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K Mima

Publications and source records attributed to K Mima.

At least 19 recordsLinked to original sources
PubMed2006

Surface acceleration of fast electrons with relativistic self-focusing in preformed plasma.

We report an observation of surface acceleration of fast electrons in intense laser-plasma interactions. When a preformed plasma is presented in front of a solid target with a higher laser intensity, the emission direction of fast electrons is changed to the target surface direction from the laser and specular directions. This feature could be caused by the formation of a strong static magnetic field along the target surface which traps and holds fast electrons on the surface. In our experiment, the increase in the laser intensity due to relativistic self-focusing in plasma plays an important role for the formation. The strength of the magnetic field is calculated from the bent angle of the electrons, resulting in tens of percent of laser magnetic field, which agrees well with a two-dimensional particle-in-cell calculation. The strong surface current explains the high conversion efficiency on the cone-guided fast ignitor experiments.

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PubMed2006

Optimum hot electron production with low-density foams for laser fusion by fast ignition.

We propose a foam cone-in-shell target design aiming at optimum hot electron production for the fast ignition. A thin low-density foam is proposed to cover the inner tip of a gold cone inserted in a fuel shell. An intense laser is then focused on the foam to generate hot electrons for the fast ignition. Element experiments demonstrate increased laser energy coupling efficiency into hot electrons without increasing the electron temperature and beam divergence with foam coated targets in comparison with solid targets. This may enhance the laser energy deposition in the compressed fuel plasma.

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PubMed2004

Fast ion acceleration in ultraintense laser interactions with an overdense plasma.

In order to study the ion acceleration processes in ultraintense laser-plasma interactions with solid targets, neutron spectra from deuteron-deuteron (D-D) nuclear reactions were measured. Spectra were obtained when (50-100 TW, 0.5-1 ps) laser light irradiated obliquely incident deuterated plastic targets as a function of laser polarization, intensity, and density scale length of the preformed plasma. The experimental data are compared with three-dimensional Monte Carlo simulations. The results indicate that the ion momentum distribution is collimated and directed into the bulk of the target to the target normal direction with an energy that is linearly proportional to the laser intensity. The distribution of the accelerated ions was observed to change from isotropic to anisotropic with laser prepulse intensity. All the results indicate that the ion acceleration is dominated by an electrostatic field generated from a charge displacement of the hot electrons at the target surface.

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PubMed2003

Anomalous resistivity resulting from MeV-electron transport in overdense plasma.

Laser produced hot electron transport in an overdense plasma is studied by three-dimensional particle-in-cell simulations. Hot electron currents into the plasma generate neutralizing return currents in the cold plasma electrons, leading to a configuration which is unstable to electromagnetic Weibel and tearing instabilities. The resulting current filaments self-organize through a coalescence process finally settling into a single global current channel. The plasma return current experiences a strong anomalous resistivity due to diffusive flow of cold electrons in the magnetic perturbations. The resulting electrostatic field leads to an anomalously rapid stopping of fast MeV electrons (almost 3 orders of magnitude stronger than that through classical collisional effects).

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PubMed2002

Proposed double-layer target for the generation of high-quality laser-accelerated ion beams.

In order to achieve a high-quality, i.e., monoenergetic, intense ion beam, we propose the use of a double-layer target. The first layer, at the target front, consists of high-Z atoms, while the second (rear) layer is a thin coating of low-Z atoms. The generation of high-quality proton beams from the double-layer target, irradiated by an ultraintense laser pulse, is demonstrated with three-dimensional particle-in-cell simulations.

Ions↗
PubMed2002

Three-dimensional particle-in-cell simulations of energetic electron generation and transport with relativistic laser pulses in overdense plasmas.

The interaction of relativistic laser light with overdense plasmas is studied by three-dimensional particle-in-cell simulations. Generation of layered current sheets and quasistatic magnetic fields is observed near the target surface owing to anisotropic laser filamentation and Weibel instabilities. Later these current sheets tear into filaments that partially merge with each other to form isolated magnetic channels penetrating into the dense plasmas. It is found that fast electron energy flow is not only inside the magnetic channels but also it is widely distributed outside the channels. This is possible because of electron anomalous diffusion across self-generated magnetic fields. Consequently, the total hot electron current exceeds a few hundred kiloamperes and is much larger than the Alfvén current. Hence a considerable amount of energy flows towards the plasma core. Significant heating of the bulk plasma electrons is also observed.

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PubMed2002

Observation of neutron spectrum produced by fast deuterons via ultraintense laser plasma interactions.

We report the first precise spectral measurement of fast neutrons produced in a deuterated plastic target irradiated by an ultraintense sub-picosecond laser pulse. The 500-fs, 50-J, 1054-nm laser pulse was focused on the deuterated polystyrene target with an intensity of 2 x 10(19) W/cm(2). The neutron spectra were observed at 55 degrees and 90 degrees to the rear target normal. The neutron emission was 7 x 10(4) per steradian for each detector. The observed neutron spectra prove the acceleration of deuterons and neutron production by d(d,n)3He reactions in the target. The neutron spectra were compared with Monte Carlo simulation results and the deuteron's directional anisotropy and energy spectrum were studied. We conclude that 2% of the laser energy was converted to deuterons, which has an energy range of 30 keV up to 3 MeV.

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PubMed2002

Stochastic heating and acceleration of electrons in colliding laser fields in plasma.

We propose a mechanism that leads to efficient acceleration of electrons in plasma by two counterpropagating laser pulses. It is triggered by stochastic motion of electrons when the laser fields exceed some threshold amplitudes, as found in single-electron dynamics. It is further confirmed in particle-in-cell simulations. In vacuum or tenuous plasma, electron acceleration in the case with two colliding laser pulses can be much more efficient than with one laser pulse only. In plasma at moderate densities, such as a few percent of the critical density, the amplitude of the Raman-backscattered wave is high enough to serve as the second counterpropagating pulse to trigger the electron stochastic motion. As a result, even with one intense laser pulse only, electrons can be heated up to a temperature much higher than the corresponding laser ponderomotive potential.

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PubMed2001

Anisotropic filamentation instability of intense laser beams in plasmas near the critical density.

The relativistic filamentation instability (RFI) of linearly polarized intense laser beams in plasmas near the critical density is investigated. It is found that the RFI is anisotropic to transverse perturbations in this case; a homogeneous laser beam evolves to a stratified structure parallel to the laser polarization direction, as demonstrated recently with three-dimensional particle-in-cell simulations by Nishihara et al. [Proc. SPIE 3886, 90 (2000)]. A weakly relativistic theory is developed for plasmas near the critical density. It shows that the anisotropy of the RFI results from a suppression of the instability in the laser polarization direction due to the electrostatic response. The anisotropic RFI is also analyzed based on an envelope equation for the laser beam. Finally, the envelope equation is solved numerically, and anisotropic filamentation and self-focusing are illustrated.

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PubMed2001

Fast heating of ultrahigh-density plasma as a step towards laser fusion ignition.

Modern high-power lasers can generate extreme states of matter that are relevant to astrophysics, equation-of-state studies and fusion energy research. Laser-driven implosions of spherical polymer shells have, for example, achieved an increase in density of 1,000 times relative to the solid state. These densities are large enough to enable controlled fusion, but to achieve energy gain a small volume of compressed fuel (known as the 'spark') must be heated to temperatures of about 108 K (corresponding to thermal energies in excess of 10 keV). In the conventional approach to controlled fusion, the spark is both produced and heated by accurately timed shock waves, but this process requires both precise implosion symmetry and a very large drive energy. In principle, these requirements can be significantly relaxed by performing the compression and fast heating separately; however, this 'fast ignitor' approach also suffers drawbacks, such as propagation losses and deflection of the ultra-intense laser pulse by the plasma surrounding the compressed fuel. Here we employ a new compression geometry that eliminates these problems; we combine production of compressed matter in a laser-driven implosion with picosecond-fast heating by a laser pulse timed to coincide with the peak compression. Our approach therefore permits efficient compression and heating to be carried out simultaneously, providing a route to efficient fusion energy production.

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PubMed2001

Characterization of 5' flanking region of alpha isoform of rat Ca2+/calmodulin-dependent protein kinase II gene and neuronal cell type specific promoter activity.

The 5' flanking region of the alpha isoform of the rat Ca2+/calmodulin-dependent protein kinase II (alpha CaM kinase II) gene was isolated in 2.3 kbp of genomic sequence. Functional analysis of alpha CaM kinase II promoter deletion mutants fused to a reporter gene in neuroblastoma, including N18TG2, NG108-15, and CAD cells revealed strong transcriptional activity localized 100-145 bp, and a potent silencer 199-275 bp upstream of the transcription start site. The promoter is inactive in non-neuronal cells including BALB/c 3T3, Chinese hamster ovary, HT1080, and C6 glioma cells. These results indicated that the alpha CaM kinase II gene is transcribed from a tissue-specific promoter which is under intense negative control.

Animals↗
PubMed2001

Structure formation and tearing of an MeV cylindrical electron beam in a laser-produced plasma.

The stability of a cylindrical, solid hot electron beam propagating in a high density plasma has been studied using a two-dimensional, hybrid Darwin code. The initially solid beam evolves into a hollow, annular beam due to the Weibel instability and generates strong magnetic fields on both sides of the annular ring. The annular structure subsequently breaks up into several beamlets via a mechanism similar to a tearing instability. It is found that the magnetic fields parallel to the direction of beam propagation also grow due to the tearing process.

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PubMed2000

Angular distributions of fast electrons, ions, and Bremsstrahlung x/gamma-rays in intense laser interaction with solid targets.

We study the angular distributions of fast electrons, ions, and bremsstrahlung x/ gamma-rays generated during the interaction of an ultrashort intense laser pulse with solid targets. A relation is found on the angular directions for fast electrons and ions as a function of the particle's kinetic energy, experienced Coulomb potential changes, and the incident angle of the laser pulse. It is valid independent of the acceleration mechanisms and the polarization of the laser pulse, as confirmed by particle-in-cell simulations. The angular distribution of bremsstrahlung x/gamma-rays is presented to show explicitly its correlation with the corresponding angular distributions of electrons.

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PubMed2000

Electron acceleration by a short relativistic laser pulse at the front of solid targets.

Acceleration of electrons in a low-density plasma in front of a solid target by a propagating short ultraintense laser pulse is studied. When the laser is reflected at the target surface the accelerated electrons, with energy scaling as the laser intensity, continue to move forward inertially and thus escape from the pulse. Electrons accelerated backwards by the reflected light can attain even higher energies due to their longer acceleration length and their high initial momentum from a relativistic return current.

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PubMed2000

Feed-out of rear surface perturbation due to rarefaction wave in laser-irradiated targets

We report experimental results on hydrodynamic perturbation transfer from the rear to the front of laser-irradiated targets. Flat polystyrene foils with rear-surface perturbations were irradiated by partially coherent light. We observed phase inversion of the rear surface after the shock breakout at the rear surface. Perturbations on the laser-irradiated surface arose due to the rippled rarefaction wave. Experimental results were well reproduced by a simple model with unperturbed hydrodynamic quantities calculated from the one-dimensional simulation.

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PubMed2000

Laser-hole boring into overdense plasmas measured with soft X-Ray laser probing

A laser self-focused channel formation into overdense plasmas was observed using a soft x-ray laser probe system with a grid image refractometry (GIR) technique. 1.053 &mgr;m laser light with a 100 ps pulse duration was focused onto a preformed plasma at an intensity of 2x10(17) W/cm (2). Cross sections of the channel were obtained which show a 30 &mgr;m diameter in overdense plasmas. The channel width in the overdense region was kept narrow as a result of self-focusing. Conically diverging density ridges were also observed along the channel, indicating a Mach cone created by a shock wave due to the supersonic propagation of the channel front.

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PubMed2000

Long-scale jet formation with specularly reflected light in ultraintense laser-plasma interactions

Long-scale jetlike x-ray emission was observed in a 100-TW laser-plasma interaction. The jet was well collimated with a divergence of 30-40 mrad and continued from the target surface into underdense regions for a distance over 4 mm in the specular direction of the laser light. A two-dimensional particle-in-cell simulation shows an electron acceleration with the specularly reflected laser light and collimation of the electron stream by a self-generated magnetic field, resulting in the electron jet to the direction of the specularly reflected light.

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