R. Kodama

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.

Calibration of imaging plate for high energy electron spectrometer

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.

Laser light and hot electron micro focusing using a conical target

The laser light propagation inside the conical target had been studied by three-dimensional particle-in-cell simulations. It is found that the laser light is optically guided inside the conical target and focused at the tip of the cone. The intensity increases up to several tens of times in a several micron focal spot. It is the convergence of hot electrons to the head of the cone that is observed as a consequence of the surface electron flow guided by self-generated quasistatic magnetic fields and electrostatic sheath fields. As a result, the hot electron density at the tip is locally ten times greater than the case of using a normal flat foil.

Fast heating scalable to laser fusion ignition.

Rapid heating of a compressed fusion fuel by a short-duration laser pulse is a promising route to generating energy by nuclear fusion, and has been demonstrated on an experimental scale using a novel fast-ignitor geometry. Here we describe a refinement of this system in which a much more powerful, pulsed petawatt (1015 watts) laser creates a fast-heated core plasma that is scalable to full-scale ignition, significantly increasing the number of fusion events while still maintaining high heating efficiency at these substantially higher laser energies. Our findings bring us a step closer to realizing the production of relatively inexpensive, full-scale fast-ignition laser facilities.

Prepulse-free petawatt laser for a fast ignitor

We have developed a prepulse-free short-pulse Nd:glass laser system of 0.9-PW peak power to heat a pre-imploded high-density plasma. An optical parametric chirped amplification system is introduced to reduce the prepulses to an amplitude (1.5/spl times/10/sup -8/) of that of the main pulse. The compressor is a double-path grating pair system 94 cm in diameter compressing the 50-cm-diameter laser beam to 470 fs. An off-axis parabolic mirror has focused the 420-J energy to an intensity of 2.5/spl times/10/sup 19/ W/spl middot/cm/sup -2/. Part of the front end of the chirped pulse is seeded into the preamplifier of the GEKKO XII laser, used to implode a pellet target, to enable the petawatt laser to irradiate the pre-imploded pellet during stagnation of a few tens of picoseconds.

Review of progress in Fast Ignition

Marshall Rosenbluth’s extensive contributions included seminal analysis of the physics of the laser-plasma interaction and review and advocacy of the inertial fusion program. Over the last decade he avidly followed the efforts of many scientists around the world who have studied Fast Ignition, an alternate form of inertial fusion. In this scheme, the fuel is first compressed by a conventional inertial confinement fusion driver and then ignited by a short (∼10ps) pulse, high-power laser. Due to technological advances, such short-pulse lasers can focus power equivalent to that produced by the hydrodynamic stagnation of conventional inertial fusion capsules. This review will discuss the ignition requirements and gain curves starting from simple models and then describe how these are modified, as more detailed physics understanding is included. The critical design issues revolve around two questions: How can the compressed fuel be efficiently assembled? And how can power from the driver be delivered efficient...

Plasma devices to guide and collimate a high density of MeV electrons

The development of ultra-intense lasers has facilitated new studies in laboratory astrophysics and high-density nuclear science, including laser fusion. 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 electrons with current densities as large as 1012 A cm-2. However, the divergence of these particle beams 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.

Observation of proton rear emission and possible gigagauss scale magnetic fields from ultra-intense laser illuminated plastic target

CR-39 film stacks are used to measure the energy and angular distribution of protons emitted from the rear surface of ultra-intense laser illuminated plastic foils. The experiment suggests that the energetic protons are dragged away from the rear surface, where the hot electron formed a virtual cathode. The two-dimensional particle in cell simulation supports this hypothesis. For 5 (100) μm thick target, 1.8×109 protons have a slope temperature of 3 (2) MeV. The ring structure of proton emission leads us to the hypothesis that a toroidal magnetic field associated with the hot electrons works on the fast ions and deflects them. From the logarithmic slope of the ring diameter versus the ion energy, the product of the magnetic field × the length over which it works on the ions is estimated to be up to 1000 MG⋅μm. The simulation shows that a strong toroidal magnetic field was excited at the target rear side with expansion of plasmas. The proton’s angular distribution from the rear surface has the logarithmic ...

Nuclear fusion: Fast heating scalable to laser fusion ignition

Rapid heating of a compressed fusion fuel by a short-duration laser pulse is a promising route to generating energy by nuclear fusion, and has been demonstrated on an experimental scale using a novel fast-ignitor geometry. Here we describe a refinement of this system in which a much more powerful, pulsed petawatt (1015 watts) laser creates a fast-heated core plasma that is scalable to full-scale ignition, significantly increasing the number of fusion events while still maintaining high heating efficiency at these substantially higher laser energies. Our findings bring us a step closer to realizing the production of relatively inexpensive, full-scale fast-ignition laser facilities.

Plasma jet formation and magnetic-field generation in the intense laser plasma under oblique incidence

Long-scale jet-like x-ray emission was observed in the experiments on the interactions of 100 TW laser light with plasmas. The jet formation is investigated by simulations with a two-dimensional particle code. When an S-polarized intense laser is irradiated obliquely on an overdense plasma, collimated MeV electrons are observed from the critical surface in the specular reflection direction. These electrons are found to be accelerated through the coronal plasma by the reflected laser light, which was modulated at the reflection point. The quasisteady magnetic channel occurs simultaneously and collimates the energetic electrons along the specular direction. In the case of P-polarized laser, it is found that an outgoing electron stream is induced at the critical surface due to Brunel mechanism. Megagauss quasistatic magnetic fields are generated and pinch the electron stream. The angle of ejected electron depends on the electron’s energy. The emission direction of the jet generated by the P-polarized light is determined by the canonical momentum conservation along the target surface.