Shogo Isayama

Generating 100-MeV protons is pivotal to developing laser-driven ion sources for many applications that range from medicine to laboratory astrophysics. An ultra-thin target is a way to increase the ion energy; however, such a target is easily destroyed by the interaction with laser prepulses. We have developed a large-area suspended graphene (LSG) target and demonstrated its durability by generating energetic protons and carbons by irradiating the thinnest target ever (2 nm) with an ultra-intense laser without a plasma mirror. Here, we show the experimental results of proton energies reaching 132 MeV by optimizing the laser conditions for the ultra-thin LSG targets. Ion diagnostics were primarily performed using a CR-39 stack detector combined with machine-learning-assisted analysis. Numerical simulations support the experimental results, where the radiation pressure of a relatively long-duration, low-intensity laser pulse slowly pushes the plasma, leading to the formation of a propagating electrostatic wave that efficiently accelerates protons, as in surfing acceleration.

This paper presents an experimental investigation of magnetic reconnection between two laser-produced expanding plasmas, focusing on the quantitative evaluation of the reconnection rate and energy conversion under varying initial conditions. By changing the separation distance between the drive laser focal spots (1 mm and 2 mm), we systematically controlled the inflow parameters. The reconnection region was probed using a two-directional laser Thomson scattering (LTS) system, which simultaneously measured the local plasma parameters parallel to the outflows and along the current sheet. Based on our established method incorporating macroscopic energy and mass conservation laws to derive the upstream magnetic field directly from LTS spectra, we characterized the temporal evolution of the current sheet and the reconnection rate. The larger spot separation allows the plasma bubbles to expand for a substantially longer time before the formation of a reconnection current sheet, resulting in different upstream conditions. Nevertheless, both configurations yielded comparable upstream magnetic fields and reconnection rates. These results suggest that the reconnection rate is relatively insensitive to the global inflow conditions and is primarily controlled by the local physics of the reconnection layer once a current sheet is formed.

Wakefield acceleration has been considered a possible mechanism for producing cosmic rays, including extremely high-energy ions, especially in astrophysical environments. In the laboratory, however, laser-driven wakefield acceleration of ions remains challenging because ions are much heavier than electrons. A central issue is how ions enter the accelerating field. One possible route with present laser facilities is to use dense plasma, in which the wake moves more slowly and ions can be trapped by the bow wake formed at the laser front. Here we experimentally demonstrate bow-wake acceleration of protons using resorcinol-formaldehyde foam targets. We obtain protons with energies up to 60 MeV. Three-dimensional particle-in-cell simulations support the interpretation that the protons are accelerated by the bow wake. These results show that dense plasma can enable ion injection into laser-driven wakes and provide a laboratory route toward bow-wake acceleration of high-energy ions.

We develop a physics-informed neural network (PINN)-based framework for simulating electron–ion two-fluid plasmas with large scale separation. As a benchmark, we examine the one-dimensional diffusion of a magnetized low-temperature plasma and compute the spatiotemporal evolution of the two-fluid variables. The governing equations, initial conditions, and boundary conditions are embedded into the composite loss function, and the forward problem is solved by minimizing this loss function. Accuracy is evaluated by comparison with conventional numerical simulations. To handle the intrinsic electron–ion multiscale nature, we adopt a species-dependent normalization that scales each variable by appropriate thermal and characteristic quantities, yielding improved stability and a mean relative error of 3.08 × 10−3. Although training instability occurs for certain hyperparameter choices, such as different learning rates or random initializations, well-converged cases can be clearly identified from the loss evolution. Therefore, this instability does not constitute a serious practical limitation. The results further indicate that learning stability depends on how strongly the solution is anchored by boundary constraints. Neumann-type boundary conditions exhibit sensitivity to learning rate and initialization, whereas Dirichlet conditions improve reproducibility and robustness. Overall, this study clarifies how normalization and boundary-condition design jointly govern training stability in PINN-based two-fluid plasma simulations.

This study demonstrates the feasibility of applying Physics‐Informed Neural Networks (PINNs) to reconstruct the spatial and temporal evolution of two‐dimensional magnetohydrodynamic (MHD) reconnection structures from limited in situ observational data. By embedding the complete set of MHD equations into the loss function, the reconstructed solutions naturally satisfy the governing physical laws. The reconstruction accuracy is systematically evaluated by varying the number, spatial distribution, and sampling interval of observation points. The analysis reveals that placing observation points both upstream and downstream of the plasmoid significantly enhances reconstruction accuracy, highlighting the importance of capturing both the early‐time evolution near the ‐point and the well‐developed downstream structures. These findings demonstrate the potential of PINNs as a powerful tool for recovering large‐scale MHD reconnection structures from sparse data, while also providing practical guidance for the design and operation of future multi‐satellite observation missions.

Energetic electrons significantly influence power coupling and transport in helicon plasmas. Although energetic electrons and Landau damping processes have been reported in previous helicon studies, their nonlinear interaction with waves and the resulting saturation dynamics have not been quantified. Here, we present the first self-consistent analysis of the bump-on-tail (BOT) instability in helicon discharges using the BOT model adapted to helicon-relevant parameters. Under experimental conditions, the instability exhibits weakly nonlinear growth and saturates without frequency chirping, resulting from the balance between energetic-electron drive and Krook-type collisional relaxation. Parameter scans show that collisionality plays a dual role—damping the wave while continually repopulating the resonant velocity region—leading to enhanced saturation amplitude and stronger modifications to the electron distribution. Increasing the energetic drive further intensifies the nonlinear wave–particle coupling and broadens the plateau in velocity space. By coupling the BOT-induced electric field to fluid and Poisson equations, we show that the resulting density and flux perturbations can be an order of magnitude larger in rotating plasmas than in stationary ones. The model thus reveals a kinetic-to-fluid coupling pathway, showing that resonant electrons can modify macroscopic transport and power deposition. These results advance the understanding of wave–particle interaction in high-density radio frequency (RF) plasmas and provide a physics basis for optimizing power coupling and stability in next-generation helicon devices.

This study explores the transitional characteristics of blue-core mode helicon discharge, which to our knowledge was not particularly focused before. Parameters are measured on a recently built advanced linear plasma device, i.e., a multiple plasma simulation linear device [Sun et al., Fusion Eng. Des. 162, 112074 (2021) and Wu et al., Plasma Sources Sci. Technol. 33, 085007 (2024)] by various diagnostics including the Langmuir probe, an optical emission spectrometer, and a standard high-speed camera. It is found that the jump direction of electron density (from low level to high level) is opposite to that of electron temperature (from a high level to a low level). Electron density increases significantly, and the radial profile becomes localized near the axis when the blue-core mode transition occurs. With increased field strength, electron density increases whereas electron temperature drops. The radial profile of electron temperature looks like a “W” shape, i.e., minimizing around the edge of the blue-core column. Electron density increases with background pressure, while electron temperature peaks around certain pressure value. High-speed videos show that the plasma column oscillates radially and experiences azimuthal instabilities with high rate once entered the blue-core mode. An electromagnetic solver based on Maxwell's equations and a cold-plasma dielectric tensor is also employed to compute the wave field and power absorption during blue-core mode transition, to provide more details that are valuable for understanding the transitional physics but not yet available in experiment. The results show that the wave field in both radial and axial directions changes significantly during the transition, its structure differs from antenna to downstream, and the power dependence of the wave magnetic field is overall opposite to that of a wave electric field. This work presents comprehensive characteristics of the transitional blue-core mode discharge and is important to both physics understanding and practical applications.

We propose a particle acceleration mechanism driven by large-amplitude Alfvén waves in a strong magnetic field. The acceleration process proceeds through multiple stages triggered by counterpropagating wave-particle resonant acceleration (CWRA) via decay instability. Initially, parent and daughter Alfvén waves resonantly accelerate particles perpendicular to the ambient magnetic field. The resultant modulational instability generates electrostatic fields within the wave packet, which are locally amplified by the ponderomotive force of the Alfvén wave packet. These fields subsequently drive further acceleration within a few relativistic gyroperiods via gyroresonant surfing acceleration (GRSA). During this, the v×B force facilitates momentum transfer from the perpendicular to the parallel direction. In the later stage, particles become trapped by the parent wave and gain additional energy through single wave resonant acceleration (SWRA). Furthermore, the accumulation of accelerated particles induces electrostatic trailing fields behind and at the tail of the wave packet, which drive trailing-field acceleration (TFA) of other electrons. The combined effects of these mechanisms, CWRA followed by GRSA and SWRA, result in highly relativistic electron energy. The electron energy accelerated through the above process is higher than that accelerated through TFA. This multistage acceleration process provides insights into the generation of high energy cosmic rays in astrophysical environments.

Short‐pulse intense lasers have the potential to model extreme astrophysical environments in laboratories. Although there are diagnostics for energetic electrons and ions resulting from laser‐plasma interactions, the diagnostics to measure velocity distribution functions at the interaction region of the laser and plasma are limited. We have been developing the diagnostics of the interaction between the intense laser and plasma using scattered intense laser. We performed experiments to measure electron density by observing the spatial distributions and the ratio of horizontal to vertical polarization components of the scattered laser beam using optical imaging. The observed ratio of polarization components is consistent with the drive laser beam, indicating the observed light originates from the drive laser. Imaging of the scattered light shows the structure of electron density, the zero moment of the electron velocity distribution function, interacting with the intense laser. We observed the change of structure due to the laser pre‐pulse that destroys the target before the arrival of the main pulse.