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Drift-kinetic PIC model for simulations of longitudinal plasma confinement in mirror traps

V. V. Glinskiy, I. V. Timofeev, V. V. Prikhodko

Physics of PlasmastodayPlasma & Confinement AI, Modeling & Simulation

The paper presents a 1D2V electrostatic particle-in-cell (PIC) model with a drift-kinetic description of all particle types, aiming to simulate classical longitudinal plasma transport in axially symmetric open traps. The model generalizes the semi-implicit PIC method with exact conservation of energy and charge to the case of collisional plasma and adapts it to boundary conditions on perfectly conducting walls with a floating potential. The implementation of Coulomb collisions is tested on the problem of temperature relaxation in a two-component plasma and demonstrates good agreement with the analytical theory. Since the quasi-neutral approximation is not assumed and Ampère's law is used to find the electric field, the model is able to correctly reproduce the ambipolar electric potential profile up to the walls. At the same time, the main advantage of implicit PIC simulations—the ability to use large grid steps, many times larger than the Debye radius—does not prevent the key elements of the Debye sheath physics from being reproduced correctly. In particular, the magnitude of the near-wall drop in electric potential as well as the Bohm criterion are found to be independent of whether the Debye scale is resolved or not. A comparison of stationary plasma profiles formed in a mirror trap in the presence of a constant particle source with similar profiles from the hybrid (fluid electrons) code MIDAS showed that the electron temperature, potential, and density of the confined plasma may differ by a noticeable amount, up to 15%–20%.

Two-fluid boundary turbulence simulations in reversed field pinch plasmas

M. Giacomin, B. Momo, I. Predebon, N. Vianello, M. Zuin

Journal of Plasma PhysicstodayPlasma & Confinement AI, Modeling & Simulation

Turbulent transport in magnetic confinement fusion devices governs the overall plasma confinement properties and regulates the plasma-material interaction at the first wall. In the plasma boundary, turbulence is typically investigated through three-dimensional two-fluid flux-driven turbulence simulations. In this work, the GBS boundary turbulence code is extended to enable turbulence simulations in reversed field pinch configurations, encompassing the reversal surface and an arbitrary level of magnetic chaos. The differential operators implemented in the code are modified to avoid the approximations of large-aspect ratio and weak poloidal magnetic field. Three-dimensional Poisson and Ampere solvers are implemented to allow for turbulence simulations in conditions of partially or fully disrupted magnetic flux surfaces. This modified version of the GBS code is then applied to simulate turbulence in the boundary of RFX-mod reversed field pinch plasmas. Turbulent eddies across the reversal surface show properties similar to those typically found in tokamak boundary turbulence simulations. Despite the good agreement found with experimental measurements, these simulations reveal a significant limitation of the fluid-based turbulence modeling of the edge region in reversed field pinch plasmas, which arises from the intrinsically short parallel connection length. This conclusion is also supported by a linear gyrokinetic analysis that identifies trapped electron modes as the dominant microinstability in this region.

National Research CouncilConsorzio RFX

Yesterday

Stellarator island divertor shape optimization for reduced peak heat fluxes

Avigdor Veksler, Aaron Bader, Heinke Frerichs, Elizabeth Joy Paul

Plasma Physics and Controlled FusionyesterdayPlasma & Confinement Materials & Plasma-Facing Components AI, Modeling & Simulation

An automated algorithm to construct island divertors for stellarators is presented and is used to find divertors that meet heat load requirements determined by material limits. The algorithm uses just two initial conditions: two starting coordinates on the island separatrix chosen by the user. We leverage the simplicity of the algorithm to explore the divertor parameter space in a fixed magnetic equilibrium. Heat loads are approximated using the field line diffusion model implemented in the FLARE code. Divertor solutions that satisfy heat load requirements while maintaining a high power fraction captured are found using a parameter scan and a Bayesian optimization routine. The optimization finds divertors that perform the same as the parameter scan, but with a 75% reduction in computational cost. The resulting divertors satisfy heat load requirements across varying cross-field heat diffusivities. Optimization over various islands in the equilibrium shows that low-elongation islands are the easiest to find divertors that satisfy heat flux requirements. This algorithm presents a simple parameterization for island divertors and facilitates further physics and optimization studies.

University of Wisconsin MadisonType One Energy

Magnetic energy dissipation and electron energy transfer in distinct sub-regions within the magnetic reconnection diffusion region

Dongke Chen, Can Huang, Junyu Zhao, Yasong S Ge, A M Du

Plasma Physics and Controlled FusionyesterdayAI, Modeling & Simulation

Magnetic reconnection efficiently converts magnetic energy into plasma kinetic energy. With magnetic free energy stored in the current layer, previous numerical simulation studies, which focused on fixed localized regions surrounding X-lines, have demonstrated that the released magnetic energy is primarily converted into enthalpy flux and redirected Poynting flux. Electron acceleration has been widely observed in the separatrix, current layer, and exhaust. However, a quantitative analysis of how individual physical terms characterize energy transfer across distinct regions and evolutionary stages remains lacking. Using full-particle simulations of single-X-line reconnection, we dynamically partition the reconnection domain into the inflow region (IR), separatrix region (SR), electron current layer (ECL), and exhaust region (ER) based on naturally occurring physical boundaries. Our results show that: 1. During antiparallel reconnection, the magnetic energy in the ECL remains nearly constant, where dissipation is balanced by inflow injection; in guide-field reconnection, the ECL exhibits a net magnetic energy increase. 2. Although the ECL features the highest energy conversion power density, the SR and ER together dominate the integrated energy conversion over the analyzed reconnection process. The introduction of a guide field reduces the fractional contribution of energy conversion within the ECL. 3. Compared with other sub-regions, the ECL channels a larger fraction of converted magnetic energy to electrons. The presence of a guide field elevates the global fraction of electron energy gain across the entire reconnection diffusion region. This work explicitly quantifies the contribution distribution of various conversion and transport terms to the energy budget in each sub-region, establishing a new quantitative framework for understanding energy release during magnetic reconnection.

Effects of fusion-born alphas on MHD instabilities in burning plasmas with spin-polarized fuels

Yueqiang Liu

Plasma Physics and Controlled FusionyesterdayPlasma & Confinement AI, Modeling & Simulation

A new magnetohydrodynamic-kinetic hybrid model is developed with inclusion of an anisotropic (in the particle pitch) particle distribution for fusion-born alphas, for the purpose of modeling drift-kinetic effects of the latter on macroscopic instabilities in burning plasmas with spin-polarized (SP) deuterium-tritium (D-T) fuels. The new model is applied to numerically analyze both the internal kink (IK) and the resistive wall mode (RWM) stability, based on two ITER D-T scenarios. The adiabatic contribution of alphas generally destabilize the IK and the RWM, with the anisotropic model generally yielding a stronger destabilization compared to the isotropic counterpart. Partial stabilization of the IK mode is generally obtained with inclusion of the non-adiabatic resonance contributions (of various sorts) of alphas in the zero-orbit width approximation. Adding the finite-orbit width correction brings full stabilization of the mode. Destabilization of an IK-peeling branch is however computed, accompanied by a substantial modification to the mode eigenfunction by anisotropic alphas. Non-adiabatic contributions of alphas are also generally stabilizing for the RWM, independent of the assumed (isotropic versus anisotropic) particle distribution model. On the other hand, triggering of a high-frequency fishbone-like mode is found in the ITER 10 MA scenario with finite plasma flow, by the adiabatic contribution of alphas due to SP fuels.

General Atomics

Characteristics of monotonic sheaths near a wall with grazing magnetic incidence

Alessandro Geraldini, Robert J. Ewart, Stephan Brunner, Felix Parra

Plasma Physics and Controlled FusionyesterdayPlasma & Confinement Materials & Plasma-Facing Components AI, Modeling & Simulation

We consider a magnetised plasma in contact with an absorbing planar wall, where the angle α between the magnetic field and the wall is small, α ≪ 1 (in radians), and the plasma is uniform in the directions tangential to the wall. The finite ratio γ of the characteristic electron gyroradius ρ e to the Debye length λ D , γ = ρ e /λ D, is retained via a grazing-incidence (α ≪ 1) gyrokinetic treatment [1,2]. Building on a previously developed iterative scheme [2,3] to solve for the steady-state electrostatic potential in the quasineutral magnetic presheath of width ∼ ρ S , we developed a scheme that simultaneously solves for both the presheath and the non-neutral Debye sheath of width ∼ λ D in the limit λ D /ρ S → 0, provided that the electrostatic potential solution is monotonic. The code, called GYRAZE, thus provides the energy-angle distribution of ions at the wall and the velocity distributions of electrons reflected by the wall for different values of wall potential.

Swiss Federal Institute of Technology LausannePrinceton UniversityPrinceton Plasma Physics Laboratory

Optimization of ARC-like tokamak stationary operation via core plasma composition and boundary shaping

Audrey Saltzman, Pablo Rodriguez-Fernandez, Aaron Ho, Garud Snoep, Jiyun Han, Jo Hall, Michail Savvas Anastopoulos Tzanis, Jon Hillesheim, Alexander J Creely, Philip B Snyder, et al.

Plasma Physics and Controlled FusionyesterdayPlasma & Confinement AI, Modeling & Simulation

Impurity composition, plasma shape, and pedestal density all provide strong levers on fusion power. Here, we explore the ways in which their variation changes fusion power and seek to find the optimum of these parameters. The key impacts of these variables are through changes in the core turbulent transport, the density of the fuel species, the pedestal pressure, and the plasma volume. ITG stabilization due to increased amounts of impurities is observed. The dependence of all of these parameters on the pedestal pressure is especially complicated because of the separate impacts on the peeling and ballooning modes, which can each limit the pedestal. Optimization of this multidimensional operating space is enabled by the use of Bayesian optimization, resulting in an operating point similar to ARC V3A with ∼30% more fusion power and a higher fusion power density. Increased shaping parameters, including elongation, triangularity, and squareness are all beneficial, as is high Zeff . When elongation is also allowed to vary, a ∼65% increase in fusion power can be achieved. While not commonly considered, we find squareness is an important lever on fusion power. The plasma performance is limited by the Greenwald density limit constraint. The workflow developed here and demonstrated with the example of ARC V3A can readily be applied to other tokamak designs.

Massachusetts Institute of TechnologyCommonwealth Fusion SystemsOak Ridge National Laboratory

Real-Time Monitoring of Deuterium-Tritium Fusion Neutron Production for Fusion–Fission Hybrid Reactors Based on Reversed-Field Pinch Plasmas

Lidia Piron, Marco Gobbin, Nicolò Ferron, Matteo Baruzzo, Eric Fredrickson, Zamir Ghani, Krassimir K. K Kirov, Morten Lennholm, Roberto Piovan, P Martin

Plasma Physics and Controlled FusionyesterdayPlasma & Confinement Control & Diagnostics AI, Modeling & Simulation

Results from the last DD and DT JET campaigns in the framework of the EUROfusion Tokamak Exploitation Work Package Activity" 2026 submitted to Nuclear FusionIn this work, we present a real-time algorithm for use in fusion-fission hybrid reactors to monitor the fast neutron production (neutron rate) generated by Deuterium-Tritium fusion reactions. The algorithm, based on insights from TFTR and JETD-Tritium operations, tracks continuously the neutrons 14.1 MeV. If this metric falls below a threshold value, due, for example, to fuel dilution, the onset of MHD instabilities or temporary actuator unavailability, the algorithm can raise an alarm. This enables plasma recovery when actuator capability permits, or ensures safe plasma termination when recovery is not feasible. The role of alpha particles is examined through test cases of the algorithm, while their confinement properties are investigated using the ORBIT code under various RFP magnetic field configurations.

Consorzio RFXUnited Kingdom Atomic Energy AuthorityNational Research Council

Control of Runaway Electron Current by Magnetic Chaos During Reconnection in Post-Disruption Plasmas

DARIO BORGOGNO, Daniela Grasso, Lovepreet Singh

Plasma Physics and Controlled FusionyesterdayPlasma & Confinement Control & Diagnostics AI, Modeling & Simulation

We numerically investigate the mutual interaction between runaway electron (RE) current and magnetic reconnection in a post-disruption plasma configuration where the plasma current is entirely carried by REs. Extending previous two-dimensional studies to full three-dimensional geometry, we show that the presence of RE current enhances the reconnection process, resulting in a reconnected area approximately 50% larger than in the absence of REs. In the 3D regime, the nonlinear interaction among multiple unstable tearing modes with different helicities drives the development of magnetic chaos, which rapidly spreads from the island separatrices to the entire reconnected region. Once global chaos is established, the chaotic magnetic field lines efficiently redistribute the RE current, leading to a progressive flattening of its radial profile. This occurs over a timescale of approximately 300τ A , corresponding to approximately 10µs for typical JET parameters, in quantitative agreement with experimental observations. These results provide a consistent physical framework for interpreting RE redistribution and loss in post-disruption plasmas, with direct implications for disruption mitigation strategies in future high-current devices such as ITER.

Polytechnic University of TurinMassachusetts Institute of Technology

On the timescales of controlled termination of tokamak plasmas

Simon Van Mulders, Olivier Sauter

Plasma Physics and Controlled FusionyesterdayPlasma & Confinement Control & Diagnostics AI, Modeling & Simulation

The RAPTOR code is used to model how the time required for controlled termination of Ohmic plasmas scales from present tokamaks like TCV and JET, to reactor-grade tokamaks like ITER and DEMO. We show that ramping the plasma current Ip down to 20% of the flat-top value over a time Δt ramp−down =τLR=Li/R, with internal inductance Li and resistance R evaluated at flat-top conditions, results in an approximately self-similar peaking of the current density for these four tokamaks, indicating the adequacy of τLR as a relevant timescale for cross-machine comparison, yielding τLR= 0.033s (TCV), 2.87s (JET), 63.2s (ITER) and 166.9s (DEMO). Note that τLR is easy to evaluate, both in systems codes and on a real-time control system. For the simulated ramp-downs with Δt ramp−down =τLR, the end-of-ramp-down normalized internal inductance ℓi3 is limited below 2. An Ip ramp-down faster than τLR=Li/R requires a reversal of the boundary loop voltage and leads to the formation of a broad plasma layer carrying current in the direction opposite to the total plasma current, concomitant with ℓi3>2, a central region with low magnetic shear and strongly peaked pressure profiles. Significant reduction of plasma volume and elongation, as foreseen for ITER and DEMO, is shown to counteract the reversal of current density and the ℓi3 increase, while easing vertical stability control, potentially enabling faster Ip ramp-down scenarios. Experimental and theoretical studies should be performed to test the feasibility of such fast termination scenarios, notably with respect to vertical position control, shape control and (resistive) beta limits. An analytical model is proposed to estimate τLR based on 0D engineering parameters. For burning plasmas, ITER baseline simulations show that a termination within 150% of the L-mode τLR can avoid negative edge current density and a large ℓi3 increase for an HL transition 1/3rd into the ramp-down.

Swiss Federal Institute of Technology Lausanne

Numerical study on the effects of conducting wall geometry and plasma toroidal rotation on resistive wall mode in CFETR

Yu Jing, Shilong Li, Yue Liu

Plasma Physics and Controlled FusionyesterdayPlasma & Confinement AI, Modeling & Simulation

Based on a 13 MA hybrid equilibrium of the CFETR tokamak, the MARS code is employed to investigate the effects of conducting wall geometry and plasma toroidal rotation on resistive wall mode (RWM) stability. Realistic wall configurations with localized geometric modifications on the low-field side (LFS), high-field side (HFS), and upper and lower poloidal ends are considered, together with a sheared toroidal rotation profile relevant to the CFETR 13 MA hybrid scenario and a uniform rotation profile for comparison. The results show that wall geometry variations on the LFS have a more significant influence on RWM stability than modifications in other wall regions, and RWM stability is highly sensitive to plasma toroidal rotation. To clarify the role of rotation-profile structure, several sheared rotation profiles with different peak positions and radial widths are designed and analyzed. Across the uniform-rotation case and the designed sheared-rotation cases, elongation of the LFS conducting wall reduces the critical rotation frequency required for RWM stabilization relative to the conformal and actual conducting wall configurations. These results indicate a coupling between LFS conducting wall geometry and plasma rotation in determining the RWM stability boundary in the CFETR 13 MA hybrid scenario.

Second quantization of the nonlinear Vlasov–Poisson system for quantum computation

Michael Quackenbush May, Hong Qin

Plasma Physics and Controlled FusionyesterdayAI, Modeling & Simulation

Current quantum algorithms for plasma physics require linearized systems, and quantum algorithms for nonlinear systems are highly system-specific. Second quantization, which is system-agnostic, has the exceptional capacity to render many systems suitable for quantum computation, i.e. finite-dimensional, linear, and unitary. We show how the Fourier mode-truncated Schrödinger–Poisson representation of the Vlasov–Poisson system can be second quantized into a finite-dimensional Hamiltonian system. With three- and five-mode examples, the second quantized system is shown to reproduce corresponding nonlinear dynamics in the Schrödinger–Poisson system in the simulated time intervals. Parallel integration of wide distributions of initial conditions of the Vlasov–Poisson system has the potential to be efficiently computable using the proposed second quantized model on a quantum computer.

Princeton Plasma Physics LaboratoryPrinceton University

Design of low-entropy compression experiment using uranium ion beam generated by the existing GSI accelerator SIS18

Naeem A Tahir, Z Major, S A Piriz, Paul Neumayer, Antonio Roberto Piriz, Vincent Bagnoud

Plasma Physics and Controlled FusionyesterdayAI, Modeling & Simulation Inertial Fusion & HEDP

In this paper, we present 2D hydrodynamic simulations of implosion of a multi-layered disc shaped target, that is driven by an intense uranium beam, which matches the parameters of the currently available beam at the GSI heavy ion synchrotron, SIS18. The purpose of these simulations is to assess the feasibility of a LAPLAS type implosion experiment, at the SIS18 accelerator facility. The target is comprised of a carbon sample that is enclosed in a tungsten ring. One face of the target is irradiated with the SIS18 beam, that has a circular focal spot, while the particle energy is 400 MeV/u. The thickness of the target is chosen in such a way that the beam is completely stopped in the tungsten region, whereas, it loses part of its energy in the carbon part, and escapes from the opposite face of the target. This means that the Bragg peak lies in the outer tungsten region, while it does not lie inside the carbon sample. In such a configuration, the higher pressure generated in the Bragg peak region due to higher energy deposition, can be utilized to achieve a higher compression in the sample, below the Bragg peak region. Our simulations show that with a suitable focal spot size, the carbon sample in that region can be transformed into the diamond phase.

Optimization of room-temperature targets for laser direct-drive fusion

QianLei Du, Fuyuan Wu, Rafael Ramis, Jie Zhang

Nuclear FusionyesterdayAI, Modeling & Simulation Inertial Fusion & HEDP

Fusion ignition is a critical milestone toward the realization of fusion energy. For laser direct-drive fusion, the cryogenic deuterium-tritium (DT) targets encounter considerable fabrication and delivery challenges, such as the survival of cryogenic targets during the injection and flight in a reactor chamber. To mitigate the cryogenic bottlenecks, we explore the potential of room-temperature solid targets composed of low-Z elements and hydrogen isotopes via machine-learning optimization. In this work, a multi-fuel module is developed with the one-dimensional hydrodynamics code (MULTI-IFE) to simulate multiple room-temperature fuels. By optimizing the target structure and laser waveform parameters, a calculated fusion energy gain of 10.18 can be achieved with 2.35 MJ of laser energy, despite the additional radiative and conductive losses that may arise from the low-Z ions in the room-temperature target. A thicker target driven by a laser energy of 9.4 MJ is able to produce 109.7 MJ fusion energy with a maximum in-flight-aspect-ratio (IFAR) smaller than 17. During the fusion burning, tritium breeding via neutron–lithium reactions increases the fusion energy output for the LiBD₂T₂ target by 5.2%. This work provides a reference for the ongoing development of direct-drive repetition-rate laser fusion energy.

Beam filamentation instability drives deuterium-tritium fusion with polarized neutron emission

Guanqi Qiu, Deji Liu, Dongchi Cai, Ronghao Hu, Zheng Gong, Xueqing Yan

arXiv (physics.plasm-ph)yesterdayAI, Modeling & Simulation Inertial Fusion & HEDP

Filamentation instabilities are generally regarded as detrimental to fast-electron transport in fusion plasmas because they increase beam divergence and redistribute deposited energy. Here, we use two-dimensional particle-in-cell simulations coupled to a spin-dependent deuterium-tritium fusion module to investigate whether filamentation can instead transfer energy from counterstreaming electrons to prepolarized fusion ions. The simulations show that magnetic filaments, together with longitudinal inductive and transverse charge-separation electric fields, accelerate initially stationary deuterons and tritons to energies at which fusion reactions occur. Over the parameter range examined, the calculated neutron yield increases with the saturated magnetic-field energy. The model further predicts anisotropic, spin-resolved neutron emission. The direction of maximum neutron polarization is approximately perpendicular to the dominant deuterium-tritium collision direction and evolves with the angular distribution of the reacting ions. These results suggest that beam filamentation can couple relativistic-electron energy to fusion ions and that polarized neutron emission may provide a reaction-weighted signature of the underlying plasma dynamics.

The Richtmyer-Meshkov Instability of Thermal, Isotope, and Species Interfaces in a five-moment multi-fluid plasma

K. C. Tapinou, V. Wheatley, D. Bond, Ingo Jahn

arXiv (physics.plasm-ph)yesterdayAI, Modeling & Simulation Inertial Fusion & HEDP

The Richtmyer-Meshkov instability (RMI) results from the impulsive acceleration of a density interface where either it or the acceleration is perturbed. Density interfaces may arise due to a change in gas species, isotope, temperature or a combination of these. We computationally investigate the effect of interface type on the plasma RMI, which is relevant for a range of applications, including inertial confinement fusion. We simulate the evolution of single-mode perturbed thermal, species and isotope interfaces in an ideal ion-electron plasma using the multi-fluid plasma (MFP) model. We find that in the MFP model, the evolution of different types of interface differ significantly, in contrast to single-fluid models where they behave similarly if the Atwood number is matched. The thermal and species interfaces produce the most severe response to shock acceleration, experiencing the secondary instabilities and enhanced primary mode growth. The isotope interface evolution is restrained in comparison to the former cases, resembling the response predicted by single-fluid models. The determining factor in the severity of the MFP RMI is the density ratio across the initial interface in the electron fluid, which is unity for an isotope interface. We observe that as the density ratio across the electron interface decreases, so do the magnitudes of the self-generated fields and consequently the severity of the growth amplification. Generally, the evolution of the RMI with different types of interface becomes more similar as the level of coupling between the ion and electron fluids is increased, characterised by reducing the plasma non-dimensional skin depth

Direct Numerical Comparison of Data Driving Strategies for Solar Flux Emergence in Magnetohydrodynamics: B-driving vs E-driving

Kyriakos Christos Tapinou, Axel Raboonik, David I. Pontin, Michael S. Wheatland, Mark C. M. Cheung, Hannah Schunker

arXiv (astro-ph.SR)yesterdayAI, Modeling & Simulation

Data driving is an important technique for modelling realistic magnetic field and plasma evolution in regions of the solar atmosphere where direct observations are either absent or insufficient. This technique uses observational data to drive the evolution of the simulation, while the modelled solar atmosphere evolves self-consistently. A common approach for data-driven simulations is B-driving, which uses the observed magnetic field at the surface of the Sun as a boundary condition. However, only prescribing B at the boundary does not generally determine its numerical evolution when the update depends on the reconstructed interface states or fluxes. To address this, E-driving provides an alternative approach by instead driving the simulation using the electric field on the boundary, which directly controls the magnetic field update. We systematically quantify the performance of B- and E-driving in a controlled set of numerical experiments. We test both methods in the Athena++ solver using (a) an analytical flux-emergence model (Y. Fan & S. Gibson 2003) and (b) a "ground-truth" reference simulation (S. Toriumi & S. Takasao 2017; S. Toriumi et al. 2020) of active region emergence. For E-driving, the electric field is incorporated into Athena++'s constrained-transport (CT) update, preserving the discrete solenoidal constraint. We assess the reproduction of (i) boundary magnetic flux, (ii) relative magnetic helicity, (iii) total and free magnetic energy, and (iv) magnetic morphology. Across these diagnostics, E-driving gives better overall reproduction of the reference evolution than B-driving. For the configurations considered in an Eulerian CT scheme, E-driving is more accurate than the commonly used ghost-cell B-driving strategies and simpler than constructing an update-consistent B-driving boundary condition.

Imaging ablator-fuel mix of hot spot in inertial confinement fusion via resonant X-ray absorption using an X-ray free electron laser

Lingen Huang, Long Yang, Alejandro Laso Garcia, Oliver S. Humphries, Michal Šmíd, Mikhail Mishchenko, Victorien Bouffetier, Carsten Baehtz, Erik Brambrink, Samuele D. Di Dio Cafiso, et al.

arXiv (physics.plasm-ph)yesterdayControl & Diagnostics AI, Modeling & Simulation Inertial Fusion & HEDP

A novel diagnostic is proposed for active probing of doped inertial confinement fusion (ICF) spherical shells, enabling direct measurements of mix and burn dynamics at next-generation implosion facilities via resonant absorption imaging with an X-ray free-electron laser (XFEL). The charge-state-sensitive imaging of embedded dopants provides spatiotemporally resolved constraints on ionization, opacity, and material mix seeded by hydrodynamic instabilities in stagnated plasmas. Proof-of-principle experiments demonstrating resonant XFEL probing of hot spot in coated copper wires driven by an ultra-short relativistic laser pulse, have established the feasibility of this approach. Furthermore, atomic and radiation-hydrodynamic simulations, combined with synthetic resonant X-ray absorption imaging, extend the diagnostic concept to laser-driven direct-drive ICF shells with copper-doped ablators and radiation-driven indirect-drive shells with tungsten-doped ablators. This unique approach, combining a laser-driven implosion facility with a high-brightness XFEL, could enable precise measurements of fusion-relevant hot dense plasmas at multi-keV temperatures, provide stringent benchmarks for radiation-hydrodynamics models, and advance the realization of inertial fusion energy.

Kinetic wave activity and proton heating in 3D hybrid simulations of decaying balanced and imbalanced Alfvénic turbulence

C. A. González, T. A. Bowen, A. Mallet, A. Tenerani

arXiv (physics.plasm-ph)yesterdayAI, Modeling & Simulation

We investigate turbulent dynamics from large to sub-proton scales, the kinetic wave activity and resulting proton heating in three-dimensional hybrid-kinetic simulations of freely decaying balanced and imbalanced Alfvénic turbulence. We considered initial fluctuations with same amplitude and spectral shape but different degrees of correlation between velocity and magnetic field perturbations (i.e., normalized cross-helicity), which define the corresponding turbulent regimes. We found that our balanced turbulence simulation exhibits stronger energy dissipation and more efficient proton heating compared to the imbalanced case. On the large fluid scales, the two simulations undergo distinct nonlinear dynamic and energy cascades, leading to different spectral properties once turbulence is well developed. At sub-proton scales, fluctuations are predominantly oblique with right-hand polarization, a property common to both regimes. In contrast, only the imbalanced simulation displays substantial wave power parallel to the guide magnetic field, with clear signatures of ion-cyclotron waves. These waves are excited in regions with strong temperature anisotropy that arise during the initial nonlinear steepening of Alfvénic fluctuations, a process that simultaneously generates parallel-propagating proton beams. Furthermore, non-Maxwellian features emerge in velocity space, indicating different heating mechanisms in balanced and imbalanced regimes and providing evidence for the role of kinetic instabilities in regulating the turbulent dynamics. Overall, these results show that cross-helicity is a key parameter controlling the large-scale evolution, kinetic activity, and energy dissipation in collisionless turbulent plasmas.

A neural characteristic mapping method: Lagrangian PINNs based on flow maps for transport-dominated problems

Martin Campos Pinto, Victor Fournet, Emmanuel Franck, Philipp Krah, Victor Michel-Dansac

arXiv (math.NA)yesterdayAI, Modeling & Simulation

We propose a novel numerical method for transport-dominated problems using Physics-Informed Neural Networks (PINNs). In such problems, the solution can develop large gradients and fine structures over time. This is particularly challenging for classical PINNs, which represent the solution itself. In our approach, rather than the solution, we choose to approximate the flow map of the equation. The solution is then recovered by pulling back the exact initial condition, so that its fine structures are produced by the map rather than represented by the network. Our method is constructed by approximating the flow map of the equation with multiple PINNs, one per time subinterval, and composing them using the semigroup property of the flow, so that each network only has to represent a map close to the identity. To highlight the advantages of our method compared to classical PINNs or classical numerical schemes, we present numerical results on the linear advection equation, the incompressible Euler equation in vorticity formulation, the Vlasov-Poisson equation, and finally a drift-kinetic equation.