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Language：English   Publisher：Review of Scientific Instruments  

Particle counting analysis is a possible way to characterize GeV-scale, multi-species ions produced in laser-driven experiments. We present a multi-layered scintillation detector to differentiate multi-species ions of different masses and energies. The proposed detector concept offers potential advantages over conventional diagnostics in terms of (1) high sensitivity to GeV ions, (2) realtime analysis, and (3) the ability to differentiate ions with the same charge-to-mass ratio. A novel choice of multiple scintillators with different ion stopping powers results in a significant difference in energy deposition between the scintillators, allowing accurate particle identification in the GeV range. Here, we report a successful demonstration of particle identification for heavy ions, performed at the Heavy Ion Medical Accelerator in Chiba. In the experiment, the proposed detector setup showed the ability to differentiate particles with similar atomic numbers, such as C6+ and O8+ ions, and provided an excellent energy resolution of 0.41%-1.2% (including relativistic effect, 0.51% - 1.6%).

DOI： 10.1063/5.0101872

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Language：English   Publisher：Scientific Reports  

Graphene is known as an atomically thin, transparent, highly electrically and thermally conductive, light-weight, and the strongest 2D material. We investigate disruptive application of graphene as a target of laser-driven ion acceleration. We develop large-area suspended graphene (LSG) and by transferring graphene layer by layer we control the thickness with precision down to a single atomic layer. Direct irradiations of the LSG targets generate MeV protons and carbons from sub-relativistic to relativistic laser intensities from low contrast to high contrast conditions without plasma mirror, evidently showing the durability of graphene.

DOI： 10.1038/s41598-022-06055-4

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DOI： 10.1063/5.0049725

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DOI： 10.1063/5.0011935

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Publisher：Plasma Physics and Controlled Fusion  

Energetic-particle-driven modes in magnetically confined fusion plasmas often exhibit nonlinear frequency sweeping (‘chirping’), reflecting complex wave–particle interactions near marginal stability. While the bump-on-tail (BOT) instability within the Berk–Breizman framework has served as a canonical model for understanding such phenomena, a unified nonlinear description remains incomplete when drag, diffusion, and Krook relaxation act simultaneously. In this work, we present a comprehensive numerical investigation of the BOT instability that explicitly retains all three collision operators together with external wave damping. Using a validated characteristic-based BOT code, we systematically scan the multi-dimensional collision parameter space and construct nonlinear regime maps and bifurcation diagrams. To organize the rich dynamics, we introduce a two-level categorization that combines global wave-energy evolution (damped, steady, periodic, and chaotic states) with chirping subtypes identified from spectral morphology. We find that diffusion and Krook relaxation regularize the nonlinear dynamics and promote ordered transitions from chaotic behaviors to periodic oscillations and steady saturation as collision strength increases, with the saturation level decreasing approximately exponentially with external damping. In contrast, drag alone does not admit steady solutions and instead drives persistent or chaotic chirping through convective deformation of resonant phase-space structures. When drag is combined with diffusion or Krook relaxation, clear transition sequences emerge: increasing drag breaks hole–clump symmetry, broadens the effective resonance region, and drives systematic transitions from transient to intermittent and persistent chirping. At fixed ratios of collision operators, higher absolute collision rates require larger wave amplitudes to balance collision-driven restoration against wave-induced phase-space flattening. Furthermore, extending our analysis into linearly stable regimes reveals robust subcritical nonlinear behavior, where finite-amplitude initial perturbations overcome background dissipation to trigger nonlinear states. These results provide unified nonlinear regime maps and mechanistic phase-space interpretations for energetic-particle-driven chirping, offering a predictive framework that is directly relevant to the diagnosis and control of chirping Alfvénic activity in fusion experiments.

DOI： 10.1088/1361-6587/ae66b5

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Language：English   Publisher：Scientific Reports  

Helicon plasma sources play a central role in applications ranging from material treatment to space propulsion and fusion, yet the physical processes governing their ignition and transient ionization remain incompletely understood. Here we develop a self-consistent, fully coupled multiphysics framework implemented in COMSOL Multiphysics, that integrates Maxwell's equations, electron energy transport, drift-diffusion kinetics, and heavy-species chemistry to capture the complete spatiotemporal evolution of helicon discharges. The model reproduces experimental measurements across pressure, magnetic field, and frequency ranges, and reveals a previously unresolved transient ionization stage characterized by a rapid density rise within ~ 10- 4 s, accompanied by a two-peak electron temperature structure that governs the formation of the dense plasma core. By tracking the RF power flow and field topology, we characterize the transient redistribution of RF energy during ignition. A short-lived phase of localized energy deposition accompanies the onset of ionization, followed by a gradual restructuring of the RF field distribution as the plasma density increases, together with rapid density growth and profile restructuring. Systematic parametric scans further reveal the sensitivity of this mode-coupling process to gas pressure, magnetic field strength, and driving frequency. These results provide a unified picture of the ignition in helicon plasmas and establish a predictive tool for the design and optimization of RF plasma sources across space propulsion, manufacturing, and fusion technologies.

DOI： 10.1038/s41598-026-47901-z

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Publishing type：Research paper (scientific journal)   Publisher：Physics of Fluids  

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.

DOI： 10.1063/5.0312985

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Language：English   Publisher：The Japan Society of Applied Physics  

<p>Magnetic reconnection, a fundamental process driving energy release across laboratory, space, and astrophysical plasmas, efficiently converts magnetic field energy into plasma kinetic and thermal energy. Here, we present the simultaneous, direct observation of plasma outflow acceleration across different spatial scales in laser-produced plasmas: the near-field (local acceleration) and the far-field (the resulting plasma jet). Local plasma flow near the X-point is measured using the Doppler shift of Thomson scattering spectra. The resultant high-speed plasma jet emanating from the reconnection region is subsequently detected by an ion collector with a Faraday cup. The near- and far-field velocity measurements exhibit a similar tendency across three different initial plasma conditions. This simultaneous diagnostic approach provides a powerful tool for the detailed experimental investigation and validation of magnetic reconnection physics.</p>

DOI： 10.56646/jjapcp.12.0_011015

CiNii Research

Publisher：Physics of Plasmas  

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.

DOI： 10.1063/5.0299693

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Publisher：High Energy Density Physics  

Graphene is an atomic thin 2D material, known as the strongest material with such a thin regime. Free-standing, large-area suspended graphene (LSG) has been developed for a target of laser-driven ion acceleration. The LSG has shown remarkable durability against laser prepulse, producing MeV protons and carbons by direct irradiation with an intense laser without a plasma mirror, yet no optimization has been concerned. Here we investigate the optimization of the laser-driven ion acceleration with LSG, paying special attention to the target conditions. We irradiate nanometer-thick LSG with an intense laser, where the incident angle and the target thickness are controlled. The maximum proton energy increases with increasing the number of LSG layers, where 25±0.3MeV protons at maximum are consistently observed with Thomson parabola spectrometer and diamond-based detectors. For comparison purposes, we perform ideal numerical simulations using particle-in-cell (PIC) code without consideration of the prepulse. In the PIC simulation, the protons are successively accelerated by the electric field associated with laser radiation pressure and the surface sheath field, yet the maximum proton energies are overestimated. The maximum proton energies from the experiment asymptotically approach the ideal PIC expectations, indicating that the thinner LSG is more affected by the prepulse. We expect higher proton energy with the optimized LSG conditions and a plasma mirror.

DOI： 10.1016/j.hedp.2025.101195

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Publisher：Contributions to Plasma Physics  

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.

DOI： 10.1002/ctpp.70020

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Publisher：2025 Conference on Lasers and Electro Optics Europe and European Quantum Electronics Conference CLEO Europe Eqec 2025  

The laser-driven proton acceleration has been extensively studied via simulations and experiments, such as target normal sheath acceleration (TNSA)^[1] and radiation pressure acceleration (RPA)^[2]. Recently, pioneers have explored the use of laser wakefield acceleration (LWFA)^[3] to accelerate protons. However, achieving higher proton energies with this method requires significantly more laser energy. In 2021, Isayama et al. introduced a three-stage hybrid acceleration scheme (RPA-LWFA-TNSA)^[4] achieving high proton energy efficiently. The configuration of this scheme is shown in Fig. 1(a). Two circularly-polarized pulses are used. The targets consist of a solid-density (SD) target and a near-critical density (NCD) target separated by a vacuum gap. The first pulse interacts with the SD target and causes the RPA acceleration. The second pulse interacts with the NCD target and induces the LWFA and TNSA acceleration mechanism.

DOI： 10.1109/CLEO/EUROPE-EQEC65582.2025.11109725

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Publisher：European Conference on Lasers and Electro Optics Proceedings Conference on Lasers and Electro Optics Europe CLEO Europe 2025 and European Quantum Electronics Conference Eqec 2025  

Scopus

Language：English   Publisher：Physical Review E  

The interaction between a thin foil target and a circularly polarized laser light injected along an external magnetic field is investigated numerically by particle-in-cell simulations. A standing wave appears at the front surface of the target, overlapping the injected and partially reflected waves. Hot electrons are efficiently generated at the standing wave due to the relativistic two-wave resonant acceleration if the magnetic field amplitude of the standing wave is larger than the ambient field. A bifurcation occurs in the gyration motion of electrons, allowing all electrons with nonrelativistic velocities to acquire relativistic energy through the cyclotron resonance. The optimal conditions for the highest energy and the most significant fraction of hot electrons are derived precisely through a simple analysis of test-particle trajectories in the standing wave. Since the number of hot electrons increases drastically by many orders of magnitude compared to the conventional unmagnetized cases, this acceleration could be a great advantage in laser-driven ion acceleration and its applications.

DOI： 10.1103/PhysRevE.110.065212

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Publisher：Physics of Plasmas  

The mechanism of generating collisionless shock in magnetized gas plasma driven by laser-ablated target plasma is investigated by using one-dimensional full particle-in-cell simulation. The effect of finite injection time of target plasma, mimicking the finite width of laser pulse, is taken into account. It was found that the formation of a seed-shock requires a precursor. The precursor is driven by gyrating ions, and its origin varies depending on the injection time of the target plasma. When the injection time is short, the target plasma entering the gas plasma creates a precursor; otherwise, gas ions reflected by the strong piston effect of the target plasma create a precursor. The precursor compresses the background gas plasma, and subsequently, a compressed seed-shock forms in the gas plasma. The parameter dependence on the formation process and propagation characteristics of the seed-shock was discussed. It was confirmed that the seed-shock propagates through the gas plasma exhibiting behavior similar to the shock front of supercritical shocks.

DOI： 10.1063/5.0230232

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Publisher：Reviews of Modern Plasma Physics  

Following the First Helicon Plasma Physics and Applications (HPPA) Workshop, which was held in 2021 remotely due to COVID-19, this Second HPPA Workshop aimed to establish a regular onsite meeting for the specific field of helicon plasma. It was held on 11–14 April 2024 in Chongqing, China, and organized by Chongqing University and co-organized by Southwestern Institute of Physics. This workshop attracted ~ 160 registrations, ~ 140 onsite participants, and ~ 27,000 number of views through the live streaming online. The 48 presentations covered most topics about helicon plasma, e.g., from fundamental physics to various applications. This paper summarizes the important findings of fundamental physics research, progresses on source and diagnostic developments, and new explorations of laboratory and industrial applications, together with enlightening comments and perspectives regarding future research for this field. It serves as a valuable reference for the helicon research community and other relevant fields.

DOI： 10.1007/s41614-024-00171-6

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Publisher：Physics of Plasmas  

We have investigated space and astrophysical phenomena in nonrelativistic laboratory plasmas with long high-power lasers, such as collisionless shocks and magnetic reconnections, and have been exploring relativistic regimes with intense short pulse lasers, such as energetic ion acceleration using large-area suspended graphene. Increasing the intensity and repetition rate of the intense lasers, we have to handle large amounts of data from the experiments as well as the control parameters of laser beamlines. Artificial intelligence (AI) such as machine learning and neural networks may play essential roles in optimizing the laser and target conditions for efficient laser ion acceleration. Implementing AI into the laser system in mind, as the first step, we are introducing machine learning in ion etch pit analyses detected on plastic nuclear track detectors. Convolutional neural networks allow us to analyze big ion etch pit data with high precision and recall. We introduce one of the applications of laser-driven ion beams using AI to reconstruct vector electric and magnetic fields in laser-produced turbulent plasmas in three dimensions.

DOI： 10.1063/5.0190062

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Publisher：AIP Advances  

In the realm of high-energy-density laboratory plasma experiments, ion radiography is a vital tool for measuring electromagnetic fields. Leveraging the deflection of injected protons, ion imaging can reveal the intricate patterns of electromagnetic fields within the plasma. However, the complex task of reconstructing electromagnetic fields within the plasma system from ion images presents a formidable challenge. In response, we propose the application of neural network techniques to facilitate electromagnetic field reconstructions. For the training data, we generate corresponding particle data on ion radiography with diverse field profiles in the plasma system, drawing from analytical solutions of charged particle motions and test-particle simulations. With these training data, our expectation is that the developed neural network can assimilate information from ion radiography and accurately predict the corresponding field profiles. In this study, our primary emphasis is on developing these techniques within the context of the simplest setups, specifically uniform (single-layer) or two-layer systems. We begin by examining systems with only electric or magnetic fields and subsequently extend our exploration to systems with combined electromagnetic fields. Our findings demonstrate the viability of employing neural networks for electromagnetic field reconstructions. In all the presented scenarios, the correlation coefficients between the actual and neural network-predicted values consistently reach 0.99. We have also learned that physics concepts can help us understand the weaknesses in neural network performance and identify directions for improvement.

DOI： 10.1063/5.0189878

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

Publisher：Physics of Plasmas  

Deep learning (DL) has recently become a powerful tool for optimizing parameters and predicting phenomena to boost laser-driven ion acceleration. We developed a neural network surrogate model using an ensemble of 355 one-dimensional particle-in-cell simulations to validate the theory of phase-stable acceleration (PSA) driven by a circularly polarized laser driver. Our DL predictions confirm the PSA theory and reveal a discrepancy in the required target density for stable ion acceleration at larger target thicknesses. We discuss the physical reasons behind this density underestimation based on our DL insights.

DOI： 10.1063/5.0178238

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Publisher：Physics of Plasmas  

We theoretically and numerically investigate the ion-acoustic features of collective Thomson scattering (CTS) in two-stream plasmas. When the electron distribution functions of two (stationary and moving) components overlap with each other at the phase velocities corresponding to the two resonant peaks of the ion-acoustic feature, the theoretical spectrum shows asymmetry because the rate of electron Landau damping is different for the two peaks. The results of numerical simulations agree well with the theoretical spectra. We also demonstrate the effect of a two-stream-type instability in the ion-acoustic feature. The simulated spectrum in the presence of the instability shows an asymmetry with the opposite trend to the overlapped case, which results from the temporal change of the electron distribution function caused by the instability. Our results show that two-stream plasmas have significant effects on CTS spectra and that the waves resulting from instabilities can be observed in the ion-acoustic feature.

DOI： 10.1063/5.0117812

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Language：English   Publisher：Physical Review E  

Magnetic reconnection in laser-produced magnetized plasma is investigated by using optical diagnostics. The magnetic field is generated via the Biermann battery effect, and the inversely directed magnetic field lines interact with each other. It is shown by self-emission measurement that two colliding plasmas stagnate on a midplane, forming two planar dense regions, and that they interact later in time. Laser Thomson scattering spectra are distorted in the direction of the self-generated magnetic field, indicating asymmetric ion velocity distribution and plasma acceleration. In addition, the spectra perpendicular to the magnetic field show different peak intensity, suggesting an electron current formation. These results are interpreted as magnetic field dissipation, reconnection, and outflow acceleration. Two-directional laser Thomson scattering is, as discussed here, a powerful tool for the investigation of microphysics in the reconnection region.

DOI： 10.1103/PhysRevE.106.055207

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Language：English   Publisher：Physical Review E  

A developing supercritical collisionless shock propagating in a homogeneously magnetized plasma of ambient gas origin having higher uniformity than the previous experiments is formed by using high-power laser experiment. The ambient plasma is not contaminated by the plasma produced in the early time after the laser shot. While the observed developing shock does not have stationary downstream structure, it possesses some characteristics of a magnetized supercritical shock, which are supported by a one-dimensional full particle-in-cell simulation taking the effect of finite time of laser-Target interaction into account.

DOI： 10.1103/PhysRevE.106.025205

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Event date： 2018.11

Language：English  

Country：Japan  

Event date： 2018.5

Language：English  

Country：Japan  

Event date： 2017.9

Language：English  

Country：China  

Event date： 2020.1

Language：English  

Country：Taiwan, Province of China  

Event date： 2019.1

Language：English  

Country：Taiwan, Province of China  

Event date： 2018.12

Language：Japanese  

Venue：大阪   Country：Japan  

Event date： 2018.1

Language：English  

Country：Taiwan, Province of China