Hideaki Takabe

Self‐consistent growth rate of the Rayleigh–Taylor instability in an ablatively accelerating plasma

The linear stability of an ablating plasma is investigated as an eigenvalue problem by assuming the plasma to be at the stationary state. For various structures of the ablating plasma, the growth rate is found to be expressed well in the form γ=α(kg)1/2 −βkVa, where α=0.9, β≂3–4, and Va is the flow velocity across the ablation front, and is found to agree well with recent two‐dimensional simulations in a classical transport regime. Short‐wavelength lasers inducing enhanced mass ablation are suggested to be advantageous to stable implosion because of the ablative stabilization.

Scalings of implosion experiments for high neutron yield

A series of experiments focused on high neutron yield has been performed with the Gekko‐XII green laser system [Nucl. Fusion 27, 19 (1987)]. Deuterium–tritium (DT) neutron yield of 1013 and pellet gain of 0.2% have been achieved. Based on the experimental data from more than 70 irradiations, the scaling laws of the neutron yield and the related physical quantities have been studied. Comparison of the experimental neutron yield with that obtained by using a one‐dimensional fluid code has led to the conclusion that most of the neutrons produced in the stagnation phase of the computation are not observed in the experiment because of fuel–pusher mixing, possibly induced by the Rayleigh–Taylor instability. The coupling efficiency and ablation pressure have been calculated using the ion temperature measured experimentally. A coupling efficiency of 5.5% and an ablation pressure of 50 Mbar have been obtained.

Observation of magnetic field generation via the Weibel instability in interpenetrating plasma flows

Astrophysical processes are often driven by collisionless plasma shock waves. The Weibel instability, a possible mechanism for developing such shocks, has now been generated in a laboratory set-up with laser-generated plasmas.

Nonrelativistic Collisionless Shocks in Unmagnetized Electron-Ion Plasmas

We show that the Weibel-mediated collisionless shocks are driven at nonrelativistic propagation speed (0.1c < V < 0.45c) in unmagnetized electron-ion plasmas by performing two-dimensional particle-in-cell simulations. It is shown that the profiles of the number density and the mean velocity in the vicinity of the shock transition region, which are normalized by the respective upstream values, are almost independent of the upstream bulk velocity, i.e., the shock velocity. In particular, the width of the shock transition region is ~100 ion inertial lengths, independent of the shock velocity. For these shocks the energy density of the magnetic field generated by the Weibel-type instability within the shock transition region reaches typically 1%-2% of the upstream bulk kinetic energy density. This mechanism probably explains the robust formation of collisionless shocks, for example, driven by young supernova remnants, with no assumption of an external magnetic field in the universe.

Direct-drive hydrodynamic instability experiments on the GEKKO XII laser

Hydrodynamic instabilities, such as the Rayleigh–Taylor (R–T) instability, play a critical role in inertial confinement fusion as they finally cause fuel-pusher mixing that potentially quenches thermonuclear ignition. Good understanding of the instabilities is necessary to limit the mixing within a tolerable level. A series of experiments has been conducted on the GEKKO XII laser facility [C. Yamanaka et al., IEEE J. Quantum Electron. QE-17, 1639 (1981)] to measure hydrodynamic instabilities in planar foils directly irradiated by 0.53 μm laser light. It has been found that (1) the imprint is reasonably explained by an imprint model based on the equation of motion with the pressure perturbation smoothed by the cloudy-day effect, and (2) the experimental R–T growth rate is significantly reduced from the classical growth rate due probably to ablative stabilization enhanced by nonlocal heat transport.

X-ray astronomy in the laboratory with a miniature compact object produced by laser-driven implosion

It has been suggested that the extreme states of matter generated by high-intensity lasers could allow conditions similar to those in the vicinity of black holes to be studied in the lab. The observation of striking similarities between the X-ray spectra emitted by a laser-driven laboratory plasma and those measured from two high-mass binary star systems suggests such potential has been realized.

Electrostatic and electromagnetic instabilities associated with electrostatic shocks: Two-dimensional particle-in-cell simulation

A two-dimensional electromagnetic particle-in-cell simulation with the realistic ion-to-electron mass ratio of 1836 is carried out to investigate the electrostatic collisionless shocks in relatively high-speed (∼3000 km s−1) plasma flows and also the influence of both electrostatic and electromagnetic instabilities, which can develop around the shocks, on the shock dynamics. It is shown that the electrostatic ion-ion instability can develop in front of the shocks, where the plasma is under counterstreaming condition, with highly oblique wave vectors as was shown previously. The electrostatic potential generated by the electrostatic ion-ion instability propagating obliquely to the shock surface becomes comparable with the shock potential and finally the shock structure is destroyed. It is also shown that in front of the shock the beam-Weibel instability gradually grows as well, consequently suggesting that the magnetic field generated by the beam-Weibel instability becomes important in long-term evolution ...

Numerical study of pair creation by ultraintense lasers

Now that intensity of lasers has reached 1020 W/cm2, electron–positron pairs can be created by the irradiation of such ultraintense lasers on a thin gold foil. The energy of electrons produced by ultraintense lasers reaches more than several tens of MeV. Such high energy electrons become a source for creating electron–positron pairs via interaction with nuclei. There are a few processes that create electron–positron pairs in this situation. Two processes, call the trident process and the Bethe–Heitler process, are considered in this study. A numerical simulation code based on a relativistic Fokker–Planck equation is developed for studying the hot electron transport. The equation is solved by assuming one-dimensional real space and two-dimensional momentum space with axial symmetry. It is found that the total positron yield increases logarithmically with the increase of the laser intensity, and the resultant energy distribution of the created positron is found to have a peak near the energy of 1–2 MeV.

Rayleigh–Taylor instability in a spherically stagnating system

The Rayleigh–Taylor instability induced on the fuel–pusher interface in the deceleration phase of shell implosion is investigated in spherical geometry. A linearized equation for the perturbation from the background dynamics described by a self‐similar motion is solved analytically and numerically. The effective growth rate of the Rayleigh–Taylor instability is not found to be sensitive to the compressibility. In a spherical system where the gravity and wavelength of the perturbation vary in time and space, the growth of the perturbation is found to be approximately expressed in the form ‖ξ‖∝ Rc exp[∫(αAkeffgeff)1/2dt], where keff and geff are the effective wavelength and gravity at the contact surface, with Rc the radius and αA the Atwood number.

Collisionless shock generation in high-speed counterstreaming plasma flows by a high-power laser

The experimental demonstration of the formation of a strong electrostatic (ES) collisionless shock has been carried out with high-speed counterstreaming plasmas, produced by a high-power laser irradiation, without external magnetic field. The nearly four times density jump observed in the experiment shows a high Mach-number shock. This large density jump is attributed to the compression of the downstream plasma by momentum transfer by ion reflection of the upstream plasma. Particle-in-cell (PIC) simulation shows the production of a collisionless high Mach-number ES shock with counterstreaming interaction of two plasma slabs with different temperatures and densities, as pointed out by Sorasio et al. [Phys. Rev. Lett. 96, 045005 (2006)]. It is speculated that the shock discontinuity is balanced with the momentum of incoming and reflected ions and the predominant pressure of the electrons in the downstream with PIC simulation.