An on-the-fly machine-learning potential (MLP) was constructed and assessed for tungsten containing small vacancy clusters. The present MLP reproduces the lattice constant, bulk modulus, elastic constants, monovacancy formation energy, and binding energies of di- and trivacancies in good agreement with density functional theory (DFT), improving accuracy for vacancy energetics compared with existing interatomic potentials. These results provide a first-principles-based foundation for extending the model to hydrogen-vacancy complexes and investigating how hydrogen trapping affects the stability and evolution of small vacancy clusters.

Additional electromagnets enable adjustment of the external DC magnetic field condition by varying the DC current applied to each electromagnet in a developed, large-diameter (LD), low-aspect-ratio (LAR) radiofrequency (RF) plasma source using a flat spiral antenna. The magnetic field topology downstream changes in three cases: diverging, uniform, and converging fields, while the field strength near the flat spiral RF antenna remains nearly unchanged. Electron density increases stepwise with increasing RF power, and the transition thresholds systematically shift toward lower RF power as the downstream magnetic field becomes more convergent. The lowered threshold power may be related to reduced axial electron loss under the converging-field condition. These results indicate that control of the downstream magnetic-field topology can modify the RF power threshold for accessing higher-density plasma branches in the LD-LAR RF plasma source.

This paper proposes a new method, called structural correlation analysis, which investigates the relation between two plasma structures.The method is applied to plasma images obtained by tomography systems installed at different axial positions in a cylindrical plasma, PANTA.The results indicate the presence of fluctuation structures with high correlation at two different axial positions, suggesting their stiffness along the axial direction, with a finite inclination between them.The analysis finds that the inclination should be caused by azimuthal rotation of a stiff helical structure slowly propagating in the axial direction.

This study numerically investigates the injection, electron-impact ionization, and subsequent confinement of H₂⁺ beams from an idealized Cyclotron Auto-Resonance Acceleration (CARA) stage in a field-reversed configuration (FRC). A 1.5-MeV beam is injected from the upstream axial boundary with axial and azimuthal velocity components. Ionization is modeled as an effective H₂⁺→ H₂²⁺ charge-state transition, while the subsequent Coulomb dissociation into two protons is not explicitly resolved. We compare a full-domain model with an idealized separatrix-only model that suppresses ionization in the scrape-off layer (SOL). Under the prescribed equilibrium and constant electron-pressure profile, the overall ionized-and-confined fraction reaches approximately 0.45 atT_e= 50 keV. At lowT_e, particles ionized inside the separatrix are confined efficiently, whereas ionization on open field lines causes substantial losses in the full-domain model. At higherT_e, the lower electron density and smaller ionization cross section lengthen the mean free path, suppress SOL ionization, and make the confinement fraction results from the two models converge.

The nonlinear wave triggered at the boundary layer betweenlaser-irradiated solid carbon and ambient hydrogen gas is analyzed usinga physics-informed neural network (PINN)-based approach.We demonstrate that the boundary layer evolves through multiple phasescharacterized by (I) solid-plasma expansion while maintaining thecontact surface, (II) formation of a mixing region, and (III) bifurcation of the gas surface into a nonlinear wave and a rarefactionfront.The one-dimensional results show that the gas surface bifurcatessimultaneously with the formation of the mixing region, in contrastto observations in two-dimensional cluster-gas systems.The PINN model quantitatively captures the dynamics across thesetransitions and verifies that the nonlinear wave launched through thisprocess is governed by an advective Korteweg-de Vries (KdV)-type equation.

A reduced transport model previously proposed for gyrokinetic turbulence simulations has been shown to reproduce turbulenttransport over a wide range of parameters [M. Nunami, et al., Phys. Plasmas 20, 092307 (2013);T. Nakayama, et al., Sci. Rep. 13, 2319 (2023)].While this model successfully captures simulation results, the origin of its functional form has remained largely empirical.In this work, we investigate a phenomenological representation of turbulent transport based on a minimal set of physical constraints.By separating turbulence-driven transport enhancement from zonal-flow-induced regulation, a general class of admissible transportfunctions is constructed.Representative regulation functions constructed within a physically motivated class of admissible forms are shown to sharea common asymptotic structure in the weak-regulation regime relevant to gyrokinetic simulations.Based on this observation, a lowest-order rational representation is identified as a minimal realization of the phenomenologicalconstraints.The resulting expression naturally includes the transport models previously proposed as special cases.This work provides a phenomenological and scaling-based foundation for reduced turbulent transport models, thereby clarifyingthe relation between previously proposed empirical models and a minimal rational realization of the phenomenological constraintsand establishing a first step toward a systematic classification of admissible transport models.Such a classification can provide objective guidelines for selecting appropriate reduced models across different physical transport regimes.

A Langmuir probe assembly with embedded high impedance resistors was developed for the TST-2 device to measure electrostatic fluctuations over a wide frequency range, from DC to lower hybrid wave (LHW) frequency range. Toroidal wavenumbers of approximately 20 m⁻¹ were measured for fluctuations in the LHW frequency range during plasma current start-up experiment using an LHW launcher. The discrepancy between the observed parallel wavenumbers and the estimated wavenumbers at the launcher surface is discussed.