Abstract
A relativistic petawatt laser light can heat a high Z metal above keV temperatures isochorically and ionize it almost fully. Copious hard x-rays are emitted from a high-Z hot plasma, which ...acts as an x-ray source, while x-ray emissions serve as an energy sink for the plasma. The isochoric heating of a solid silver foil irradiated by a relativistic laser light is studied with the help of 1D and 2D particle-in-cell simulations including Coulomb collisions, ionizations, and radiation processes. We find that the radiation power from the keV silver plasma is comparable to the incident laser power when the laser intensity is less than 10
19
W cm
−2
. Thus, the plasma formation and particle acceleration are affected by the radiation, e.g. a highly compressed shock formation. Once the laser heating is over, the silver plasma loses its energy on a timescale of picoseconds, during which hard x-rays flash. The radiation spectra of the keV silver plasma are also presented.
To enhance the core heating efficiency in electron-driven fast ignition, we propose fast-electron beam guiding using externally applied longitudinal magnetic fields. Based on particle-in-cell ...simulations for FIREX-class experiments, we demonstrate sufficient beam guiding performance in collisional dense plasma by kilotesla-class external magnetic fields for the case with moderate mirror ratio ( 10). Boring of the mirror field was found through the formation of magnetic pipe structure due to the resistive effects, which indicates a possibility of beam guiding in high mirror field for higher laser intensity and/or longer pulse duration.
Abstract
A laser inertial fusion energy (IFE) reactor requires repetitive injection of fuel pellets and laser engagement to fuse fusion fuel beyond a few Hz. We demonstrate 10 Hz free-fall bead ...pellet injection and laser engagement with γ-ray generation. Deuterated polystyrene beads with a diameter of 1 mm were engaged by counter illuminating ultra-intense laser pulses with an intensity of 5 × 10
17
W cm
−2
at 10 Hz. The spatial distribution of free-fall beads was 0.86 mm in the horizontal direction and 0.18 mm in the vertical direction. The system operated for more than 5 min and 3500 beads were supplied with achieved frequencies of 2.1 Hz for illumination on the beads and 0.7 Hz for γ-ray generation; these frequencies were three times greater than with the previous 1 Hz injection system. The duration of operation was limited by the pellet supply. This injection and engagement system could be used for laser IFE research platforms.
To enhance the core heating efficiency in fast ignition laser fusion, the concept of relativistic electron beam guiding by external magnetic fields was evaluated by integrated simulations for FIREX ...class targets. For the cone-attached shell target case, the core heating performance deteriorates by applying magnetic fields since the core is considerably deformed and most of the fast electrons are reflected due to the magnetic mirror formed through the implosion. On the other hand, in the case of a cone-attached solid ball target, the implosion is more stable under the kilo-tesla-class magnetic field. In addition, feasible magnetic field configuration is formed through the implosion. As a result, the core heating efficiency doubles by magnetic guiding. The dependence of core heating properties on the heating pulse shot timing was also investigated for the solid ball target.
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.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A novel direct core heating fusion process is introduced, in which a preimploded core is predominantly heated by energetic ions driven by LFEX, an extremely energetic ultrashort pulse laser. ...Consequently, we have observed the D(d,n)^{3}He-reacted neutrons (DD beam-fusion neutrons) with the yield of 5×10^{8} n/4π sr. Examination of the beam-fusion neutrons verified that the ions directly collide with the core plasma. While the hot electrons heat the whole core volume, the energetic ions deposit their energies locally in the core, forming hot spots for fuel ignition. As evidenced in the spectrum, the process simultaneously excited thermal neutrons with the yield of 6×10^{7} n/4π sr, raising the local core temperature from 0.8 to 1.8 keV. A one-dimensional hydrocode STAR 1D explains the shell implosion dynamics including the beam fusion and thermal fusion initiated by fast deuterons and carbon ions. A two-dimensional collisional particle-in-cell code predicts the core heating due to resistive processes driven by hot electrons, and also the generation of fast ions, which could be an additional heating source when they reach the core. Since the core density is limited to 2 g/cm^{3} in the current experiment, neither hot electrons nor fast ions can efficiently deposit their energy and the neutron yield remains low. In future work, we will achieve the higher core density (>10 g/cm^{3}); then hot electrons could contribute more to the core heating via drag heating. Together with hot electrons, the ion contribution to fast ignition is indispensable for realizing high-gain fusion. By virtue of its core heating and ignition, the proposed scheme can potentially achieve high gain fusion.
In implosion experiments of a cone-guided target using Gekko XII laser, the lasers on the cone side are not irradiated to avoid the irradiation of the cone. In such condition, the implosion process ...is done highly asymmetrically. Thus we evaluated the effects of the asymmetric implosion on the compression ratio of the fuel in Gekko XII irradiation orientation by three-dimensional hydro simulations. In this paper, we discuss the degradation of the compression ratio by asymmetric implosion and show that the compression ratio can be enhanced by adjusting the laser intensity between each beam to reduce the asymmetry of the implosion.
A tailored-pulse-imploded core with a diameter of 70 μm is flashed by counterirradiating 110 fs, 7 TW laser pulses. Photon emission (>40 eV) from the core exceeds the emission from the imploded ...core by 6 times, even though the heating pulse energies are only one seventh of the implosion energy. The coupling efficiency from the heating laser to the core using counterirradiation is 14% from the enhancement of photon emission. Neutrons are also produced by counterpropagating fast deuterons accelerated by the photon pressure of the heating pulses. A collisional two-dimensional particle-in-cell simulation reveals that the collisionless two counterpropagating fast-electron currents induce mega-Gauss magnetic filaments in the center of the core due to the Weibel instability. The counterpropagating fast-electron currents are absolutely unstable and independent of the core density and resistivity. Fast electrons with energy below a few MeV are trapped by these filaments in the core region, inducing an additional coupling. This might lead to the observed bright photon emissions.
The effect of pre-plasma on core heating in cone-guiding fast ignition is evaluated by two-dimensional particle-in-cell (PIC) and Fokker–Planck (FP) simulations. If the long-scale pre-plasma exists ...in the cone, the generated fast electron energy becomes too high for effective core heating. As a result, the energy coupling from laser to core η
L→core
is reduced by 80% compared with the case without a pre-plasma. Even for the case without a pre-plasma, η
L→core
obtained in the simulation is smaller than that required for 5 keV heating in FIREX-I. In order to enhance η
L→core
, we propose a new target design ‘extended double cone with short inner cone wall’ for fast electron guiding to imploded core and show sufficient improvement of heating efficiency using this new cone on the basis of PIC and FP hydro-simulations.
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