Wednesday, 4 November 2026: 11 new papers, 4 cross-lists and 6 replacements in Plasma Physics (physics.plasm-ph), in arXiv's order, with full abstracts. The same papers as arXiv's own listing, set as a newspaper; every title links to arXiv.
The transmission of shock waves generated by underwater electrical foil explosions across solid materials with varying acoustic impedances and thicknesses is investigated. Copper foils with thicknesses of 10 and 18 $\mu$m are exploded on planar substrates of aluminum, copper, and 3D-printed PLA with thicknesses up to 5 mm using a multi-gigawatt pulsed-power electrical discharge. The resulting shocks are visualized in water using high-speed shadowgraphy at 5 million frames per second, while high-resolution fiber optic hydrophone measurements provide direct quantitative measurements of shock strength after transmission through the impedance-mismatched materials. One-dimensional magnetohydrodynamic simulations are used to reproduce the foil explosion dynamics and provide insight into the thermodynamic, acoustic, and electrical properties of the foil during the explosion. The results show that shock transmission is governed not only by the impedance mismatch at the solid--water interface but also by the finite thickness and dynamic response of the supporting material. These findings provide a quantitative framework for understanding and controlling shock transmission in underwater electrical foil explosions and establish the feasibility and limitations of using solid substrates as structural supports in complex shock-generating geometries.
An important physical quantity that is part of the Saha ionisation equation is the partition function of the neutral hydrogen atoms. An immense uncertainty is caused by the divergency of the neutral atom partition function arising due to the fast growing degeneracy factor of its excited n-state. We suggested a regularisation and renormalisation procedure for the partition function of the neutral hydrogen atoms. The renormalised partition function is well defined and allows to calculate the population of the energy levels and the ionisation state of the hydrogen plasma as a function of its temperature and pressure. In the low-temperature limit the behaviour of these functions smoothly transforms to the Saha equation, which is used in the description of the low-temperature plasma. At higher temperatures the renormalised ionisation equation describes the deviation from the Saha equation. The application of the renormalised ionisation equation to the solar core and atmosphere is considered.
Learned temporal models are moving beyond offline forecasting toward interactive environments for decision making, yet logged-trajectory accuracy need not predict closed-loop behavior under policies optimized in the learned model. This forecasting-control gap matters in tokamak magnetic control, where coupled plasma-circuit dynamics and actuator constraints can amplify errors through feedback. Using a calibrated free-boundary Grad-Shafranov solver, we collect four datasets in a 2x2 design over physics-parameter and current-reference coverage. At matched budgets of 500, 1k, 3k, and 5k rollouts, we train 16 action-conditioned Transformers; PPO policies optimized in the 5k surrogates are then evaluated without adaptation on six reference-solver conditions. Only joint-coverage D4 shows monotonically decreasing cross-condition mean error, whereas D1 improves in distribution but degrades out of distribution. Binary completion masks substantial closed-loop differences: D2-D4 each complete 83.3% of conditions, but reach on average 99.0%, 94.7%, and 90.5% of the evaluation horizon, while their worst conditions reach 94.2%, 68.2%, and 43.2%. Policies trained with action noise complete 5/24 matched conditions, compared with 19/24 under nominal training, and have a lower mean horizon-capped episode length. Based on one trained policy per configuration and one evaluation realization per condition, the results support combining prediction error, episode completion, condition-level duration, and tracking quality.
We propose an instantaneous control framework for the stabilization of kinetic plasma instabilities using external fields. The control problem is formulated as a PDE-constrained optimization that accounts for nonlinear plasma dynamics, while the control is determined at each time step to avoid global-in-time optimization. The Discretize-then-Optimize (DtO) approach is adopted, with the forward equations first discretized using a particle-in-cell (PIC) method. The subsequent control computation is derived from the discrete particle update, ensuring consistency with the forward solver. The control objective is formulated in terms of electromagnetic field energy and low-order moments rather than the full distribution function, making the optimization more amenable to practical implementation. The framework is applied to both electrostatic and electromagnetic settings and evaluated on several benchmark problems, including the two-stream, bump-on-tail, Dory--Guest--Harris, and Weibel instabilities. Across these tests, the control successfully suppresses field energy growth and keeps the distribution function close to its initial state. These results demonstrate the effectiveness of the proposed strategy in suppressing kinetic plasma instabilities.
The dynamics of a vortex electron in a transverse uniform magnetic field involves not only the cyclotron motion of the wave packet center but also the evolution of its internal wavefunction structure. To study this process, we develop a quantum framework and investigate the dynamics of vortex electrons in both the nonrelativistic and relativistic regimes. A discrete Bessel-Gaussian initial wave packet is expanded in the basis of Dirac eigenstates to obtain the time-dependent wavefunction and to analyze its spatiotemporal evolution. In the nonrelativistic regime, the results deviate significantly from the rigid point-dipole model, as the intrinsic kinetic orbital angular momentum exhibits dynamics more complex than simple Larmor precession, owing to the nonstationary internal current distribution of the wave packet. In the relativistic regime, the nonlinear dependence of the energy levels on the quantum numbers destroys the equal spacing of the spectrum. Consequently, the relative phases among the eigenstate components of the wave packet evolve nonuniformly, resulting in wave-packet spreading and deformation, accompanied by modified intrinsic orbital angular momentum dynamics. We introduce a dimensionless parameter to quantify this relativistic dephasing, providing an approximate criterion for selecting physical parameters and evolution times to mitigate the resulting wave-packet distortion. These results clarify the limitations of the rigid point-dipole model, highlight the essential role of internal wavefunction structure, and provide theoretical guidance for understanding and manipulating relativistic vortex electrons.
We present a two-dimensional axisymmetric physics-informed neural network (PINN) of the discharge channel of the P5 Hall thruster at 300 V and 5.4 A, which solves eleven coupled plasma fields and assimilates 145 internal probe measurements per field reconstructed from the data of Haas and Gallimore. The electric potential is obtained from Poisson's equation in charge-separation form, n_e = n_i(1+eta), with eta bounded by construction, which keeps the stiff electrostatic constraint active and trainable. The reference solution agrees with the training profiles to relative L2 errors of 0.017, 0.073 and 0.205 for the potential, electron temperature and ion density, and held-out tests show the potential to be far more robust than the ion density to the withdrawal of measurement lines. We then test the solution against requirements the data fit does not reveal: global particle and current balances, the electron current implied by the momentum equation, the field-aligned electron force balance and the axial ion momentum budget. These expose a neutral inflow six times the measured flow, a bound in the potential representation, and a momentum-implied electron current, more than 99% field-aligned, exceeding the discharge current by orders of magnitude. Correcting these, with a divergence-free magnetic field fitted to measured data, uncertainty-weighted data, the electron pressure gradient and progressively imposed field-aligned and current constraints, yields a solution that conserves particles to within 3%, holds the current between 5.1 and 5.4 A and places 97% of the potential and 88% of the temperature measurements within their uncertainties. Within the tested single-velocity fluid formulation, however, the ion density, potential and ion velocity could not be matched together, and the field-aligned electron balance remains the main obstacle to electron self-consistency.
Although plasma-based wakefield acceleration has achieved remarkable success for electrons, efficient positron acceleration remains a long standing challenge, as positrons suffer severe defocusing in the accelerating phases of such structures. Here, we propose a slit plasma wakefield acceleration (SPWA) scheme, in which a relativistic electron beam propagating through a slit channel formed by two parallel plasma slabs excites crossing plasma-electron filamentary structures within the slit, generating intense wakefields tailored to simultaneously provide longitudinal acceleration and transverse focusing for a trailing positron bunch. A theoretical model is developed to characterize the wakefield structure responsible for positron acceleration and focusing. Three-dimensional particle-in-cell simulations validate the model, and demonstrate that the SPWA scheme achieves a positron trapping efficiency of nearly 100%, while maintaining low emittance growth and narrow energy spread for an injected positron beam. These findings establish a new and robust approach to high-efficiency positron acceleration, with promising implications for future compact electron-positron colliders and high-energy positron applications.
The Gas-Dynamic Magnetic Trap (GDMT), a new linear magnetic facility for confining high-temperature hydrogen plasma, is currently under development at the Budker Institute of Nuclear Physics (BINP SB RAS). The total hydrogen flow that must be removed from the end expanders by the vacuum system is approximately 10^22 molecules/s, which corresponds to a required pumping speed on the order of 500 m3/s. Non-evaporable getters (NEG) are considered as a possible pumping method for this application. This article presents experimental studies of Ti-Zr-Al getters manufactured by Polema LLC. The pumping speed, sorption capacity, and sticking factors of the getter surface were measured. The sticking factors for hydrogen and deuterium are 2.23% and 1.54%, respectively. The getter pumping speed is maintained over 60 air-vent/reactivation cycles. Based on these results, a conceptual design of a NEG-based pumping system for the GDMT end expanders is proposed, providing a specific pumping speed of 74 m3/(m3s) for the getter panel. The proposed pumping system design and the required heating power are discussed.
Plasma mirrors (PM) enhance the temporal contrast of high-power lasers, but declining reflectivity at subrelativistic intensities limits their energy throughput. Here we show that prepulse-generated preplasma drives this decline in p polarization primarily through absorption. Particle-in-cell simulations using a measured prepulse profile reproduce the reflectivity decrease, with near- and far-field energy accounting showing a weak contribution from scattering. Optical-cycle-resolved electron tracking reveals successive electron bunches extracted from the plasma surface, supporting a vacuum-heating interpretation. Under the simulated conditions, s-polarized absorption remains weak and no comparable decline occurs. Applying the model to two representative double-plasma-mirror (DPM) systems, BELLA and CoReLS, it captures both the p-polarized and s-polarized reflectivity responses across the operating range. Yet, the model still misses the high-fluence s-polarized reflectivity rollover, and further investigation of the scattering channel might be needed. These findings connect prepulse history to polarization-dependent energy loss, informing the choice of plasma-mirror operating conditions.
A stream function formulation for the calculation of the eddy current distribution in a toroidally segmented conducting shell (Armadillo) is presented and verified. The governing Poisson-like equation is solved spectrally via a Discrete Sine Transform, which enforces the zero-current boundary condition at each gap exactly by construction, and the related radial magnetic field at the plasma surface is obtained by Biot-Savart integration. The toroidal eddy current flows in the same direction on every sector with a cosine-like poloidal distribution, while the poloidal return current, required by current continuity, peaks near each gap and reverses sign between adjacent sectors. The scaling of both components with the number of gaps is governed by the competition between the poloidal geometric scale of the shell and the toroidal segmentation scale. The transition between the two regimes is set by the aspect ratio of the device, around eight gaps for an RFX-mod2-like geometry, as predicted by the analytical wall time model of the Armadillo configuration. At the gap, the poloidal return current produces a radial field of opposite sign to the value far from the gap; the resulting non-axisymmetric error field peaks near this transition and becomes negligible for highly segmented shells. These results provide a first characterisation of the eddy currents and error fields of the Armadillo configuration, needed for future resistive wall mode and tearing mode stability analyses.
A critical challenge for tokamak-based fusion power plants is the formation of high-current, relativistic runaway electron (RE) beams during abrupt confinement loss events called disruptions. RE beams may significantly damage machine components if left unmitigated, jeopardizing the operational and economic viability of fusion power plants. We present the first self-consistent extended magnetohydrodynamic (MHD) computational study of RE mitigation with a passively driven RE mitigation coil (REMC) in a high-current tokamak. RE generation and transport are coupled with REMC-driven MHD activity in a 3D simulation that models the thermal and current quench phases of a mid-plane disruption, together with the vertical displacement and final termination of the plasma and RE beam on SPARC -- a high-field, high-current tokamak designed to achieve a fusion gain Q > 1. We demonstrate that REMC-driven magnetic stochasticity can reduce the RE current by over an order-of-magnitude when compared to unmitigated disruptions. The final termination is mediated via a post-thermal-quench "cold" vertical displacement of the plasma, which, together with magnetic stochasticity, increases RE losses via scrape-off once the plasma contacts the first wall. These results demonstrate a viable mechanism for RE mitigation using passively driven plasma instabilities on high-current tokamaks.
Magnetic reconnection is important in both space and laboratory contexts because it changes the topology of magnetic fields and converts magnetic energy into particle energies. Establishing where this energy conversion occurs is critical for understanding how reconnection partitions energy. Using a novel machine learning approach based on simulations that explicitly identifies the separatrix regions for the first time, the partition of energy flux densities within the magnetic reconnection site is evaluated through statistical study of magnetotail reconnection events observed in situ. The separatrices are found to have the largest electron energy flux densities, roughly twice those in the exhaust, and Poynting flux densities about four times those in the inflows, whereas typical ion enthalpy flux densities are largest in the exhaust. The separatrices are therefore sites of intense electron and electromagnetic energy transport, although their small thickness means that the exhaust may still carry most of the total energy flux.
Recent breakthroughs in artificial-intelligence-assisted mathematics show that open-ended machine search can outperform human mathematicians, yet transferring this paradigm to physics requires refactoring the research workflow. We introduce Simjecture, an open-source framework in which language-model agents pursue human-defined physical conjectures. The framework tackles two central challenges in this transfer: first, because physics is empirical and no model is final, it structures inquiry around counterexample-driven falsification and prospective hypothesis repair; second, because physical theories must interact with the real world, it uses computational physics as an executable testbed while separating the qualification of numerical instruments from scientific claim review so solvers and diagnostics can be adapted within a recorded investigation. We apply the framework in an 82.6 h investigation to tackle the long-standing problem of radiated energy exceeding the imploding-shell kinetic energy in a Z-pinch. The investigation reveals competing effects of energy conversion, retention, and release timing. Stronger coherent helical deformation increases resistive conversion while reducing radiation and increasing retained energy. A broader trailing-plasma profile delays the later radiation maximum and recovers much of its early bolometric deficit, with a spectral response that changes during the pulse. These results provide new insight into energy conversion in high-energy-density plasma physics and demonstrate a new paradigm for physics research.
This article presents an overview of key scientific challenges in the astrophysics and astrochemistry of the interstellar and intergalactic media. Addressing many of these challenges will become possible with the Russian space observatory Spektr-UF, which is currently being prepared for launch. The ultraviolet (UV) spectral range is critically important for studying the evolution of the Universe, as it contains fundamental absorption and emission lines of most chemical elements and the most important molecules. These lines make it possible to reliably determine the physical conditions and chemical composition of various astrophysical objects, as well as the characteristics of the processes occurring within them. The article examines the prospects for UV observations across a broad range of scientific problems, from investigating the epoch of helium reionization and the thermal history of the Universe to mapping ongoing star formation in galaxies. Dedicated sections address the study of the chemical evolution of galaxies at intermediate redshifts, the distribution of "hidden" baryons and metals, the small-scale structure of interstellar clouds, the physics of photodissociation regions and protoplanetary disks, and the modeling of UV emission from nebulae surrounding fast-moving pulsars. The high sensitivity and spectral resolution of the Spektr-UF instruments are expected to provide fundamentally new data across all these research areas, filling existing gaps in our understanding of the interstellar and intergalactic media.
Project 8, a neutrino mass experiment, employs the novel frequency technique of cyclotron radiation emission spectroscopy (CRES) to perform a model independent measurement of neutrino mass via tritium beta decay. Our pioneering experiment will consist of a beamline that dissociates molecular tritium, surface cools atoms through accommodation, evaporatively cools and slows atoms, and traps them in a CRES resonant cavity to advance towards our sensitivity goal of 40 meV/c^2 for our final electron-weighted neutrino mass measurement. We are prototyping an electron cyclotron resonance (ECR) dissociator that uses a tunable 1 kW 2.45 GHz RF generator and electromagnets to ignite a hydrogen plasma in an aluminum resonant cavity. Our ECR source is designed to produce a high atom flux (~10^19 atoms/second) near room temperature. We present the engineering development, gas diagnostics, and spectroscopy data from our hydrogen plasma.