity accumulation. A small W impurity peaking has been found, with decreasing values at higher Zeff. Further analyses incorporate reduced momentum transport modeling, and sensitivity studies of neoclassical impurity transport, confirming the robustness of the results. Overall, these findings support the viability of high-performing H-mode operation in ARC, ensuring divertor protection, enabled through Argon and Neon impurity seeding.

Accelerating integrated modeling with surrogate-based optimization: the MAESTRO workflow

Pablo Rodriguez-Fernandez, Nathan T Howard, Jo Hall, Audrey Saltzman, Adrian Martin Sanabria, Aaron Ho, Garud Snoep, Javier Pimentel Aldaz, Christopher Holland, Marco Muraca, et al.

This paper introduces the MAESTRO workflow, that enables the coupling of the PORTALS framework [P. Rodriguez-Fernandez et al, Nucl. Fusion 2024] with external solvers for the plasma equilibrium, pedestal physics, divertor constraints and heating. The surrogate-based optimization nature of the transport solver is ideally suited for external coupling, allowing efficient steady-state predictions of plasma profiles with full physics models. Improvements in the surrogate modeling of quasilinear transport models with PORTALS are presented, which enable the efficient handling of discontinuities in the transport fluxes that can arise from numerical issues or physical instabilities with extreme stiffness. The combination of physics-informed methods and advanced numerical techniques allows the MAESTRO workflow to provide accurate and efficient predictions of steady-state plasma profiles, which are critical for fusion reactor design and optimization.

Experimental evidence for increased particle fluxes due to a change in transport at the separatrix near density limits on Alcator C-Mod

Marco Andrés Miller, Jerry W Hughes, Thomas Eich, George R Tynan, Peter Manz, Amanda E Hubbard, Brian LaBombard, Jamie Dunsmore

Experimental inferences of cross-field particle flux at the separatrix, Γ ⊥ sep , show rapid growth near H-mode and L-mode density limits at high magnetic field on Alcator C-Mod. Increases in Γ ⊥ sep correlate well with proximity to high density operational boundaries as proposed by the separatrix operational space model. Γ ⊥ sep grows as the L-mode density limit and the H-L-mode back transition boundaries are approached, consistent with expectations of plasma instability-driven turbulence suggested by theory, confirming the power dependence of density limits. Γ ⊥ sep is well-organized by the characteristic wavenumber for resistive ballooning mode turbulence, k RBM , from interchange-drift-Alfvén fluid turbulence theory, with additional dependence on the cylindrical safety factor, q cyl , yielding an empirical limit to plasma operation of k RBM 2 q cyl = 1. This limit corresponds to the point where the perpendicular heat flux, Q ⊥ , reaches the level of the parallel heat flux, Q ∥ , i.e. Q ⊥ ≈ Q ∥ , beyond which point thermal equilibrium is not satisfied, resulting in a fold catastrophe.

Alpha particle losses due to ripple and field errors in a SPARC like tokamak

Hugo Emilio Ferrari, Agustín Mairotta, Cesar F Clauser, Ricardo Farengo, Justo Andres Gonzalez Litardo, Alexandra LeViness, Steve Scott

In this work we study the alpha particle losses due to ripple and coil misalignments in the SPARC tokamak. To calculate the effects of ripple and coil misalignment, we developed a code named B3D. The ripple field calculated by this code is combined with 2D equilibrium fields and predicted plasma profiles and used in the FOCUS code to calculate the alpha particle losses. We benchmarked our results against previous studies with the ASCOT and SPIRAL codes, obtaining excellent agreement. When the effect of coil misalignment is included, the alpha particle losses increase and localized regions with significantly larger values of the power density through the last closed flux surface appear. This appears to be due to the low order modes induced in the magnetic field by the misalignment.

An Inverse Grad-Shafranov Neural Network Approach to Tokamak Magnetic Control

Allen M. Wang, Adriano Mele, Cosmas Heiß, Cristian Galperti, Zander Keith, Alessandro Pau, Antoine Merle, Olivier Sauter, Daniel Gonzalez Castiñeiras, Francesco Carpanese, et al.

A new approach to tokamak magnetic control enabling high-precision plasma shaping and novel real-time adaptability is experimentally demonstrated on the Tokamak a Configuration Variable (TCV). The method is motivated by the insight that, under appropriate assumptions, a real-time inverse Grad-Shafranov solver approximates an optimal control policy for plasma boundary regulation. Building on this, a control architecture is developed in which classical controllers enforce operational constraints while a fast surrogate model provides a real-time inverse mapping from the desired plasma boundary to Poloidal Field Coil currents. Experimental results on TCV demonstrate improved plasma shaping with respect to the standard discharge preparation procedure --- albeit without explicit real-time shape feedback --- while enabling flexible response to asynchronous events. It is shown that a single network provides satisfactory performance across a range of plasma magnetic configurations. Real-time adaptivity is demonstrated in simulation, and partially in experiment, through adaptive strike point motion and early termination in response to a real-time trigger. These results suggest a viable path toward magnetic control architectures that reduce reliance on dense diagnostic coverage while maintaining high-accuracy plasma shaping, with potential relevance for future fusion power plant operation.

Physics Attention Transformer Surrogate for Rapid Vertical Instability Growth Rate Prediction: Alcator C-Mod to SPARC

Arunav Kumar, Cesar Clauser, Theodore Golfinopoulos, Cristina Rea, Francesco Capersene, Dan Boyer, SPARC Team, Alcator C-Mod Team

In this work, we investigate rapid prediction of the dominant $n{=}0$ vertical instability growth rate in C-Mod and SPARC equilibria, where nonrigid free boundary response models are too slow for control cycle use. Using a Physics Attention Transformer trained on MEQ-FGE-L labels, we predict both the scalar growth rate and the associated two dimensional perturbed toroidal current density. We find mean absolute errors of 5.4~s$^{-1}$ on held out C-Mod equilibria and 12.7~s$^{-1}$ on synthetic SPARC cases, with spatial eigenfunction errors near 5\%. We also compared PAT with operator based ML models : FNO2D and DeepONet, where we found PAT predicts a much lower normalised growth rate error and improved spatial reconstruction. These results indicate that PAT can reproduce MEQ-FGE-L outputs at control relevant latency and could support future studies of growth rate headroom monitoring and proximity aware shape control.

On the feasibility of model-based feedback control of vertical instability growth rate using out-vessel coils in ARC-like scenarios

Arunav Kumar, Cesar Clauser, Theodore Golfinopoulos, Jon C. Hillesheim

In this work, we propose a model-based feedback controller that regulates the vertical instability growth rate ($γ_{gr}$) of a high-elongation, double-null tokamak directly, using only out-vessel poloidal field (PF) coils. High elongation raises the achievable plasma current and fusion performance but makes the plasma vertically unstable, and in a fusion power plant the in-vessel coils that present devices rely on for stabilization may be absent, leaving only distant out-vessel circuits. The controller couples a machine learning surrogate of non-rigid, profile agnostic vertical instability metric to a constrained quadratic program: the surrogate supplies real-time $γ_{gr}$ estimates and, via automatic differentiation, the actuator sensitivities, while the program allocates coil voltages to track a target growth rate, maintain double-null divertor balance, and respect electromechanical limits. We tested this method on the ARC~V3A power plant design configuration across 24 closed-loop simulations spanning equilibrium variations, actuator degradations, and transient disturbances. We achieved full or marginal success in 83\% of these cases (full in 50\%, marginal in a further 33\%) and lose control in the remaining 17\%; the failures map the boundary of out-vessel controllability (occurring at the highest growth rates) and under actuator limits. The controller does not regulate boundary shape explicitly: separatrix geometry follows indirectly from growth rate and flux balance control and would require a separate shape control layer for sustained scenario evolution.

First step toward multi machine ELM energy scalings and extrapolations to SPARC and ITER

R. Perillo, A. Redl, T. Eich, C. J. Lasnier, A. Nelson, R. Rizkallah, D. Silvagni, A. Stagni, J. A. Boedo, A. McLean, et al.

It is shown that the ELM energy loss normalized by the plasma stored energy (ΔEELM/Wplasma) for high-density small/QCE ELM regimes scales inversely with the separatrix turbulence parameter a_t. In contrast, the neoclassical electron collisionality at the pedestal top, nu*e,neo, expected to regulate ΔEELM/Wplasma according to the Loarte scaling (Plasma Phys. Control. Fusion 2003 45 1549), does not adequately capture ΔEELM/Wplasma data for peeling-ballooning-limited type-I ELMs and ballooning-limited small/QCE ELMs, limiting its applicability for extrapolation to one scenario window. A multi-machine database including seven tokamaks and with ΔEELM/Wplasma ranging from 0.5% to 14%, has been analyzed. A regression analysis on only type-I ELMs yields ((ΔE_ELM)/W_plasma )_(Type-I ) [%]=6.8*T_(e,ped)^0.03 n_(e,ped)^(-0.4) \k{appa}^(-0.4) R_major^0.4, corresponding to ΔEELM/Wplasma =4.5% for nominal SPARC pedestal parameters and 12% for the ITER D-T Q=10 scenario. For the small/QCE ELM class, however, as a_t increases, the pedestal moves toward a ballooning-limited boundary, the toroidal mode number increases, the ELM frequency rises following the scaling f_ELM=46e^((2.25*a_t)), and ΔEELM/Wplasma decreases via the relation (ΔE_ELM)/W_plasma [%]=1.6e^(-(α_t/2)). For SPARC QCE-relevant a_t=0.86 and ITER high-fueling scenario a_t = 0.64, the scaling favorably predicts ΔEELM/Wplasma of 1.0% and 1.2%, respectively, with values below 1% if the small/QCE ELM regime is pushed beyond a_t >1. The small/QCE ELM-fitted results represent an initial step toward future analysis on broader datasets, which will be necessary to improve the accuracy of projections for future reactor-relevant scenarios.

Characterization of a prototype parallel-plate 238U fission chamber with DD and DT fusion neutron sources

V. Hagenlocker, R.A. Tinguely, X. Wang, J.L. Ball, B. Buschmann, M. Gatu Johnson, R. Gocht, P. Raj

Deuterium retention of boron powder from deuterium gas or ion exposure

Shota Abe, Adam Q. Kuang, Christopher P Chrobak, Alessandro Bortolon, Camilo Jaramillo-Correa, Bruce E Koel

We report retention of deuterium (D), a proxy for tritium (T), from D2 neutral gas and D+ ion exposures on commercial boron (B) powder, a proxy for B dust potentially formed in fusion reactors. D removal behavior from B powder is reported as a means of estimating a potential T inventory in B dust in advanced fusion reactors, such as SPARC and ITER, the latter currently plans to employ B wall conditioning by glow-discharge boronization. B wall conditioning forms surface coatings of chemical compounds on tungsten (W) plasma-facing components (PFCs), thus suppressing plasma contamination by impurities such as oxygen, carbon, and tungsten. However, B-based slag or dust particles are expected to form. Such B dust particles can retain hydrogen isotope species, thereby causing a T inventory issue. In this work, the commercial B powder was exposed to D2 neutral gas or D+ ions and analyzed by temperature-programmed desorption to quantify D retention and the desorption temperature. The experiment confirmed D retention from D2 neutral gas exposure. A strong D2 desorption peak at 700 K, corresponding to B-D bonding, was observed for all D2 gas and D+ ion exposure cases. For D2 gas exposures, D retention was significantly enhanced at a B powder temperature of 550 K. B powder bakeout under vacuum at 600 K for 22 hours after D2 exposure exhibited efficient D removal. In contrast, the B powder bakeout at 420 K, even for 1 day, did not remove D efficiently. D retention yields from D+ ion exposure, emulating charge-exchange involving D atoms, were determined. Experiments confirmed that D retention was suppressed by oxidation of the B powder surface, which naturally occurs in reactor environments.

The power exhaust constrained SPARC separatrix operational space

B. Lomanowski, T. Eich, J. D. Lore, J. -S. Park, T. Body, P. Stangeby

In this work we extrapolate the separatrix operational space (SepOS) projections to SPARC and introduce detachment access criteria, thus formulating the combined power exhaust constrained SepOS (i.e., PE-SepOS) to evaluate integrated power exhaust solutions at scale. Through the interpretation of SPARC SOLPS-ITER datasets and foundational work already demonstrated in experiments we formulate an as simple as possible description of the SOL net power and momentum losses in dissipative regimes to link the main power exhaust quantities with the SepOS parameters. Through this framework, we demonstrate the utility of a normalized PE-SepOS framework in identifying accessible operational points for given exhaustible Psep requirements. In applying the PE-SepOS framework to project the SPARC operational space, we find inherent trade-offs, namely: i) accessing high impurity radiation scenarios leads to pronounced reductions in ne,sep (e.g., 50% reductions at 2% Ne concentration) as a consequence of power limitation, and ii) given present understanding of access criteria to the quasi-continuous exhaust regime (QCE), a compromise between high radiative fraction and high density/neutral pressure is required for QCE access at sufficiently high density, high alpha_t conditions, with the divertor dissipative regime transitioning to pronounced detachment. Taking advantage of universal trends enabling projections of density and impurity seeding scans, the PE-SepOS thus provides a framework for mapping out the edge plasma operational space in a scalable manner, subject to validation during early SPARC operations with suitable divertor and edge plasma observables.

Assessment of 0-D L-H Power Threshold Scaling and Regression Stability in DIII-D with Applied 3D Magnetic Fields

Michael O Hanson, George R Tynan, Dmitri M Orlov

A database of 192 L-H transitions in DIII-D is used to assess the effects of applied three-dimensional (3D) magnetic fields on the H-mode power threshold and the stability of zero-dimensional (0-D) empirical regressions. The dataset includes nominally axisymmetric discharges and discharges with resonant or non-resonant magnetic perturbations. Filtering criteria reduce uncertainties associated with absorbed power, neutral-beam modulation, and fast-ion losses, while applied-field components are quantified using equilibrium reconstruction and spectral analysis. Measured threshold powers show substantial scatter and systematic deviations from the 2008 ITPA multi-machine scaling, including for discharges without applied perturbations. TRANSP modeling indicates that empirical estimates can substantially underpredict fast-ion losses, particularly at low plasma current and with non-axisymmetric fields. Adding global 3D-field metrics does not robustly isolate the effects of applied perturbations. A fully unconstrained regression retains a 70% residual root-mean-square error and produces nonphysical parameter dependencies, including a plasma surface-area exponent of 2.79. Extrapolations to ITER-relevant conditions consequently have broad confidence intervals. These results show that hidden-variable dependencies can strongly affect empirical threshold parameterizations even in a restricted single-machine dataset. Machine-specific conditions, local edge physics, and power-accounting uncertainties limit the predictive capability of purely 0-D scalings. Improved predictions for ITER and future devices will require better fast-ion-loss treatment and physics-based, edge-localized quantities.

Empirical impact of near-separatrix plasma and neutral transport on the pedestal in the transition between EDA and ELMy H-modes on Alcator C-Mod

M.A. Miller, J.W. Hughes, S. Saarelma, T. Eich, J. Dunsmore, J. Han, P. Manz, J.W. Connor, G.R. Tynan, A.E. Hubbard, et al.

The transition between the ELMy H-mode and the EDA H-mode is studied on Alcator C-Mod using an experimental database and recently-developed predictive pedestal models. High-resolution Thomson scattering measurements are used to inspect both the separatrix and pedestal operational space using two independent fit functions, each tailored to analyze either the pedestal or the separatrix and near-SOL profiles. The pedestal density, n e ped , and the separatrix density, n e sep , are compared to main chamber neutral measurements. n e ped is sensitive to neutral sources only in the ELMy H-mode regime and not in the EDA H-mode regime. Density fluctuation spectra reveal that quasi-coherent structures become stronger at higher densities and more coherent in the EDA relative to the inter-ELM phases of ELMy H-modes, before weakening again at the highest values of n e ped . The Saarelma–Connor pedestal density prediction model is validated for ELMy H-modes up to n e ped = 2.0 × 10 20 m − 3 . An additional transport channel driven by resistive ballooning modes (RBMs), D RBM , scaling directly with α t , a collisional turbulence control parameter, and inversely with k RBM 2 q ^ cyl , the product of the square of the characteristic RBM turbulence wavenumber and the cylindrical safety factor, is shown to improve the prediction for EDA H-modes, finding good model agreement up to n e ped = 3.0 × 10 20 m − 3 . EPED scans in n e ped are also performed, at three different values of n e sep / n e ped . Increasing this ratio moves the peeling to ballooning branch transition to lower n e ped . It also increases p ped in the peeling branch and decreases it in the ballooning branch. Agreement with the EPED prediction is found for large ELM H-modes. SPARC pedestal density predictions for an ELMy and an EDA/QCE-like H-mode are performed using the extension to the standalone Saarelma–Connor model and are found consistent with assumptions used in previous EPED modeling. Inclusion of D RBM significantly weakens the predicted density gradient near the separatrix, lowering n e ped by approximately 20%.

The impact of kinetic and global effects on ideal ballooning 2nd stable pedestals of conventional and low aspect-ratio tokamaks

M.S. Anastopoulos Tzanis, M. Yang, A. Kleiner, J.F. Parisi, G.M. Staebler, P.B. Snyder

The EPED model (Snyder et al 2011 Nucl. Fusion 51 103016) has had success in describing the pedestal structure of Type-I ELM and QH-mode plasmas in tokamaks, by combining kinetic ballooning mode (KBM) and peeling–ballooning (PB) constraints. Within EPED, the KBM constraint is often approximated by a technique using calculated ideal ballooning mode (IBM) thresholds, fit to a functional form designed to capture non-local effects. It has been noted that quantitative differences between local ideal MHD and gyro-kinetic (GK) ballooning stability can be larger at low aspect ratio. KBM critical pedestals are consistent with observations in initial studies on conventional and spherical tokamaks. In this work, the application of a reduced model for the calculation of the kinetic ballooning stability boundary is presented based on a novel and newly developed gyro-fluid system (GFS) code (Staebler et al 2023 Phys. Plasmas 30 102501). GFS is observed to capture KBMs in DIII-D as well as the NSTX pedestals, opening a route to integrating this model into EPED. Finally, high but finite n global ballooning modes are observed to limit the access to the local 2nd stability and thus provide a transport mechanism that constrains the pedestal evolution with β p , ped . The high n global ballooning stability is approximated by its ideal MHD analog using ELITE. It is shown that nearly-local high n modes with k y ρ s ∼ 0.25 − 0.5 can provide a proxy for the critical β p , ped when a 2nd stable access exists on DIII-D plasmas. The use of GFS and ELITE scaling in EPED provides improved agreement to EPED1 in an initial comparison with DIII-D pedestal data.