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Woong-Ryol Lee, Taehyun Tak, Luca Zabeo, Timo Ravensbergen, Anze Zagar, Piotr Perek, Walter Van Herck, Peter De Vries, Seung-Ju Lee, Lana Abadie, et al.
J. L. Peebles, P. V. Heuer, D. H. Barnak, Y. V. Zhang, J. R. Davies
Proton radiography of laser direct-drive spherical implosions has shown anomalous structures that correspond to strong electric or magnetic fields extending throughout the corona. These fields have the ability to affect laser–target interactions and act as an energy sink. To better understand these fields, simplified experiments were conducted in planar geometry on the OMEGA extended performance laser at the Laboratory for Laser Energetics. Field structure was measured using dual-axis proton radiography for experiments that varied target material, target size, pulse shape, and intensity. Proton radiographs were analyzed and quantitatively demonstrate that the growth of these features is dominated by incident laser energy and target Z. The data are consistent with a secondary, electrostatic instability established by the expansion-driven Weibel instability as the primary driver for these fields in these interactions.
D. L. Balabanski, A. S. Cucoaneș, V. Horný, J. F. Ong, S. Popa, V. A. Popescu, P. Tomassini, D. Ursescu
High-power laser systems have the potential to provide a compact and decentralized complementary technology to conventional cyclotrons for the production of medical radioisotopes. Within the Dr. Laser project at Extreme Light Infrastructure - Nuclear Physics, we focus on the production of 11C, a key positron-emitting radionuclide that can be incorporated into a wide range of molecules without altering their biological functionality. This work presents a comprehensive simulation framework for optimizing the entire laser-driven radioisotope production chain. The framework integrates hydrodynamic and particle-in-cell simulations for secondary ion generation from sub-micron liquid targets, as well as nuclear activation calculations for natural and enriched boron. The article also discusses optical techniques for beam focusing and the 11C extraction from target materials, such as boron nitride, boron oxide, and sodium borohydride. The simulation results indicate that current 100-TW-class, 10-Hz laser systems can achieve end-of-irradiation activities of 50–100 MBq. After processing, these activities are expected around 10–20 MBq, which is sufficient for preclinical research studies. Our simulations also show that a system based on 100 TW laser and sub-micron liquid-sheet targets is able to provide stable Target Normal Sheath Acceleration conditions, well suited for efficient laser-driven 11C production.
Suneui Lee, Naoko Ashikawa, Daiki Kikuchi, Takumi Chikada, Yuka Shimizu, Yoshinao Matsunaga, Yoshifumi Kume, Colin Baus, Christian Day, Satoshi Konishi
Xiaodan Cheng, Yiyang Wang, Dongdong Xiong, Hongli Jiang, Bingbing Yin, Yi Yang
T W Morgan, Martin Balden, Jos Scholte, Selanna Roccella, J H You
ITER is anticipated to operate with occasional slow transients, resulting in an increase in expected heat loads to the divertor from 10 to 20 MW m −2 and thus in the surface temperature increasing to > 2000 ◦C. At the same time mitigated ELMs may be expected to strike the surface. To investigate the consequences of this, two sets of experiments were carried out in Magnum-PSI, representing the first experiments to explore this regime. In the first set, seven ITER-like tungsten monoblocks were exposed to either hydrogen plasma, or impurity-seeded hydrogen plasma at a surface temperature of 2050 ◦C. Simultaneously five of the seven monoblocks were exposed to 10 5 ELM-like pulses using a 1 ms duration laser at an energy density of 0.1 − 0.19 MJ m −2 . Very strongly roughened and cracked laser exposed areas with localized melted regions were observed. These areas protruded several hundreds of micrometers above the original surface. In the second experiment, three tungsten plates, with either small, large or no castellations cut into the surface, were exposed to a hydrogen plasma at 2100 ◦C with a wider range of ELM-like pulse numbers (10 2 − 10 5 ) and energies (0.13 − 0.31 MJ m −2 ). The surface was found to evolve by roughening and pre-crack formation towards similar strongly protruding roughened structures as pulse number increased, while increasing the transient energy resulted in increasing the fraction of surface melting observed in the loaded region. Mass loss measurements of the plates indicated that erosion was significant, between 0.05-0.13 nm per pulse for the different plates. Extrapolating these results to ITER suggests that this mass loss rate would be relatively high and would constitute a significant tungsten source.
Evgeniia Ponomareva, Nima Fakhrayi Mofrad, Andrea E Sand
Understanding material behavior under ion bombardment is a crucial aspect in predicting material surface modification and the lifetime of plasma-facing materials in fusion devices. Atomistic simulations are commonly employed to calculate key ion irradiation outcomes such as sputtering and reflection yields. However, within this simulation framework, the energy dissipation of ions in the electronic system is often neglected or treated using a simple velocity-dependent friction force. Although this approximation can be adequate for slow heavy ions, light ions in the sub-keV range lose a substantial fraction of their energy to electronic excitations, requiring a more realistic energy transfer description. Here, we incorporate trajectory-resolved electronic stopping obtained from ab initio calculations into molecular dynamics simulations of hydrogen and helium irradiation of tungsten and iron. We show that density-dependent electronic stopping leads to systematic variations in sputtering yields, especially for tungsten and lower impact energies, whereas reflection yields are found to be largely insensitive to the model choice. Material-dependent features are identified and mainly attributed to different relative contributions of projectile and recoil electronic energy loss. This work provides a physically motivated approach to improve the accuracy of the sputtering data used in plasma-surface interaction modeling.
Nathaniel Chen, Andrew Rothstein, Ricardo Shousha, Hiro Farre-Kaga, Peter Steiner, Azarakhsh Jalalvand, Egemen Kolemen
Machine learning models are increasingly used in feedback control loops for nuclear fusion, where inference speed and predictable timing are critical. We summarize lessons from models deployed for control on the DIII-D tokamak and develop a benchmark to compare inference backends across ten neural networks and model components from fusion control and diagnostic pipelines. For models greater than five million parameters, the CPU backends take tens to thousands of milliseconds, while GPU inference is substantially faster, suggesting an upper limit on CPU-oriented development for control. These results show why the deployment backend must be selected together with the model and its control-cycle budget.
Sidney D. V. Williams, Kevin Mitchell, Ethan Custodio, Dmitri M. Orlov
Magnetic field-line transport in magnetically confined plasmas is commonly modeled as diffusion through stochastic magnetic layers generated by resonant perturbations. However, experimentally relevant magnetic configurations frequently exhibit transport rates that are substantially slower than predicted by fully stochastic models, indicating the presence of unresolved dynamical barriers. In this work, we investigate the role of nested resonant island chains and their associated invariant manifolds in regulating transport within the edge magnetic topology of DIII-D tokamak discharge #171491 subjected to strong n=3 resonant magnetic perturbations. Using field-line tracing, Poincare analysis, and manifold calculations, we identify a hierarchy of homoclinic and heteroclinic tangles associated with both period-one and period-ten hyperbolic points. A systematic construction based on primary intersection points is developed to extract partial transport barriers from the full manifold geometry and divide the chaotic region into dynamically distinct domains. Monte Carlo simulations reveal that field-line escape exhibits a bi-exponential character when only the outermost period-one boundary is considered, reflecting the presence of a long-lived "sticky" region associated with the period-ten island chain. When the inner period-ten boundary is explicitly incorporated into the Monte Carlo, the escape process becomes approximately single exponential and is accurately described by turnstile lobe transport. The measured escape rate agrees with predictions obtained from the turnstile lobe area and the symplectic structure of the field-line map. These results demonstrate that nested tangles provide a quantitative transport skeleton governing magnetic-field-line escape and establish a framework for analyzing transport barriers, cantori, and lobe dynamics in realistic fusion-plasma magnetic configurations.
Hongxuan Zhu, R. Gaur, X. Wei, Z. Lin, A. Bhattacharjee
It is well known that quasi-symmetric (QS) stellarators are isomorphic to tokamaks in terms of their neoclassical-transport properties, and the corresponding transport coefficients can be calculated in the same manner as in tokamaks. However, less is known regarding the turbulent-transport properties of QS stellarators, e.g. the transport coefficients from the ion-temperature-gradient (ITG) mode. In this work, a systematic theory of the ``equivalent tokamaks'' for QS stellarators is presented based on the local gyrokinetic formulation and the near-axis expansion theory. It is shown that to zeroth order in the minor radius, the equivalent tokamaks can be chosen to have circular flux surfaces and can be characterized by three geometric quantities: the aspect ratio, the rotational transform, and the magnetic shear. To achieve first-order accuracy, however, not all QS stellarators have equivalent tokamaks, but good approximations can be found for some cases either as global or local equilibria. Local and global gyrokinetic simulations of ITG transport are performed for a selection of QS configurations, and quantitative agreement in the turbulent transport levels is found between the stellarators and their equivalent tokamaks.
O. P. Morozov, V. A. Kurshakov, I. V. Timofeev
The paper proposes a generalization of the fully implicit energy- and charge-conserving electromagnetic particle-in-cell method to the case where the lightest type of plasma particle (electrons) is described in the drift-kinetic approximation. This allows us to remove the very strict time step limitation of this method requiring to resolve the gyrorotation of electrons. Since the drift-kinetic model is only applicable to light particles whose contribution to plasma polarization is small, this model can be further simplified by neglecting the electron polarization drift and including in Maxwell's equations, in addition to the current of gyrocenters, only the magnetization current. In order for such a hybrid model to retain conservative properties in finite-difference form, a method of self-consistent interpolation of $\nabla B$ in the mirror force from grid to particle and the magnetization vector from particle to grid is proposed. Unlike existing drift-kinetic models, this model is not limited to considering small perturbations near a given equilibrium, and therefore allows one to study the formation of plasma equilibria in regimes with a finite ratio of plasma to magnetic field pressure. Testing of the parallel code implemented in C++ using the PETSc library confirmed the fulfillment of the finite-difference laws of energy and charge conservation, as well as the ability of the model (in the drift-kinetic version for all types of particles) to correctly reproduce the diamagnetic effect and longitudinal ion-acoustic wave.
Gabor Demeter
The plasma channel of the AWAKE plasma wakefield acceleration experiment is created by laser ionization of rubidium vapor in a 10~m long vapor source. Verifying the properties of the channel --- the radius of its fully ionized core and the width of its boundary sheath --- that are vital for ensuring good-quality wakefields is important as is validating numerical models of ionizing pulse propagation. Near-resonant schlieren imaging can detect the plasma channel in the low-density vapor, but there is no direct inversion to obtain channel parameters from the images. We use symbolic regression to find compact analytical formulas that connect measurable properties of the schlieren signal to plasma profile parameters. Using databases of simulated signals generated with three different plasma edge-profile families, we obtain formulas with one and two fitting constants to predict the equivalent channel radius with an accuracy below twice the camera pixel size and formulas to predict the sheath width. Re-evaluating previous measurements with these formulas, we confirm a theoretically predicted power-law relationship between the channel radius and the energy of the ionizing laser pulse. The formulas can be adapted to new experimental conditions by refitting the constants on a modest set of new samples, providing a lightweight, interpretable alternative to evaluation with deep neural networks.
Katharina Noesges, Tim Bolles, Máté Vass, Ihor Korolov, Thomas Mussenbrock
We develop a nonlinear self-consistent two-sheath circuit model for low-pressure capacitively coupled plasmas that applies to geometrically symmetric as well as asymmetric discharges. The quasineutral plasma bulk is represented by an inductive-resistive element and coupled to stationary particle and electron-energy balances. Both boundary sheaths are treated dynamically using a lncosh sheath charge-voltage model with bounded differential elastance, based on a Riccati closure for the differential sheath width. The model recovers the quadratic depletion-sheath relation in the small-charge limit, while the characteristic sheath scales are determined from the RF-averaged sheath voltages using a collisionless Child-Langmuir/Bohm closure. The resulting four-variable RF subsystem contains the two sheath charges, the blocking-capacitor voltage, and the discharge current. In the symmetric monofrequent limit, the two sheath nonlinearities compensate strongly and the dc self-bias vanishes, whereas geometrical or electrical asymmetry breaks this compensation and enhances harmonic generation and plasma-series-resonance oscillations.
J. M. Finn, E. G. Evstatiev
A new method of analyzing grid, or aliasing, instabilities is described, in particle-in-cell (PIC) codes for plasma kinetic theory. It starts with a meshfree approach with $N_{p}$ macroparticles, with $N_{p}<\infty$. Linearization of these equations is done about a uniform density non-drifting equilibrium prescribed by macroparticles on a uniform \emph{lattice}. Macroparticle positions perturbations are induced about this lattice. The resulting linear equations are analyzed for stability and dispersion relations. The linearized equations are then discretized on a grid of $N_{g}$ points with $N_{g} \le N_{p}$, in both momentum conserving (MCP) and energy conserving (ECP) formulations. For a cold stationary plasma, this leads a \emph{dynamical matrix}. For ECP, the matrix is symmetric and positive definite (SPD) and immediately shows stability. For MCP, the dynamical matrix is neither symmetric nor positive definite, preventing immediate conclusions on stability. For both MCP and ECP and for a cold stationary plasma, the resulting matrix elements vary relative to the meshfree value, a result of aliasing. This deviation is periodic in the displacement of the particle lattice relative to the grid. In the MCP discretization, eigenvalues occur in complex conjugate pairs for general placement of the particle lattice relative to the grid. These conjugate pairs indicate nonnegative growth rates. The nature of the aliasing is studied and shown to be related to the trapezoidal rule error over the grid. The scaling of the linear growth rates with respect to $N_{g}$ and especially with respect to $N_{ppc}$ is studied. These analytical results are compared with PIC simulations and found to be in excellent agreement. The connection with a cold drifting \emph{beam} is discussed briefly, specifically, grid-induced instabilities for both the MCP and ECP.
Bernard Parent, Brendan Perry
Fluid models of discharges fail where electron runaway dominates. On the left branch of the Paschen curve and in high-voltage cathode sheaths, the electron population splits into a thermal bulk and a ballistic beam that no single electron fluid can represent. We present a multigroup fluid model that divides ballistic electrons into groups indexed by their birth potential: the electrostatic potential at which they were liberated, offset by the forward kinetic energy they carried. Because the birth potential is a constant of ballistic motion, the electric field never moves electrons between groups and only collisions do. Each group requires a single continuity equation because its velocity follows algebraically from the local potential, and the number of groups is set by the energy resolution alone independently of the mesh. A runaway criterion based on the local field assigns each newly liberated electron to the bulk or a ballistic group, so the model applies even where no sheath exists. Benchmarked against a kinetic solver in argon, it reproduces the left branch of the Paschen curve---which local-field and local-energy models miss or misplace---and tracks the current--voltage characteristic of a cathode sheath to within a few percent at reduced fields exceeding $10^4$ Td, where the local-energy model overpredicts the current up to thirtyfold. Since as few as 10 to 30 groups suffice, depending on the field, the approach brings near-kinetic accuracy to multi-species computational fluid dynamics solvers at the cost of that many additional species, without added velocity-space dimensions or particle noise.
J.H. Jeong, S.C. Hong, B.S. Kim, J.Y. Jang, W. Cho, J.G. Kwak, S.J. Wang
Tianyu Zheng, Chunsen Ye, Zhengyue Wang, Guannan Chen, Yu Qiu, Pengrui Zhang, Wei Sun, Jinhe Sun
V. V. Mikhailenko, V. S. Mikhailenko, Hae June Lee
Electrostatic parametric instabilities in the inhomogeneous near-scrape-off layer (SOL) tokamak plasma, driven by the combined action of a high-harmonic fast wave (HHFW) with a frequency near the 30th ion-cyclotron harmonic and electron and ion temperature gradients, are investigated numerically. The results indicate the parametric decay of the HHFW into a high-harmonic ion-cyclotron (HHIC) (Bernstein) wave and HHIC quasimode. The instability exists only over a finite range of HHFW wavelengths. The development of the parametric HHIC quasimode decay instability in the near-SOL plasma leads to the onset of parametric turbulence accompanied by anisotropic ion heating, with the ion heating rate across the magnetic field significantly exceeding that along the magnetic field.
M. Bonitz, H. Kählert, D. Krimans, C. Makait, P. Hamann, J. Vorberger, Zh. Moldabekov, S. X. Hu, V. V. Karasiev, D. Kraus, et al.
The year 2025 has been designated by UNESCO as the International Year of Quantum Science and Technology. One hundred and twenty-five years ago, Max Planck's discovery of radiation quanta started the quantum era, and 100 years ago, quantum mechanics was developed by Schrödinger, Heisenberg, Bohr, Pauli, Dirac, Born, Fermi, and many others. By now, quantum mechanics is the theoretical foundation of most fields of physics and chemistry, and it is the basis for modern nanotechnology. How about plasma physics? How important are quantum effects in plasmas? In what experiments are quantum effects observed, and where do they govern the behavior of plasmas? How can these effects be treated theoretically and via computer simulations? Starting with a brief historical overview, we discuss the broad parameter range that is the characteristic of plasmas and outline where quantum effects are relevant. This is the case primarily for warm dense matter and inertial fusion plasmas. We provide an overview of the theoretical quantum methods that are available for these dense plasmas and how their respective advantages can be combined in order to achieve predictive capability. The key is a downfolding approach that is based on first-principles simulations.
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.
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