K. Mima

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 108u2009K (corresponding to thermal energies in excess of 10u2009keV). 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.

Magnetic instability by the relativistic laser pulses in overdense plasmas

The magnetic instability driven by the relativistic electron stream generated by ultra-intense laser is investigated with the help of a two-dimensional particle-in-cell simulation, which includes the relativistic binary collision. The linear growth rate of the instability is also studied using the two-stream fluid model, which consists of a fast electron current and a return current. The growth rate is evaluated numerically from the linearized equations of the electron fluids with the Maxwell equations. The kinetic effects of electrons on the magnetic instability are found to reduce the growth rate. The growth rate is maximum at the wavelength near the plasma skin length because of the plasma kinetic effect. When the initial plasma temperature is high, the growth rates of shorter wavelengths are significantly reduced. In the collisional plasma, the growth rates of modes whose wavelength is shorter than the plasma skin length are suppressed and the spectral peak of the growth rate shifts to long wavelength...

Studies of ultra-intense laser plasma interactions for fast ignition

Laser plasma interactions in a relativistic parameter regime have been intensively investigated for studying the possibility of fast ignition in inertial confinement fusion (ICF). Using ultra-intense laser systems and particle-in-cell (PIC) simulation codes, relativistic laser light self-focusing, super hot electrons, ions, and neutron production, are studied. The experiments are performed with ultra-intense laser with 50 J energy, 0.5–1 ps pulse at 1053 nm laser wavelength at a laser intensity of 1019u200aW/cm2. Most of the laser shots are studied under preformed plasma conditions with a 100 μm plasma scale length condition. In the study of laser pulse behavior in the preformed plasmas, a special mode has been observed which penetrated the preformed plasma all the way very close to the original planar target surface. On these shots, super hot electrons have been observed with its energy peak exceeding 1 MeV. The energy transport of the hot electrons has been studied with making use of Kα emissions from a see...

Collimated electron jets by intense laser beam-plasma surface interaction under oblique incidence

Oblique incidence of a

High-energy ion generation by short laser pulses

p

Stimulated photon cascade and condensate in a relativistic laser-plasma interaction

-polarized laser beam on a fully ionized plasma with a low density plasma corona is investigated numerically by particle-in-cell and Vlasov simulations in two dimensions. A single narrow self-focused current jet of energetic electrons is observed to be projected into the corona nearly normal to the target. Magnetic fields enhance the penetration depth of the electrons into the corona. A scaling law for the angle of the ejected electrons with incident laser intensity is given.

Overdense propagation of a relativistically intense laser light

This paper reviews the many recent advances at the Center for Ultrafast Optical Science (CUOS) at the University of Michigan in multi-MeV ion beam generation from the interaction of short laser pulses focused onto thin foil targets at intensities ranging from 1017 to 1019 W/cm2. Ion beam characteristics were studied by changing the laser intensity, laser wavelength, target material, and by depositing a well-absorbed coating. We manipulated the proton beam divergence using shaped targets and observed nuclear transformation induced by high-energy protons and deuterons. Qualitative theoretical approaches and fully relativistic two-dimensional particle-in-cell simulations modeled energetic ion generation. Comparison with experiments sheds light on ion energy spectra for multi-species plasma, the dependences of ion-energy on preplasma scale length and solid density plasma thickness, and laser-triggered isotope yield. Theoretical predictions are also made with the aim of studying ion generation for high-power lasers with the energies expected in the near future, and for the relativistic intensity table-top laser, a prototype of which is already in operation at CUOS in the limits of several-cycle pulse duration and a single-wavelength spot size.

High Energy Ion Generation

The propagation of a linearly polarized relativistic laser pulse in an underdense plasma is studied by fluid-Maxwell and particle-in-cell simulations. A nonlinear interplay between backward and forward stimulated Raman scattering instabilities produces a strong spatial modulation of the light pulse and the down cascade in its frequency spectrum. The Raman cascade saturates by a unique photon condensation at the bottom of the light spectra near the electron plasma frequency, related to strong depletion and possible break-up of the laser beam. In the final stage of the cascade-into-condensate mechanism, the depleted downshifted laser pulse is gradually transformed into a train of ultra-short relativistic light solitons.

Fast heating scalable to laser fusion ignition: Nuclear fusion

One-dimensional (1D) propagation of a relativistically intense circularly polarized electromagnetic (EM) wave in an over-critical density plasma is investigated. Cases of fast group velocity to which ions cannot follow the motion and of slow propagation in which ion dynamics plays an important role are discussed. However, electrons can be treated as in static force balance keeping local charge neutrality. It is shown that plane waves are always unstable in the overdense plasma. In particular, two types of modulational instability are found in the case of slow propagation and their growth rates are obtained. It is also shown that an envelope solitary wave solution can be obtained in an overdense region. Density limit for the solitary wave propagation is obtained as a function of its amplitude. The solitary wave is a rarefaction wave for the case of fast propagation, while it becomes of compressional character propagating with supersonic speed for the case of slow propagation. A general expression for the propagation speed as a function of the plasma density and the solitary wave amplitude is obtained for the compressional solitary wave, and the upper and lower limits of the density (or the amplitude) for given amplitude (or density) are obtained. A three-dimensional (3D) effect is briefly discussed and a boundary value problem is formulated for the case in which the plasma fills a half space with the other half space being in vacuum. For the case of an EM wave with ultrarelativistic intensity the transmission coefficient into an over-critical density plasma is found to be a universal function of the ratio of the incident wave amplitude to the plasma density.

Progress of direct drive laser fusion research at ILE, Osaka

We report on multi-MeV ion beam generation from the interaction of a 10 TW, 400 fs, 1.053 μm laser focused onto thin foil targets at intensities ranging from 1017 to 1019 W/cm2. Ion beam characteristics were studied by changing laser intensity, the preformed plasma scale-length and target material initial conductivity. We manipulated the proton beam divergence by using shaped targets. We observed nuclear transformation induced by high-energy protons and deuterons. A fully relativistic two-dimensional particle-in-cell simulation modeled energetic ion generation. These simulations identify the mechanism for the hot electron generation at the laser-plasma interface. Comparison with experiments sheds light on the dependence of ion-energy on preplasma scale length and solid density plasma thickness as well as relates ion energies for multi-species plasma.