Yoichiro Hironaka

SILICATE DUST SIZE DISTRIBUTION FROM HYPERVELOCITY COLLISIONS: IMPLICATIONS FOR DUST PRODUCTION IN DEBRIS DISKS

Fragments generated by high-velocity collisions between solid planetary bodies are one of the main sources of new interplanetary dust particles. However, only limited ranges of collision velocity, ejecta size, and target materials have been studied in previous laboratory experiments, and the collision condition that enables the production of dust-sized particles remains unclear. We conducted hypervelocity impact experiments on silicate rocks at relative velocities of 9 to 61 km s{sup -1}, which is beyond the upper limit of previous laboratory studies. Sub-millimeter-diameter aluminum and gold spheres were accelerated by laser ablation and were shot into dunite and basalt targets. We analyzed the surfaces of aerogel blocks deployed near the targets using an electron probe micro analyzer and counted the number of particles that contained the target material. The size distributions of ejecta ranged from five to tens of microns in diameter. The total cross-sectional area of dust-sized ejecta monotonically increased with the projectile kinetic energy, independent of impact velocity, projectile diameter, and projectile and target material compositions. The slopes of the cumulative ejecta-size distributions ranged from -2 to -5. Most of the slopes were steeper than the -2.5 or -2.7 that is expected for a collisional equilibrium distribution in a collisionmorexa0» cascade with mass-independent or mass-dependent catastrophic disruption thresholds, respectively. This suggests that the steep dust size-distribution proposed for the debris disk around HD172555 (an A5V star) could be due to a hypervelocity collision.«xa0less

Shock Hugoniot and temperature data for polystyrene obtained with quartz standard

Equation-of-state data, not only pressure and density but also temperature, for polystyrene (CH) are obtained up to 510 GPa. The region investigated in this work corresponds to an intermediate region, bridging a large gap between available gas-gun data below 60 GPa and laser shock data above 500 GPa. The Hugoniot parameters and shock temperature were simultaneously determined by using optical velocimeters and pyrometers as the diagnostic tools and the α-quartz as a new standard material. The CH Hugoniot obtained tends to become stiffer than a semiempirical chemical theoretical model predictions at ultrahigh pressures but is consistent with other models and available experimental data.

Impact experiments with a new technique for acceleration of projectiles to velocities higher than Earth's escape velocity of 11.2 km/s

[1]xa0The impact velocities of asteroids on Earth and other terrestrial planets can be greater than 10 km/s, and impacts at these high velocities can produce significant effects on the planetary surfaces. However, since macroscopic (>∼0.1 mm) projectiles with an aspect ratio of ∼1 are not easily accelerated to more than 10 km/s in laboratories, there are few detailed experimental studies. In this paper, we demonstrate that impact velocities greater than 10 km/s can be achieved with glass and aluminum projectiles of 0.1–0.3 mm in diameter using a high-power laser, GEKKO XII-HIPER at Institute of Laser Engineering, Osaka University. The velocity of the projectiles is estimated based on the images taken by high-speed X-ray streak and framing cameras. Projectiles collide into copper or LiF plate targets. The copper plates are recovered for analysis. The sizes of craters on the copper plates are not far from the extrapolations from previous work with lower velocities. A tantalum witness plate placed near the copper plates records a large number of secondary craters from each impact. In the case of the impacts of the LiF plates, we observe two emission lines of Li gas using a spectrometer with a streak camera. Thus, we can simulate the hypervelocity impacts with velocities higher than 10 km/s in laboratories.

Fast ignition realization experiment with high-contrast kilo-joule peta-watt LFEX laser and strong external magnetic field

A petawatt laser for fast ignition experiments (LFEX) laser system [N. Miyanaga et al., J. Phys. IV France 133, 81 (2006)], which is currently capable of delivering 2u2009kJ in a 1.5 ps pulse using 4 laser beams, has been constructed beside the GEKKO-XII laser facility for demonstrating efficient fast heating of a dense plasma up to the ignition temperature under the auspices of the Fast Ignition Realization EXperiment (FIREX) project [H. Azechi et al., Nucl. Fusion 49, 104024 (2009)]. In the FIREX experiment, a cone is attached to a spherical target containing a fuel to prevent a corona plasma from entering the path of the intense heating LFEX laser beams. The LFEX laser beams are focused at the tip of the cone to generate a relativistic electron beam (REB), which heats a dense fuel core generated by compression of a spherical deuterized plastic target induced by the GEKKO-XII laser beams. Recent studies indicate that the current heating efficiency is only 0.4%, and three requirements to achieve higher efficiency of the fast ignition (FI) scheme with the current GEKKO and LFEX systems have been identified: (i) reduction of the high energy tail of the REB; (ii) formation of a fuel core with high areal density using a limited number (twelve) of GEKKO-XII laser beams as well as a limited energy (4u2009kJ of 0.53-μm light in a 1.3u2009ns pulse); (iii) guiding and focusing of the REB to the fuel core. Laser–plasma interactions in a long-scale plasma generate electrons that are too energetic to efficiently heat the fuel core. Three actions were taken to meet the first requirement. First, the intensity contrast of the foot pulses to the main pulses of the LFEX was improved to >109. Second, a 5.5-mm-long cone was introduced to reduce pre-heating of the inner cone wall caused by illumination of the unconverted 1.053-μm light of implosion beam (GEKKO-XII). Third, the outside of the cone wall was coated with a 40-μm plastic layer to protect it from the pressure caused by imploding plasma. Following the above improvements, conversion of 13% of the LFEX laser energy to a low energy portion of the REB, whose slope temperature is 0.7u2009MeV, which is close to the ponderomotive scaling value, was achieved. To meet the second requirement, the compression of a solid spherical ball with a diameter of 200-μm to form a dense core with an areal density of ∼0.07u2009g/cm2 was induced by a laser-driven spherically converging shock wave. Converging shock compression is more hydrodynamically stable compared to shell implosion, while a hot spot cannot be generated with a solid ball target. Solid ball compression is preferable also for compressing an external magnetic field to collimate the REB to the fuel core, due to the relatively small magnetic Reynolds number of the shock compressed region. To meet the third requirement, we have generated a strong kilo-tesla magnetic field using a laser-driven capacitor-coil target. The strength and time history of the magnetic field were characterized with proton deflectometry and a B-dot probe. Guidance of the REB using a 0.6-kT field in a planar geometry has been demonstrated at the LULI 2000 laser facility. In a realistic FI scenario, a magnetic mirror is formed between the REB generation point and the fuel core. The effects of the strong magnetic field on not only REB transport but also plasma compression were studied using numerical simulations. According to the transport calculations, the heating efficiency can be improved from 0.4% to 4% by the GEKKO and LFEX laser system by meeting the three requirements described above. This efficiency is scalable to 10% of the heating efficiency by increasing the areal density of the fuel core.

Laser-shock compression and Hugoniot measurements of liquid hydrogen to 55 GPa

KYOKUGEN, Center for Quantum Science and Technology under Extreme Conditions,Osaka University, Toyonaka, Osaka 560-8531, Japan(Dated: January 7, 2011)The principal Hugoniot for liquid hydrogen was obtained up to 55 GPa under laser-driven shockloading. Pressure and density of compressed hydrogen were determined by impedance-matching toa quartz standard. The shock temperature was independently measured from the brightness of theshock front. Hugoniot data of hydrogen provide a good benchmark to modern theories of condensedmatter. The initial number density of liquid hydrogen is lower than that for liquid deuterium, andthis results in shock compressed hydrogen having a higher compression and higher temperature thandeuterium at the same shock pressure.

Femtosecond time-resolved x-ray diffraction from optical coherent phonons in CdTe(111) crystal

Coherent phonons excited in a CdTe(111) crystal by 70 fs laser irradiation have been investigated by femtosecond time-resolved x-ray diffraction. The longitudinal optical phonon with a frequency of approximately 5 THz near the Brillouin zone center has been detected as modulation in intensities of x-ray diffraction. Atomic displacement in the [111] direction in the coherent longitudinal optical phonon has been estimated.

In-situ spectroscopic observations of silicate vaporization due to >10 km/s impacts using laser driven projectiles

[1]xa0We present the results of shock-induced silicate vaporization experiments using laser driven hypervelocity projectiles. In-situ spectroscopic observations of shock-heated quartz and diopside were conducted. We observed both atomic emission lines and blackbody continuum. Because emission lines occur only in a gas phase, this observation indicates that the incipient vaporization of silicates actually occurs at >10 km/s. We estimated the peak-shock temperatures from the blackbody spectra. The obtained results suggest that the temperature dependence of the isochoric specific heat Cv depends rather strongly on material at extremely high pressures. Such difference in Cv dependence on temperature will influence the impact vaporization efficiency. Thus, investigation on the Cv of the other major silicates is necessary for understanding impact-related phenomena. Furthermore, the observed high intensity of emission lines shows the possibility that a variety of the thermodynamic variables of expanding silicate vapor can be measured with a higher speed spectrometer.

Shock‐induced silicate vaporization: The role of electrons

[1]xa0We conducted a spectroscopic study of shock-heated silicate (diopside) and obtained the time evolution of the spectral contents, the line widths of emission lines, and the time- and irradiance-averaged peak shock temperatures. The peak shock pressures ranged from 330 to 760 GPa. Time-resolved emission spectra indicated that the initial spectrum was blackbody radiation; the spectrum evolved to yield several ionic emission lines, which in turn evolved to yield atomic lines at the later stages. The shock-heated diopside was highly dissociated and ionized, even though it is likely to have been subjected to high-pressure conditions near the liquid–vapor phase boundary. The time evolution of the spectra, from ions to atoms, strongly suggests that electron recombination occurred in the expanding shock-induced diopside vapor. The time- and irradiance-averaged peak shock temperatures at >330 GPa were lower than the theoretical Hugoniot curve, with a constant isochoric specific heat, indicating endothermic shock-induced ionization. Thus, we conclude that electrons behave as an important energy reservoir in energy partitioning via endothermic shock-induced ionization and subsequent exothermic electron recombination. This electron behavior leads to a higher degree of vaporization after isentropic release and a lower cooling rate due to the exothermic electron recombination in expanding impact-induced silicate vapors than previously expected. These results will affect the predictions associated with hypervelocity impact events in planetary science, such as the origin of the Moon and chemical reactions and production of silicate dust particles in impact-generated silicate vapor clouds.

Time-resolved spectroscopic observations of shockinduced silicate ionization

We conducted time-resolved spectroscopic observations of shock-heated quartz and forsterite using a high-power laser. The results revealed that ionization occur easily under shockinduced warm dense conditions. We compare the obtained temperatures on the Hugoniot with a few theories. The comparison suggests that the contribution of shock-induced ionization to the isochoric specific heat during shock compression is ~1 kb/atom for quartz and forsterite. Shock-induced ionization and subsequent electron recombination leads to a larger amount of vapor and may lead to dynamical and chemical evolution of silicate vapor clouds different from our current understandings.

Measurement of preheating due to radiation and nonlocal electron heat transport in laser-irradiated targets

This paper reports an experimental study on preheating of laser-irradiated targets. We performed temperature measurements at the rear surface of laser-irradiated targets under conditions of two different laser wavelengths (0.35 or 0.53u2002μm) and several intensities (2×1013–1×1014u2002W/cm2) in order to verify an effect of radiation and nonlocal electron heat transport. The preheating temperature was evaluated by observing self-emission, reflectivity, and expansion velocity at the rear surface of planar polyimide foils. The experimental results show that the x-ray radiation is dominant for preheating for 0.35-μm laser irradiation, but contribution of nonlocal electron heat transport is not negligible for 0.53-μm laser irradiation conditions.