Language：English  

DOI： 10.1063/5.0049725

Language：English  

DOI： 10.1063/5.0011935

Language：English  

Language：English  

Language：English  

Language：English  

Language：English  

Language：English  

Language：English  

Language：English  

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

Web of Science

Scopus

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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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

Web of Science

Scopus

PubMed

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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Scopus

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

Web of Science

Scopus

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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Scopus

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

Web of Science

Scopus

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

Web of Science

Scopus

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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Scopus

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

Web of Science

Scopus

PubMed

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

Web of Science

Scopus

PubMed

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