arXiv subjects

M. Murakami

Publications and source records attributed to M. Murakami.

At least 19 recordsLinked to original sources

arXiv2026

Dual-pulse micronozzle acceleration of sub-GeV-class protons

We propose a dual-pulse micronozzle acceleration scheme that enables phase-locked acceleration of laser-driven protons, mitigating the trade-off between maximum proton energy and laser-to-proton conversion efficiency. A delay-tuned synchronization window injects a compact proton front generated by a shaping prepulse into a quasistatic axial electric field driven by a delayed main pulse in a micronozzle cavity. Phase locking preserves the relative phase between the proton bunch and the accelerating field, suppresses thermal debunching, and prolongs the acceleration stage. At main-pulse intensities of order 10^21 W/cm^2, sub-GeV proton cutoff energies are obtained with a total laser-to-proton conversion efficiency of about 20%. The efficiency for protons above 100 MeV exceeds about 13%, indicating preferential energy loading into a compact proton population. Simulations with an unconfined dual-pulse hydrogen rod show that the improvement results from temporal synchronization and geometric confinement, which sustain a long-lived axial accelerating channel. An analytical synchronization model agrees with the simulations. Three-dimensional particle-in-cell simulations confirm that phase locking and spectral hardening are preserved in slit-nozzle geometries, with cutoff energies about 60% higher than those of an unconfined hydrogen rod. These results establish phase-locked acceleration as a practical design principle for compact, high-yield sub-GeV proton drivers for secondary-particle applications.

physics.plasm-ph↗

arXiv2026

Scaling laws of multi-shock implosions toward the quasi-isentropic limit

We present a unified theoretical and numerical framework for self-similar multi-shock implosions achieving ultrahigh compression in a uniform solid spherical target. Extending the classical Guderley model to N stacked, spherically converging shocks, we derive selfsimilar solutions and the scaling law for the final density. One dimensional Lagrangian hydrodynamic simulations confirm this relation over a broad range of parameters, from the weakly to the strongly nonlinear regime. The results show that cumulative compression increases systematically with the number of stacked shocks while entropy generation is strongly suppressed, asymptotically approaching a quasi isentropic limit as N increases infinity. This volumetric scheme strongly suppresses the Rayleigh Taylor instability that plagues shell based implosions and thus provides a robust, largely instability-resistant compression pathway applicable to inertial confinement fusion and other high energy density systems. The framework bridges similarity theory with realistic multi-shock dynamics, guiding the design of advanced laser-driven compression schemes.

physics.plasm-ph↗

arXiv2025

Generation of ultrahigh field by micro-bubble implosion

Breaking the 100-MeV barrier for proton acceleration will help elucidate fundamental physics and advance practical applications from inertial confinement fusion to tumour therapy. Herein we propose a novel concept of bubble implosions. A bubble implosion combines micro-bubbles and ultraintense laser pulses of 10^20-10^22W/cm^2 to generate ultrahigh fields and relativistic protons. The bubble wall protons undergo volumetric acceleration toward the centre due to the spherically symmetric Coulomb force and the innermost protons accumulate at the centre with a density comparable to the interior of a white dwarf. Then an unprecedentedly high electric field is formed, which produces an energetic proton flash. Three-dimensional particle simulations confirm the robustness of Coulomb-imploded bubbles, which behave as nano-pulsars with repeated implosions and explosions to emit protons. Current technologies should be sufficient to experimentally verify concept of bubble implosions.

physics.plasm-ph↗

arXiv2025

Magnetic field amplification driven by the gyro motion of charged particles

Spontaneous magnetic field generation plays important role in laser-plasma interactions. Strong quasi-static magnetic fields affect the thermal conductivity and the plasma dynamics, particularly in the case of ultra intense laser where the magnetic part of Lorentz force becomes as significant as the electric part. Kinetic simulations of giga-gauss magnetic field amplification via a laser irradiated microtube structure reveal the dynamics of charged particle implosions and the mechanism of magnetic field growth. A giga-gauss magnetic field is generated and amplified with the opposite polarity to the seed magnetic field. The spot size of the field is comparable to the laser wavelength, and the lifetime is hundreds of femtoseconds. An analytical model is presented to explain the underlying physics. This study should aid in designing future experiments.

physics.plasm-ph↗

arXiv2025

Microbubble implosions in finite hollow spheres

Microbubble implosion (MBI) is a recently proposed novel mechanism with many interesting and exciting potential applications. MBI predicts that the inner layers of a spherical target with a hollow cavity can be compressed into a core with a density 105 times that of the solid density. Furthermore, this ultra-compressed core mostly consists of ions. This leads to the generation of ultra-high electric fields, which may be applicable to gamma-ray lensing or pair creation. However, MBI has yet to be studied for finite hollow spheres whose electrons are free to redistribute themselves after being given an initial temperature. This paper studies MBI under finite sphere conditions. Using an electron distribution model, the electron distribution after receiving an initial temperature is studied. Then, the optimal parameters required to fill a hollow cavity with electrons are calculated. The dynamics of MBI is simulated using a hybrid one-dimensional code. The simulation demonstrates that MBI occurs even for finite spheres, and high-density compression is still achievable with this setup. It also shows the optimal target structure, which maximizes ion flashing.

physics.plasm-ph↗

arXiv2025

Progress in relativistic laser-plasma interaction with kilotesla-level applied magnetic fields

We report on progress in the understanding of the effects of kilotesla-level applied magnetic fields on relativistic laser-plasma interactions. Ongoing advances in magnetic-field-generation techniques enable new and highly desirable phenomena, including magnetic-field-amplification platforms with reversible sign, focusing ion acceleration, and bulk-relativistic plasma heating. Building on recent advancements in laser-plasma interactions with applied magnetic fields, we introduce simple models for evaluating the effects of applied magnetic fields in magnetic-field amplification, sheath-based ion acceleration, and direct laser acceleration. These models indicate the feasibility of observing beneficial magnetic-field effects under experimentally relevant conditions and offer a starting point for future experimental design.

physics.plasm-ph↗

arXiv2025

Laser-driven electrodynamic implosion of fast ions in a thin shell

Collision of laser-driven subrelativistic high density ion flows provides a way to create extremely compressed ion conglomerates and study their properties. This paper presents a theoretical study of the electrodynamic implosion of ions inside a hollow spherical or cylindrical shell irradiated by femtosecond petawatt laser pulses. We propose to apply a very effective mechanism for ion acceleration in a selfconsistent field with strong charge separation, based on the oscillation of laser accelerated fast electrons in this field near the thin shell. Fast electrons are generated on the outer side of the shell under irradiation by the intense laser pulses. It is shown that ions, in particular protons, may be accelerated at the implosion stage to energies of tens and hundreds of MeV when a sub-micrometer shell is irradiated by femtosecond laser pulses with an intensity of 10^21 - 10^23 W cm^-2. .

physics.plasm-ph↗

arXiv2025

Generation of giga-electron-volt proton beams by micronozzle acceleration

Our proposed ion acceleration scheme, micronozzle acceleration (MNA), generates proton beams with extremely high kinetic energies on the giga-electron-volt (GeV) order. The underlying physics and performance of MNA are studied with two-dimensional particle-in-cell simulations. In MNA targets, a micron-sized hydrogen rod is embedded inside a hollow micronozzle. Subsequent illumination of the target along the symmetric axis by an ultraintense ultrashort laser pulse forms a strong electrostatic field with a long lifetime and an extensive space around the downstream tail of the nozzle. The electric field significantly amplifies the kinetic energies of the accelerated protons, and > GeV protons are generated at an applied laser intensity of 10^22 W/cm^2 .

physics.plasm-ph↗

arXiv2025

Gigagauss magnetic field generation by bladed microtube implosion

We demonstrate the generation of ultrahigh magnetic fields in the order of gigagauss using a bladed microtube target whose inner surface is periodically slanted in a sawtooth-like pattern. When irradiated by ultra-intense, ultrashort laser pulses, hot electrons with MeV energies are produced at the outer surface and swiftly transported to the inner surface, initiating a rapid implosion of plasma toward the central axis. The unique blade-induced asymmetry gives rise to vortex-shaped flows of ions and electrons near the center, forming strong azimuthal loop currents that generate ultrahigh magnetic fields at the center. Two-dimensional particle-in-cell simulations, supported by a simple analytical model, elucidate the underlying physics and reveal key scaling laws governing the field strength and spatial confinement.

physics.plasm-ph↗

arXiv2025

Self-similar multishock implosions for ultrahigh compression of matter

We present a class of self-similar solutions describing ultrahigh compression of a uniform-density target by spherically converging, stacked shock waves. Extending the classical Guderley model, we derive a scaling law for the final density of the form $\rho_{r}/\rho_{0} \propto \hat{P}^{\beta (N-1)}$, where $N$ is the number of shocks, $\hat{P}$ the stage pressure ratio, and $\beta$ a numerical exponent determined by the adiabatic index $\gamma$. One-dimensional hydrodynamic simulations confirm the validity of this scaling across a broad parameter range. Notably, the relation remains accurate even in the strongly nonlinear regime up to $\hat{P} \sim 70$, well beyond the perturbative limit, highlighting the robustness and practical relevance of the model. Owing to its volumetric geometry, this compression scheme inherently avoids the Rayleigh--Taylor instability, which typically compromises shell-based implosions, and thereby establishes a theoretical benchmark for instability-free compression in inertial confinement fusion.

physics.flu-dyn↗

arXiv2024

Mass composition of ultra-high energy cosmic rays from distribution of their arrival directions with the Telescope Array

We use a new method to estimate the injected mass composition of ultrahigh cosmic rays (UHECRs) at energies higher than 10 EeV. The method is based on comparison of the energy-dependent distribution of cosmic ray arrival directions as measured by the Telescope Array experiment (TA) with that calculated in a given putative model of UHECR under the assumption that sources trace the large-scale structure (LSS) of the Universe. As we report in the companion letter, the TA data show large deflections with respect to the LSS which can be explained, assuming small extra-galactic magnetic fields (EGMF), by an intermediate composition changing to a heavy one (iron) in the highest energy bin. Here we show that these results are robust to uncertainties in UHECR injection spectra, the energy scale of the experiment and galactic magnetic fields (GMF). The assumption of weak EGMF, however, strongly affects this interpretation at all but the highest energies E > 100 EeV, where the remarkable isotropy of the data implies a heavy injected composition even in the case of strong EGMF. This result also holds if UHECR sources are as rare as $2 \times 10^{-5}$ Mpc$^{-3}$, that is the conservative lower limit for the source number density.

astro-ph.HE↗

arXiv2024

Isotropy of cosmic rays beyond $10^{20}$ eV favors their heavy mass composition

We report an estimation of the injected mass composition of ultra-high energy cosmic rays (UHECRs) at energies higher than 10 EeV. The composition is inferred from an energy-dependent sky distribution of UHECR events observed by the Telescope Array surface detector by comparing it to the Large Scale Structure of the local Universe. In the case of negligible extra-galactic magnetic fields the results are consistent with a relatively heavy injected composition at E ~ 10 EeV that becomes lighter up to E ~ 100 EeV, while the composition at E > 100 EeV is very heavy. The latter is true even in the presence of highest experimentally allowed extra-galactic magnetic fields, while the composition at lower energies can be light if a strong EGMF is present. The effect of the uncertainty in the galactic magnetic field on these results is subdominant.

astro-ph.HE↗

arXiv2024

Observation of Declination Dependence in the Cosmic Ray Energy Spectrum

We report on an observation of the difference between northern and southern skies of the ultrahigh energy cosmic ray energy spectrum with a significance of ${\sim}8\sigma$. We use measurements from the two largest experiments$\unicode{x2014}$the Telescope Array observing the northern hemisphere and the Pierre Auger Observatory viewing the southern hemisphere. Since the comparison of two measurements from different observatories introduces the issue of possible systematic differences between detectors and analyses, we validate the methodology of the comparison by examining the region of the sky where the apertures of the two observatories overlap. Although the spectra differ in this region, we find that there is only a $1.8\sigma$ difference between the spectrum measurements when anisotropic regions are removed and a fiducial cut in the aperture is applied.

astro-ph.HE↗

arXiv2020

Sign reversal in magnetic field amplification by relativistic laser-driven microtube implosions

We demonstrate and explain the surprising phenomenon of sign reversal in magnetic field amplification by the laser-driven implosion of a structured target. Relativistically intense laser pulses incident on the outer surface of a microtube target consisting of thin opaque shell surrounding a $μ$m-scale cylindrical void drive an initial ion implosion and later explosion capable of generating and subsequently amplifying strong magnetic fields. While the magnetic field generation is enhanced and spatially smoothed by the application of a kilotesla-level seed field, the sign of the generated field does not always follow the sign of the seed field. One unexpected consequence of the amplification process is a reversal in the sign of the amplified magnetic field when, for example, the target outer cross section is changed from square to circular. Using 2D particle-in-cell simulations, we demonstrate that sign reversal is linked to the stability of the surface magnetic field of opposite sign from the seed which arises at the target inner surface during laser irradiation. The stability of the surface magnetic field and consequently the sign of the final amplified field depends sensitively on the target, laser, and seed magnetic field conditions, which could be leveraged to make laser-driven microtube implosions an attractive platform for the study of magnetic fields in high energy density plasma in regimes where sign reversal either is or is not desired.

physics.plasm-ph↗

arXiv2020

Generation of megatesla magnetic fields by intense-laser-driven microtube implosions

A microtube implosion driven by ultraintense laser pulses is used to produce ultrahigh magnetic fields. Due to the laser-produced hot electrons with energies of mega-electron volts, cold ions in the inner wall surface implode towards the central axis. By pre-seeding uniform magnetic fields on the kilotesla order, the Lorenz force induces the Larmor gyromotion of the imploding ions and electrons. Due to the resultant collective motion of relativistic charged particles around the central axis, strong spin current densities of ~ peta-ampere/cm2 are produced with a few tens of nm size, generating megatesla-order magnetic fields. The underlying physics and important scaling are revealed by particle simulations and a simple analytical model. The concept holds promise to open new frontiers in many branches of fundamental physics and applications in terms of ultrahigh magnetic fields.

physics.plasm-ph↗

arXiv2020

Birefringence in thermally anisotropic relativistic plasmas and its impact on laser-plasma interactions

One of the paradigm-shifting phenomena triggered in laser-plasma interactions at relativistic intensities is the so-called relativistic transparency. As the electrons become heated by the laser to relativistic energies, the plasma becomes transparent to the laser light even though the plasma density is sufficiently high to reflect the laser pulse in the non-relativistic case. This paper highlights the impact that relativistic transparency can have on laser-matter interactions by focusing on a collective phenomenon that is associated with the onset of relativistic transparency: plasma birefringence in thermally anisotropic relativistic plasmas. The optical properties of such a system become dependent on the polarization of light, and this can serve as the basis for plasma-based optical devices or novel diagnostic capabilities.

physics.plasm-ph↗

arXiv2020

Structural, transport, optical and electronic properties of Sr$_2$CoNbO$_6$ thin films

We study the effect of substrate induced strain on the structural, transport, optical and electronic properties of Sr$_2$CoNbO$_6$ double perovskite thin films. The reciprocal space mapping, $ϕ$-scan and high-resolution $θ$-2$θ$ scans of x-ray diffraction patterns suggest the epitaxial nature and high-quality of the films deposited on various single crystal ceramic substrates. A systematic enhancement in the dc electronic conductivity is observed with increase in the compressive strain, while a sharp reduction in case of tensile strain, which are further supported by change in the activation energy and density of states near the Fermi level. The optical band gap extracted from two distinct absorption bands, observed in the visible-near infrared spectroscopy show a non-monotonic behavior in case of compressive strain while significant enhancement with tensile strain. Unlike the bulk Sr$_2$CoNbO$_6$ (Co$^{3+}$ and Nb$^{5+}$), we observe different valence states of Co namely 2+, 3+ and 4+, and tetravalent Nb (4$d^1$) in the x-ray photoemission spectroscopy measurements. Moreover, a reduction in the average oxygen valency with the compressive strain due to enhancement in the covalent character of Co/Nb--O bond is evident. Interestingly, we observe sharp Raman active modes in these thin films, which indicates a significant enhancement in structural ordering as compared to the bulk.

cond-mat.mtrl-sci↗

arXiv2019

Spin-orbit torque-mediated spin-wave excitation as an alternative paradigm for femtomagnetism

Laser-induced femtosecond demagnetization, femtomagnetism, offers a potential route to develop faster magnetic storage devices. It is generally believed that the traditional spin-wave theory, which is developed for thermally driven slow demagnetization, can not explain this rapid demagnetization by design. Here we show that this traditional spin-wave theory, once augmented by laser-induced spin-orbit torque, provides a highly efficient paradigm for demagnetization, by capturing low-energy spin-wave excitation that is absent in existing mechanisms. Our paradigm is different from existing ones, but does not exclude them. Microscopically, we find that optical spin-orbit torque generates massive spin waves across several hundred lattice sites, collapsing the long-range spin-spin correlation within 20 fs. Our finding does not only explain new experiments, but also establishes an alternative paradigm for femtomagnetism. It is expected to have far-reaching impacts on future research.

cond-mat.mtrl-sci↗