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We report an experimental observation suggesting plasma channel formation by focusing a relativistic laser pulse into a long-scale-length preformed plasma. The channel direction coincides with the laser axis. Laser light transmittance measurement indicates laser channeling into the high-density plasma with relativistic self-focusing. A three-dimensional particle-in-cell simulation reproduces the plasma channel and reveals that the collimated hot-electron beam is generated along the laser axis in the laser channeling. These findings hold the promising possibility of fast heating a dense fuel plasma with a relativistic laser pulse.

The development of ultra-intense lasers1 has facilitated new studies in laboratory astrophysics2 and high-density nuclear science3, including laser fusion4,5,6,7. Such research relies on the efficient generation of enormous numbers of high-energy charged particles. For example, laser–matter interactions at petawatt (1015 W) power levels can create pulses of MeV electrons8,9,10 with current densities as large as 1012 A cm-2. However, the divergence of these particle beams5 usually reduces the current density to a few times 106 A cm-2 at distances of the order of centimetres from the source. The invention of devices that can direct such intense, pulsed energetic beams will revolutionize their applications. Here we report high-conductivity devices consisting of transient plasmas that increase the energy density of MeV electrons generated in laser–matter interactions by more than one order of magnitude. A plasma fibre created on a hollow-cone target guides and collimates electrons in a manner akin to the control of light by an optical fibre and collimator. Such plasma devices hold promise for applications using high energy-density particles and should trigger growth in charged particle optics.

Energetic electrons and protons are observed when a target consisting of a reentrant cone with a disk at the tip is irradiated by a petawatt (PW) laser at an intensity of approximately ${10}^{19}\phantom{\rule{0.3em}{0ex}}\mathrm{W}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. The angular distribution of the electrons and protons, dependent on the open angle of the reentrant cone, is found to differ from that in the case when a target with planar geometry is used. Two jet beams are observed, in directions parallel to the cone axis and normal to the cone-shaped wall. The number and cutoff energies of the generated protons are also related to the open angle of the cone. The efficiency of the generation of energetic electrons from the cone target is 2-3 times higher than that from a simple plane target. These results indicate a guiding of the PW laser beam in the cone geometry.

The characteristics of the forward hot electrons produced by subpicosecond laser-plasma interactions are studied for different laser polarizations at laser intensities from subrelativistic to relativistic. The peak of the hot electron beam produced by p-polarized laser beam shifts to the laser propagation direction from the target normal direction as the laser intensity reaches the relativistic. For s-polarized laser pulse, hot electrons are mainly directed to the laser axis direction. The temperature and the maximum energy of hot electrons are much higher than that expected by the empirical scaling law. The energy spectra of the hot electrons evolve to be a single-temperature structure at relativistic laser intensities from the two-temperature structure at subrelativistic intensities. For relativistic laser intensities, the forward hot electrons are less dependent on the laser polarization under the laser conditions. The existing of a preplasma formed by the laser amplified spontaneous emission pedestal plays an important role in the interaction. One-dimensional particle-in-cell simulations reproduce the most characteristics observed in the experiment.

Plasma shielding is an important concept to study if the material damage could be suppressed with plasma layer properly prepared by absorbing the incoming plasma flux onto a divertor target in MFE or the first wall in IFE reactors. First experimental evidence of this effect is reported. Two plasma plumes (n∼ 1012/cm3, Te ∼ 1 eV) are created with two laser beams. The laser ablation plasma plumes are created at laser energy density up to 10 J/cm2 and are crossed each other. 12∼59 % of the incoming plasma particles are shielded with the collisions of the other plasma plume. By observing the material dependence of colliding effects, the effect to plasma shielding is discussed. c © 2012 The Japan Society of Plasma Science and Nuclear Fusion Research

Relativistic electron motion in a plasma due to an intense laser pulse modifies the refractive index and leads to two effects: relativistic induced transparency and relativistic self-focusing. A combination of the above two effects enables transmission of laser energy deep into plasmas which is useful for fast ignition of inertial fusion. This so-called super-penetration sensitively depends on the focal position of the laser intensity due to the inhomogeneous density profile of the plasma and convergence of the laser pulse by final focusing optics. Experiments were conducted at vacuum focused laser intensities between 3.3 and 4. 4 × 1018 W cm−2 at peak plasma densities between 23 and 75nc, where nc is the critical density of the plasma. We introduced a scenario: the laser beam diameter at nc/4 density must be smaller than the plasma wavelength to achieve whole beam self-focusing. An optimum focus was found experimentally by measuring the plasma channel, laser transmittance and electron spectra. All three data are consistent with one another and numerical calculations based on a paraxial approximation model suggest that this optimum focus corresponds to the scenario described above.

An electron spectrometer (ESM) using imaging plate (IP) is designed and tested to measure relativistic electrons which are generated from laser-plasma interactions. The measurable energy range extends to 1 GeV or even higher. The absolute sensitivity of IP for 1 GeV electrons was calibrated using electrons from Linac in SPring-8. An IP has enough sensitivity for 1-GeV electrons. The electron spectrometer, which is measurable to ~ 1 GeV, has been developed using IP detector.

We demonstrated efficient fast heating of a solid wire target attached on a hollow entrant cone target with an ultra-intense laser. The heating temperatures were 1.2-2.1 keV and the heated temperatures were independent of the wire length, implying strong collimation of high energy density electrons produced in an interaction of the ultra-intense laser light with the cone-wire target along the wire. The high energy density electrons propagated along the wire target with maintaining the high energy density of electrons.

In fast ignition for laser thermonuclear fusion, an ultra intense laser (UIL) pulse irradiates an imploded plasma in order to fast-heat a high-density core with hot electrons generated in laser-plasma interactions. An UIL pulse needs to make plasma channel via laser self-focusing and to propagate through the corona plasma to reach close enough to the core. Hot electrons are used for heating the core. Therefore the propagation of laser light in the high-density plasma region and spatial distribution of hot electron are important in issues in order to study the feasibility of this scheme. We measure the spatial distribution of hot electron when the laser light propagates into the high-density plasma region by self-focusing.

We studied the influence of electrostatic and magnetic fields at the rear surface of solid targets on the hot electron emission generated by ultra-intense laser pulses. The number of emitted electrons increases in experiments when a pre-plasma is created on the target rear surface. The formation of the electrostatic potential is clearly retarded in particle-in-cell simulations resulting in the elongation of time window for hot electron emission. The increase in the number of electrons is consistent with the Alfven current within the elongated time window.

Multi MeV proton beam is generated via target normal sheath acceleration when the target is irradiated with an ultra-intense laser pulse. In addition, a unique structure, “zonal pattern”, of energetic protons is observed in the perpendicular directions of the target edges using triangular targets. The sheath field production on the target edges may be responsible for this zonal pattern. Two dimensional particle-in-cell simulations show that the electrostatic field initially produced at around the cross point at the laser axis and the rear surface expands on the target surface in time. The field enhancement occurs at the target edges when the sheath field reaches there. The enhanced field can accelerate protons in a zonal pattern. c

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.

Introduction: The purpose of this study was to quantitatively evaluate cortical bone thickness in various locations in the maxilla and the mandible. In addition, the distances from intercortical bone surface to root surface, and distances between the roots of premolars and molars were also measured to determine the acceptable length and diameter of the miniscrew for anchorage during orthodontic treatment. Methods: Three-dimensional computed tomographic images were reconstructed for 10 patients. Cortical bone thicknesses were measured in the buccal and lingual regions mesial and distal to the first molar, distal to the second molar, and in the premaxillary region at 2 different levels. Differences in cortical bone thickness at 3 angles (30°, 45°, and 90°) were also assessed. Distances of the intercortical bone surface to the root surface and the root proximity were also measured at the above areas. Results: Significantly less cortical bone thickness was observed at the buccal region distal to the second molar compared with other areas in the maxilla. Significantly more cortical bone was observed on the lingual side of the second molar compared with the buccal side. In the mandible, mesial and distal to the second molar, significantly more cortical bone was observed compared with the maxilla. Furthermore, significantly more cortical bone was observed at the anterior nasal spine level than at Point A in the premaxillary region. Cortical bone thickness resulted in approximately 1.5 times as much at 30° compared with 90° Significantly more distance from the intercortical bone surface to the root surface was observed at the lingual region than at the buccal region mesial to the first molar. At the distal of the first mandibular molar, significantly more distance was observed compared to that in the mesial, and also compared with both distal and mesial in the maxillary first molar. There was significantly more distance in root proximity in the mesial area than in distal area at the first molar, and significantly more distance was observed at the occlusal level than at the apical level. Conclusions: These data show that the safest location for placing miniscrews might be mesial or distal to the first molar, and an acceptable size of the miniscrew is less than approximately 1.5 mm in diameter and approximately 6 to 8 mm in length.

We observe a hollow structure and a fine ring in the proton images from a petawatt scale laser interaction with a ``cone-fiber'' target. The protons related to the hollow structure are accelerated from the cone-tip surface and deflected later by a radial electric field surrounding the fiber. Those associated with the fine ring are accelerated from the fiber surface by this radial electric field. This field is found to decay exponentially within 3 ps from about $5\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }\text{ }\mathrm{V}/\mathrm{m}$. Two-dimensional particle-in-cell simulations produce similar proton angular distributions.

A high energy electron spectrometer has been designed and tested using imaging plate (IP). The measurable energy range extends from 1to100MeV or even higher. The IP response in this energy range is calibrated using electrons from L-band and S-band LINAC accelerator at energies 11.5, 30, and 100MeV. The calibration has been extended to 0.2MeV using an existing data and Monte Carlo simulation Electron Gamma Shower code. The calibration results cover the energy from 0.2to100MeV and show almost a constant sensitivity for electrons over 1MeV energy. The temperature fading of the IP shows a 40% reduction after 80min of the data taken at 22.5°C. Since the fading is not significant after this time we set the waiting time to be 80min. The oblique incidence effect has been studied to show that there is a 1∕cosθ relation when the incidence angle is θ.

Hot electrons and optical emission are measured from the rear surface of a metallic foil. The spectra of the optical emission in the near infrared region have a sharp spike around the wavelength of the incident laser pulse. The optical emission is ascribed to coherent transition radiation due to microbunching in the hot electron beam. It is found that the optical emission closely correlates with the hot electrons accelerated in resonance absorption.