Recent Publications

Two-dimensional transport mechanisms in the vicinity of the X-points in snowflake divertors

Dominic Power, Maxim V Umansky, Fabio Federici, Vlad Soukhanovskii

Snowflake divertor (SFD) experiments show enhanced radial transport near the X-points, more than may be explained by standard two-dimensional edge transport models without drifts. Here we investigate transport mechanisms in SFDs through two complementary approaches: i) modelling of recent SFD experiments performed at MAST-U with the edge transport code UEDGE with ExB and ∇B drifts enabled, and ii) reduced modelling of transport and turbulence in the vicinity of the X-points with a model developed here and implemented in BOUT++. Through these two approaches we assess the two leading candidates for this transport: equilibrium ExB drifts driven by strong poloidal gradients near the primary X-point, and the 'churning mode' (CM), a toroidally-symmetric plasma vortex close to the X-points. UEDGE modelling shows that equilibrium ExB drifts do play a role in increasing particle and energy transport across the primary separatrix, increasing the proportion of heat flux delivered to secondary strike points (SPs) and reducing peak heat fluxes at the primary SPs. This mechanism is less effective in the upper divertor due the reversed drift directions in combination with the device geometry. Including drifts in the model is also shown to improve agreement with the experimental radiated power profiles. To self-consistently compare the impact of drifts and the CM we develop a reduced-MHD model which includes both effects, and which is implemented in BOUT++. We find that equilibrium ExB drifts are dominant when the ratio of plasma pressure in the null region to poloidal magnetic pressure at the midplane is low, β pm ≤ 0.5-1.0%, which were the conditions obtained in the MAST-U experiments. The CM was found to be dominant for β pm ≥ 1.0%, leading to increased transport across the null region and activation of secondary divertor legs.

The trigger for plasma detachment bifurcation in the edge divertor region of tokamaks

Menglong Zhao, Tom Rognlien, Ben Zhu, Filippo Scotti, Xinxing Ma, Adam G McLean

We identify a new trigger and dynamical pathway of plasma detachment bifurcation in tokamak divertors using steady-state and time-dependent UEDGE simulations. We report two key findings: first, the existence of a regime in which the direction of the E × B flow in the private-flux region reverses to point from the inner to the outer divertor in a deeply detached divertor plasma; and second, that this E × B flow reversal provides the causal mechanism for the abrupt drop in electron temperature (Te cliff) observed at the outer target in DIII-D. Specifically, in DIII-D H-mode plasmas with the ion B × ∇B drift driven into the active divertor, the Te cliff arises from a bifurcation-induced Te drop above the magnetic X-point accompanied by reversal of the E × B flow pattern in the private-flux region. Time-dependent simulations reveal a two-phase transition mechanism: the inner-divertor radiation front first extends across the last closed magnetic flux surface and stabilizes above the X-point, causing the local Te to drop from ∼ 70 eV to ∼ 10 eV and inducing an E × B flow reversal in a thin layer below the X-point in less than 0.5 ms (Phase I); this flow reversal subsequently triggers the sharp drop in outer-divertor target temperature on a timescale of 1–2 ms (Phase II), establishing deep plasma detachment a few ms thereafter. We show that a bifurcation transition occurs when the high-field-side radiation front crosses the separatrix while the outer divertor remains attached, with the Te cliff manifesting distinctly when the outer-target Te ≳ 10 eV prior to the bifurcation transition, implying that the bifurcation is fundamentally linked to in–out divertor asymmetry and asymmetric radiation-front evolution.

Comparative study of nitrogen and neon seeding impacts on divertor detachment and core confinement in DIII-D high-βp plasmas with ITER-similar shape

Donggui Wu, Lingyi Meng, Liang Wang, Andrea M Garofalo, Xianzu Gong, H Q Wang, Siye Ding, Xiang Jian, Xiaohe Wu, Juan Huang, et al.

Divertor detachment experiments with nitrogen (N2) and neon (Ne) seeding have been performed in DIII-D high-βp scenario at q95~6, and the impacts of impurity seeding on divertor detachment and plasma confinement have been studied. Experimental results demonstrate that both N2 and Ne seeding could effectively reduce the electron temperature at the divertor target (Tet). The reduction in particle flux (jsat) is significantly more pronounced with N2 seeding, indicating a superior capability of N2 to facilitate deep particle detachment compared to Ne under these conditions. SOLPS-ITER simulations corroborate these findings, as the parallel momentum balance analysis reveals that N2 particles are predominantly retained in the divertor region, facilitating strong radiative dissipation near the target with minimal core penetration. In contrast, Ne particles could penetrate upstream, ionize inside the separatrix and lead to higher core radiation. Consequently, the stronger power and momentum loss in the scrape-off layer with N2 seeding is the key to achieve lower Tet and jsat compared to Ne seeding. Furthermore, statistical results suggest that the plasma confinement is maintained with both impurities, as the formation of large-radius internal transport barrier (ITB) compensates for pedestal degradation. Notably, the ITB formed with Ne seeding case is substantially stronger than that with N2 seeding. TGYRO simulations indicate that for Ne seeding case, toroidal rotation shear and the E×B shear play minor roles in ITB formation. Instead, the dominant stabilizing mechanism is identified as the redistribution of the current density and the reduction of magnetic shear at large radius, which is strongly promoted by Ne seeding but only weakly influenced by N2 seeding. These results provide a comprehensive perspective on the impact of N2 and Ne seeding on detachment and plasma confinement sustainment in DIII-D high-βp operations, offering valuable insights into optimizing the core-edge integration in future fusion reactors.

Xcimer’s Athena Fusion Pilot Plant: A Preconceptual Design Overview

Susana Reyes, Conner Galloway, Alexander Valys, Amanda Bye, Kirk Flippo, Alison Christopherson, Austin Steinforth, Wayne Meier, Francisco Ogando, Rene Raffray, et al.

Validation of first-principles turbulence simulation in diverted negative triangularity plasmas on DIII-D

Philipp Ulbl, Alessandro Marinoni, Baptiste J. Frei, Filippo Scotti, Samuel D. Stewart, Gabriele Merlo, Frank Jenko, Kathreen E. Thome

We present a first-principles gyrokinetic simulation of the edge and scrape-off layer (SOL) region of negative triangularity (NT) plasmas in the DIII-D tokamak, achieving remarkable agreement with experimental data for density, ion temperature, and electron temperature profiles. Notably, our simulation validates edge and SOL ion temperature profiles in electrostatic turbulence dominated conditions for the first time. The NT edge region is found to be dominated by trapped electron modes, with a transition toward ion temperature gradient-like modes at the outer edge. The SOL heat fluxes on divertor targets exhibit an Eich profile shape, consistent with experimental observations, with a similar but slightly narrower width than measured experimentally. The simulation demonstrates the presence of geodesic acoustic modes, and our analysis reveals the characteristics of their interaction with turbulent fluctuations. This successful validation marks a significant step toward self-consistent predictions of turbulence, edge profiles, and heat exhaust in future fusion devices.

X-ray spectroscopy of hot dense plasmas using LLNL's upgraded Titan 2$ω$ short-pulse laser

A. J. Fairchild, D. T. Bishel, T. Cordova, H. Flores-Alimboyoguen, M. F. Gu, P. Hamilton, N. Hell, M. J. MacDonald, E. W. Magee, R. C. Mancini, et al.

We are developing an experimental platform at Lawrence Livermore National Laboratory's Jupiter Laser Facility to characterize X-ray emission from highly charged ions in the high-density ($n_e > 10^{23}$ cm$^{-3}$), high-temperature ($T_e \sim 1$ keV) regime, leveraging the recently upgraded frequency-doubled short-pulse Titan laser. These measurements will provide experimental benchmarks for high electron density effects on atomic structure, including Stark broadening, ionization potential depression, density-dependent line shifts, and suppression of dielectronic recombination, effects that are relevant to spectral modeling packages used by both the high-energy-density and astrophysics communities. Here, we describe the current status of the platform and present results from first experiments using the upgraded frequency-doubling capability, including calibrated K-shell spectra acquired from five time-integrated X-ray channels. Comparisons with MERL line shape calculations and with spectra recorded at AWE's Orion Laser Facility indicate that the upgraded Titan laser reliably produces the necessary plasma conditions, establishing a foundation for systematic benchmarking of high-density atomic physics in future campaigns.

Moments in time: Numerical analysis of a method for time-resolved neutron spectroscopy

C. B. Stuart, B. Appelbe, A. J. Crilly, C. Forrest, A. DeVault, M. Gatu Johnson, B. J. Lahmann, D. Schlossberg, S. P. Regan, G. Gregori

Time-resolved neutron information is essential for understanding the dynamics of inertial confinement fusion (ICF) implosions, providing key indicators that distinguish igniting from non-igniting plasmas and revealing the underlying causes of shot-to-shot performance variations. The moments method framework offers a practical pathway to extract such temporally resolved quantities using only a small number of neutron time-of-flight detectors. For example, one only needs four detectors to extract the burn-averaged rate of change of ion temperature, a well-defined physically useful quantity. In this work, we evaluate and validate the moments method across several levels of complexity. We first employ a controlled toy model to test the foundational assumptions and mathematical structure of the approach. We then demonstrate that the method generalizes to realistic one-dimensional simulation data of direct drive implosions with a 25 kJ laser driver that include time-dependent ion temperature evolution and hydrodynamic burn dynamics. Together, these results establish the moments method as an experimentally accessible and robust tool for extracting unique and independent information such as the burn-averaged rates of change of fluid velocity and ion temperature with time from neutron time-of-flight measurements, broadening the diagnostic capability of existing neutron detector systems. Fielding this diagnostic on ICF campaigns would improve our understanding of failure modes, helping to determine causes for improved or reduced performance during implosions, informing future target and laser drive design.

Modulated deuteron spectra observed with the magnetic recoil neutron spectrometer at the National Ignition Facility

Bao Nguyen, Yousef Lawrence, Christopher Wink, Timothy Mark Johnson, Niels Vanderloo, Benjamin Reichelt, Amber Hennessy, Daniel Thomas Casey, Dave Schlossberg, Nathan Masters, et al.

The Magnetic Recoil Spectrometer (MRS) at the National Ignition Facility is used to measure the neutron spectrum from deuterium–tritium-fuelled inertial confinement fusion implosions via neutron–deuteron elastic scattering and magnetic dispersion of recoil deuterons. From the MRS-determined neutron spectrum, the yield ( upper Y Subscript n Y n $Y_n$ ), apparent ion temperature ( upper T Subscript i T i $T_i$ ) and areal density ( rho upper R ρ R $\rho R$ ) are determined. However, anomalous energy modulations in recoil deuterons have been observed in several high-yield indirect drive experiments ( upper Y Subscript n Baseline tilde 10 Superscript 16 Baseline minus 10 Superscript 18 Y n ∼ 10 16 − 10 18 $Y_n\sim 10^{16}{-}10^{18}$ ). These observations raise concerns about their potential impact on the MRS-inferred performance metrics. Analytic calculations and particle-in-cell simulations are used to examine the possible beam–plasma instabilities, which indicate the two-stream instability as a plausible mechanism behind energy modulations. Based on a statistical analysis of synthetic deuteron spectra, the modulation-induced errors are found to be within the errors of the determined upper Y Subscript n Y n $Y_n$ , upper T Subscript i T i $T_i$ and rho upper R ρ R $\rho R$ values and thus do not have a significant impact on the MRS measurement.

Controlling $γ$-photon production with additively manufactured microstructured targets

Joshua Luoma, Andreas Kemp, Thomas G. White, Dean Rusby, Hui Chen, Gennady Shvets

Nonlinear inverse Compton scattering (NICS) in high-density targets offers a promising route for generating high flux, broadband sources of MeV $γ$-photons which are important for applications like radiography, photonuclear physics, and pair production. Microstructured targets enable control over the NICS process to help shape the photon energy, angular distribution, and total yield. This work presents particle-in-cell (PIC) simulations exploring target microstructures compatible with two-photon polymerization (TPP) additive manufacturing, which is a promising technique that allows high-volume and reproducible printing of sub-wavelength features as small as 200 nm. We investigate how microstructures influence NICS radiation and merge simulation analysis with simple analytical models that guide target design. These results establish pathways to optimize targets for low-divergence, high-energy photon beams with 2.0$\cdot$10$^{8}$ photons/J exceeding 100 MeV and targets for high yields delivering upwards of 2.0$\cdot$10$^{11}$ photons/J above 1 MeV in the multi-PW regime.

Poloidal asymmetries of main chamber neutral fueling in DIII-D ELMy H-mode plasmas

Laszlo Horvath, Florian Laggner, Raúl Gerrú, Filippo Scotti, Alessandro Bortolon, Eric Emdee, Shaun R Haskey, Ryan Hood, Jerry W Hughes, Tomas Odstrcil, et al.

Understanding poloidal asymmetries in main chamber neutral fueling is essential for characterizing the balance between particle sources and transport in the formation of the H-mode density pedestal. The primary goal of this work is to study changes in these fueling asymmetries and their relationship to divertor recycling across different magnetic configurations in DIII-D lower single-null type-I ELMy H-mode plasmas, using direct measurements of the pedestal ionization source obtained with the LLAMA (Lyman-alpha Measurement Apparatus) diagnostic. By combining these direct measurements of the neutral population and studying how it influences the pedestal density structure and overall pedestal performance, this work aims to provide new insights into the role of neutrals in H-mode pedestal physics. We find that the main chamber ionization on the high-field-side (HFS) strongly dominates when the ion ∇B drift is in the favorable direction. In contrast, reversing the drift direction to the unfavorable direction significantly reduces HFS fueling, that is only partially compensated by a moderate increase in low-field-side (LFS) fueling. The resulting changes in the edge particle source are accompanied by a decrease in pedestal electron density and an increase in pedestal temperature, such that the pedestal pressure remains approximately unchanged. Measurements of divertor recycling and divertor radiation indicate that these fueling asymmetries are also linked to the divertor regime. In particular, inner divertor detachment in the favorable drift configuration decouples main chamber fueling on the HFS from target recycling, while attached divertor conditions in unfavorable drift cases lead to a strong correlation between divertor recycling and main chamber fueling asymmetries. These results suggest that the ion ∇B drift direction, by modifying main chamber fueling, can influence the pedestal density structure and overall pedestal performance.

Efficient laser ion acceleration in near-critical density plasmas in the picosecond pulse regime

Joshua Luoma, Andreas Kemp, Andrew Longman, Dean Rusby, Gennady Shvets

Kilojoule-class short-pulse (< 10 ps) lasers are excellent tools for generating relativistic ion beams, but achieving high laser-to-ion conversion efficiency remains an open challenge. Recent experiments show a significant increase in conversion efficiency when leveraging targets with an average plasma density approaching the relativistic-critical limit. We present an analytical model that explains this increase by extending the theory of target-normal sheath acceleration (TNSA) to include laser penetration of relativistic-critical density targets, enabling energy transfer to the ion front by heating the expanding electron sheath. This physical process increases the sheath field strength and significantly improves ion cutoff energy and laser-to-ion conversion efficiency relative to classical TNSA. The model predicts a maximum ion conversion efficiency of 37% by optimizing laser transmission and target areal density. Key scalings of the model agree with particle-in-cell (PIC) simulations and published experimental data. A series of 2D and 3D simulations of hydrocarbon plasmas confirm the robustness of the acceleration process, demonstrating a path for maximizing ion yields using relativistic-critical targets.

Indirect-Drive Fusion Target Design for Commercial Fusion Energy

C. R. Weber, A. L. Kritcher, S. Bhandarkar, T. Briggs, T. Chapman, T. M. Fears, B. A. Hammel, D. D. -M. Ho, O. Hurricane, B. Kozioziemski, et al.

This paper presents the physics basis for commercially relevant laser indirect-drive (LID) (radiation-driven) inertial fusion energy (IFE) using a 10 MJ laser driver. To date, this approach, proven at the National Ignition Facility (NIF), remains the first and only controlled fusion method to demonstrate the key physics required for fusion energy production, including a self-sustained burning plasma, substantially de-risking the path to commercial fusion energy. Building directly on these results, we present scaled designs to larger target sizes and fusion gains relevant for commercial power generation ($G\sim 26$--$43$). The designs remain close to experimentally demonstrated ignition physics, modifying target components to improve scalability, manufacturability, and cost-effectiveness for fusion energy applications while preserving ignition-relevant implosion physics and fusion power plant compatibility. The baseline platform retains a high-density carbon ablator and clean cryogenic DT fuel layering while extending ignition platforms to substantially larger fuel masses (exceeding 10 times that of current ignition experiments at the NIF) and higher burn fractions ($\sim$40\%) with total areal densities at stagnation of $\sim$3~g/cm$^2$. Benchmarked simulations anchored to NIF ignition experiments (using HYDRA and LASNEX) predict that these designs achieve robust ignition and propagating burn at substantially higher fusion yields (265--427~MJ) with significant ignition margin (2--4$\times$ relative to NIF) against hydrodynamic instabilities and representative power-plant non-idealities, including low-mode asymmetry, polycrystalline DT ice roughness, HDC ablator voids, and target-support and fill-hole perturbations. We also show that implosion symmetry and laser-plasma interactions (LPI) can be controlled with our novel multi-beam configuration using thousands of laser beam-lines.

Self-organized positron reorienting and pinching mechanism for the experimental detection of the linear Breit-Wheeler process

Yutong He, Alexey Arefiev, Mario Manuel, Hui Chen, Christopher Ridgers

The linear Breit-Wheeler (LBW) process ($γ+γ\rightarrow e^{-}+e^{+}$) is a fundamental prediction of quantum electrodynamics, but yet to be observed under laboratory conditions using real photons. In recent years, a few experimental schemes utilizing high-intense ($\sim10^{22}$W/cm$^2$) laser-plasma interactions to observe the LBW process have been proposed. However, a high level of signal-to-noise-ratio are expected in these schemes, hindering the first-ever experimental detection of the LBW process by real photons. In this paper, we present a simple experimental setup which could enhance the expected positron signals by 2-3 orders of magnitude compared to previously proposed schemes, reaching the level of $10^{6}$MeV$^{-1}$str$^{-1}$. Moreover, such high positron signal is achieve in the direction opposite to the laser propagation, where a significantly quieter background is expected compared to the previously focused direction of laser propagation. The key to achieve this result is a newly discovered self-organized positron reorienting and pinching mechanism, enabled by the in-situ strong plasma fields from the laser-plasma interaction.

Exploration of an intrinsic low-collisionality, high performance, grassy ELM regime in the DIII-D tokamak

Zeyu Li, H Q Wang, Xi Chen, Xueqiao Xu, Rongjie Hong, Filipp Khabanov, V. S. Chan, Patrick H Diamond, Brian S Victor, Nami Li, et al.

Recent DIII-D tokamak experiments have demonstrated the integration of intrinsic low-collisionality grassy edge-localized mode (ELM) regime with high-performance hybrid core scenario, possibly offering a core-edge compatible solution for ITER and future fusion reactors. This regime features grassy ELMs (with ELM energy loss over pedestal stored energy <2%) at ITER-relevant pedestal top collisionality (ν_e^*~0.1), ITER similar shape, while maintaining high core confinement (H_98y2~1.5) in non-inductive hybrid scenarios. A small ELM-focused database is constructed to investigate the parametric dependence of the small/grassy ELM regimes. Access and sustainment of this regime appear to be favored by high poloidal beta (β_p>1.5), higher ratio of separatrix-to-pedestal density (n_(e,sep)⁄n_(e,ped) >0.4) and low pedestal top collisionality (υ_(e,ped)^*=0.1-0.4). In this scenario, the pedestal width exceeds the prediction from EPED–KBM scaling, consistent with the expectation of a turbulence-limited pedestal. Linear modeling using ELITE indicates that this grassy ELM regime is along the peeling boundary. Relative to a large-ELM phase, the grassy-ELM phase exhibits a broader inner-target heat-flux width and a substantially reduced ELM-induced transient heat-flux increment. These results motivate further evaluation of low-collisionality grassy ELMs as a potentially reactor-relevant operating regime, while full-duration sustainment and compatibility with divertor detachment remain unresolved.

Evolution of laser-driven magnetic fields from proton tomography

J. Griff-McMahon, V. Valenzuela-Villaseca, C. A. Walsh, S. Malko, B. McCluskey, K. Lezhnin, H. Landsberger, L. Berzak Hopkins, G. Fiksel, M. J. Rosenberg, et al.

Self-generated magnetic fields are commonly produced in high-power laser–plasma interactions. These fields can inhibit plasma heat-flow, which makes them important in inertial fusion and controlled laboratory astrophysics experiments. In this work, we characterize the time evolution of self-generated magnetic fields using multi-view proton tomography at two timings. Tomographic reconstructions of the magnetic field show a clear transition from fields located close to the target at early time to more extended coronal fields at later time. The tomographic inversion and mesh radiography also enable a direct measurement of the magnetic flux evolution. Comparisons with extended-magnetohydrodynamic simulations show only moderate agreement in field structure but good agreement in magnetic flux. This suggests that the field generation model is largely correct under these conditions, while the magnetic transport model requires additional development to reproduce the observed field structure.

DLS-Extended: a reduced model to assess the impact of impurity radiation location on optimal magnetic geometry choices for the STEP divertor

Mike Kryjak, Cyd Cowley, David Moulton, Ryoko Tatsumi Osawa, Stuart Scott Henderson, Omkar Myatra, Benjamin Dudson, Peter Alec Hill, Liam Pattinson, Christopher Paul P Ridgers

Alternative Divertor Configurations (ADCs) often make use of high total flux expansion (fR) and connection length (L∥) to improve the access to and the controllability of detachment, a regime vitally important for reactor-class tokamaks such as the Spherical Tokamak for Energy Production (STEP). Achieving detachment on such high-power devices is enabled through radiation from seeded impurity species such as argon and neon, which can lead to radiative losses throughout the flux tube, altering the impact of magnetic geometry on detachment. The Detachment Location Sensitivity (DLS) model analytically predicts detachment access and sensitivity based on upstream conditions, the magnetic geometry and seeded impurity radiation. It assumes the radiation region to have no spatial extent. In this work, we relax this assumption and introduce DLS-Extended, a new reduced 1D model which can capture the effects of the spatial distribution of impurity radiation along the field line. The prediction of radiation extent was verified against SOLPS-ITER simulations of an initial STEP design with a good match. DLS-Extended predicts that the radiation extent found in STEP weakens the detachment access benefit of fR and strengthens the benefit of L∥, while the detachment stability of the inner and outer legs is increased and decreased, respectively. This is due to a number of novel radiaton-driven effects and has implications for the optimum strike point position in reactor scale devices. These findings highlight the need for more experimental studies in the presence of broad-radiating impurities. DLS-Extended is distributed under the LGPL-3 open source license and is publicly available on GitHub.

Modeling hohlraum wall expansion with higher-moment multi-species fluid model

C. D. Decker, C. J. Bruulsema, W. A. Farmer, J. Harte, D. P. Higginson, A. J. Kemp, W. Riedel, J. S. Ross, D. J. Strozzi, G. Zimmerman

We examine the validity of a multi-species 13-moment fluid approach to modeling hohlraum wall expansion occurring in inertial confinement fusion targets used at the National Ignition Facility. We compare our simulation to experiments conducted at the Omega laser facility [Pape et al., Phys. Rev. Lett. 124, 025003 (2020)], which produced counter-propagating gold–carbon plasmas, a phenomenon arising during hohlraum wall expansion. The two experiments we use to benchmark this fluid model produced counter-propagating gold–carbon plasmas in (1) vacuum and (2) a helium atmosphere. We find that simulations using this advanced fluid model replicate certain aspects of the vacuum experiment, such as material interpenetration, and distinct anisotropic gold and carbon ion temperatures are not captured with single-species lower-moment fluid models. However, our simulations underpredict the temperature of scarce gold ions that penetrate deep into the carbon—a phenomenon potentially requiring a kinetic treatment to model. When applied to the helium gas filled experiment, this advanced fluid model matches the amount of material mixing and gold stagnation point but overpredicts ion temperatures in the gold. Finally, we examine the applicability of single fluid models (as well as lower-order multi-species models) by comparing simulated plasma quantities vs the helium gas-fill density. As expected, we find the models converge as the helium gas density is increased. We find that for gas-fill densities larger than 0.6 mg/cm3 the single fluid model gives similar bulk plasma densities and temperatures as the multi-fluid 13-moment model. However, for gas-fill densities lower than 1.0 mg/cm3, simulations show substantial low-z material mixing in the gold—requiring multi-species modeling. Moreover, accurately modeling the low-z mixing to within 30% requires the full multi-species 13-moment fluid model.

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

The CENTAUR Collaboration, Samuel W. Freiberger, Evan Bursch, Javier Chiriboga, Hiro J. Farre-Kaga, Eliot Felske, Sophia Guizzo, John Labbate, Shreyas Seethalla, Frederick Sheehan, et al.

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.

Macroparticles with different weights relax to different temperatures in Particle-In-Cell simulations

Remi Lehe, Arianna Formenti, Justin R. Angus, Jean-Luc Vay

A distinctive feature of Particle-In-Cell (PIC) simulations is the use of macroparticles, each representing many physical particles. The associated macroparticle weight can vary from one species to another, or even from one macroparticle to another. Here we show that, when macroparticles have different weights, the plasma evolves toward an unphysical thermal equilibrium in which the temperatures are unequal: lower-weight macroparticles reach a higher temperature than physically expected, and higher-weight macroparticles a lower one. We show that this unphysical equilibrium is difficult to avoid, but that reaching it takes time, and that the associated timescale depends on the effective collisionality of the PIC algorithm. We derive an equation that predicts the full time evolution of the temperatures, in good agreement with PIC simulations, and use it to identify strategies to delay the establishment of this unphysical thermal equilibrium.

Neutronic Analyses for the Design of the Chamber of Xcimer’s Athena Fusion Pilot Plant

Francisco Ogando, Robert Earley, Wayne Meier, Rene Raffray, Patrick Sauvan, Jaime Marian, Susana Reyes, Conner Galloway