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"Mima, Kunioki"
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Plasma density limits for hole boring by intense laser pulses
High-power lasers in the relativistic intensity regime with multi-picosecond pulse durations are available in many laboratories around the world. Laser pulses at these intensities reach giga-bar level radiation pressures, which can push the plasma critical surface where laser light is reflected. This process is referred to as the laser hole boring (HB), which is critical for plasma heating, hence essential for laser-based applications. Here we derive the limit density for HB, which is the maximum plasma density the laser can reach, as a function of laser intensity. The time scale for when the laser pulse reaches the limit density is also derived. These theories are confirmed by a series of particle-in-cell simulations. After reaching the limit density, the plasma starts to blowout back toward the laser, and is accompanied by copious superthermal electrons; therefore, the electron energy can be determined by varying the laser pulse length.
Different energy transport mechanisms come into play when intense laser pulses interact with dense plasma. Here the authors provide a limit on the plasma density reachable with an intense laser and an insight into the hole boring process.
Journal Article
Single-shot laser-driven neutron resonance spectroscopy for temperature profiling
The temperature measurement of material inside of an object is one of the key technologies for control of dynamical processes. For this purpose, various techniques such as laser-based thermography and phase-contrast imaging thermography have been studied. However, it is, in principle, impossible to measure the temperature of an element inside of an object using these techniques. One of the possible solutions is measurements of Doppler brooding effect in neutron resonance absorption (NRA). Here we present a method to measure the temperature of an element or an isotope inside of an object using NRA with a single neutron pulse of approximately 100 ns width provided from a high-power laser. We demonstrate temperature measurements of a tantalum (Ta) metallic foil heated from the room temperature up to 617 K. Although the neutron energy resolution is fluctuated from shot to shot, we obtain the temperature dependence of resonance Doppler broadening using a reference of a silver (Ag) foil kept to the room temperature. A free gas model well reproduces the results. This method enables element(isotope)-sensitive thermometry to detect the instantaneous temperature rise in dynamical processes.
Non-contact thermometry is one of the key technologies for modern science and industry. Here, authors demonstrated measurement of temperature of an element using neutron resonance spectroscopy with Doppler broadening with single intense short neutron pulse provided from high peak power.
Journal Article
Status and future prospects of laser fusion and high power laser applications
In Asia, there are many institutes for the R&D of high power laser science and applications. They are 5 major institutes in Japan, 4 major institutes in China, 2 institutes in Korea, and 3 institutes in India. The recent achievements and future prospects of those institutes will be over viewed. In the laser fusion research, the FIREX-I project in Japan has been progressing. The 10kJ short pulse LFEX laser has completed and started the experiments with a single beam. About 1kJ pulse energy will be injected into a cone target. The experimental results of the FIREX experiments will be presented. As the target design for the experiments, a new target, namely, a double cone target was proposed, in which the high energy electrons are well confined and the heating efficiency is significantly improved. Together with the fusion experiments, Osaka University has carried out laboratory astrophysics experiments on photo ionizing plasmas to observe a unique X-ray spectrum from non-LTE plasmas. In 2008, Osaka university has started a new Photon research center in relation with the new program: Consortium for Photon Science and Technology: C-PhoST, in which ultra intense laser plasmas research and related education will be carried out for 10 years. At APRI, JAEA, the fundamental science on the relativistic laser plasmas and the applications of laser particle acceleration has been developed. The application of laser ion acceleration has been investigated on the beam cancer therapy since 2007. In China, The high power glass laser: Shenguan-II and a peta watt beam have been operated to work on radiation hydro dynamics at SIOFM Shanghai. The laser material and optics are developed at SIOFM and LFRC. The IAPCM and the IOP continued the studies on radiation hydrodynamics and on relativistic laser plasmas interactions. At LFRC in China, the construction of Shenguan III glass laser of 200kJ in blue has progressed and will be completed in 2012. Together with the Korean program, I will overview the above Asian programs.
Journal Article
Absolute Local Quantification of Li as Function of State-of-Charge in All-Solid-State Li Batteries via 2D MeV Ion-Beam Analysis
Direct observation of the lithiation and de-lithiation in lithium batteries on the component and microstructural scale is still difficult. This work presents recent advances in MeV ion-beam analysis, enabling quantitative contact-free analysis of the spatially-resolved lithium content and state-of-charge (SoC) in all-solid-state lithium batteries via 3 MeV proton-based characteristic x-ray and gamma-ray emission analysis. The analysis is demonstrated on cross-sections of ceramic and polymer all-solid-state cells with LLZO and MEEP/LIBOB solid electrolytes. Different SoC are measured ex-situ and one polymer-based operando cell is charged at 333 K during analysis. The data unambiguously show the migration of lithium upon charging. Quantitative lithium concentrations are obtained by taking the physical and material aspects of the mixed cathodes into account. This quantitative lithium determination as a function of SoC gives insight into irreversible degradation phenomena of all-solid-state batteries during the first cycles and locations of immobile lithium. The determined SoC matches the electrochemical characterization within uncertainties. The presented analysis method thus opens up a completely new access to the state-of-charge of battery cells not depending on electrochemical measurements. Automated beam scanning and data-analysis algorithms enable a 2D quantitative Li and SoC mapping on the µm-scale, not accessible with other methods.
Journal Article
Feasibility study of laser-driven neutron sources for pharmaceutical applications
We predict the production yield of a medical radioisotope
^{67}$
Cu using
^{67}$
Zn(n, p)
^{67}$
Cu and
^{68}$
Zn(n, pn)
^{67}$
Cu reactions with fast neutrons provided from laser-driven neutron sources. The neutrons were generated by the p+
^9\\mathrm{Be}$
and d+
^9$
Be reactions with high-energy ions accelerated by laser–plasma interaction. We evaluated the yield to be (3.3
$\\pm$
0.5)
$\\times$
10
^5$
atoms for
^{67}$
Cu, corresponding to a radioactivity of 1.0
$\\pm$
0.2 Bq, for a Zn foil sample with a single laser shot. Using a simulation with this result, we estimated
^{67}$
Cu production with a high-frequency laser. The result suggests that it is possible to generate
^{67}$
Cu with a radioactivity of 270 MBq using a future laser system with a frequency of 10 Hz and 10,000-s radiation in a hospital.
Journal Article
Picosecond snapshot imaging of electric fields induced on a cone guide target designed for fast ignition scenario
In this study, we experimentally evaluate the ion transportation through a cone guide target, which accelerates ions up to MeV energies via target normal sheath acceleration, and transports them onto the position of imploding fuel in the fast ignition scenario of nuclear fusion. We measured the electric and magnetic fields (EM-fields) induced by return current streaming along the cone wall by proton radiography, and we report that the EM-fields are predominantly induced within a temporal window up to 30 ps after the laser injection. The magnitude of the electric field is maximized around 13 ps, reaching $4.0\\times 10^{10} \\mathrm {V}\\ \\mathrm {m}^{-1}$, when the magnetic field is below 200 T. The present scheme provides insights on the EM-fields evaluation in the time region that is difficult to treat with simulations due to the computing resources.
Journal Article
Suppression of beam merging and hosing instabilities in magnetized fast ignition fusion
The magnetized two-stream instability has been investigated in linear and nonlinear regimes. Using a linear analysis of the Vlasov equation in two dimensional wave number space, we studied the competition between modes with wave numbers parallel and transverse to the beam direction. In the analysis, it is found that a near transverse mode is still unstable even for large electron temperature. The nonlinear analysis is performed using a hybrid simulation code. When a sufficiently strong magnetic field is applied along the beam direction, the instabilities are well suppressed and the electron stream becomes laminar. When the magnetic field strength is not large enough, however, electron flow stagnates and the total number of beam electrons penetrating the plasma is largely reduced.
Journal Article
Double layer acceleration by laser radiation
It is shown that it is possible to accelerate micro-foils to velocities from 108 cm/s up to relativistic velocities without the disturbance of the Rayleigh-Taylor instability. The acceleration occurs due to the radiation pressure of proper high power lasers. In these systems, the ablation force is negligible relative to the ponderomotive force that dominates the acceleration. The laser irradiances of 1017 W/cm2 < IL < 1021 W/cm2 with a pulse duration of the order of 10 picoseconds can accelerate a micro-foil by the laser radiation pressure to velocities as high as 109 cm/s before breaking by Rayleigh Taylor (RT) instability. Similarly, laser irradiances of IL > 1021 W/cm2 with pulse duration of the order of 10 femtoseconds can accelerate a micro-foil to relativistic velocities without RT breaking. Due to the nature of the accelerating ponderomotive force, in both the relativistic and non-relativistic cases, the structure of the accelerated target contains a double layer (DL) at the interface of the laser-target interaction. The DL acts as a piston during the acceleration process. The influence of the DL surface tension on the RT instability is also analyzed in this paper.
Journal Article
Generation of pre-formed plasma and its reduction for fast-ignition
We investigated generation of pre-formed plasma on plates and inside cone targets due to a pre-pulse before the arrival of the main ultra-intense laser pulse in the fast-ignition scheme of the inertial confinement fusion. We estimated the pre-pulse level to be 130 mJ for LFEX laser used in the 2009 FIREX experiment, and the density gradient scale length of the pre-formed plasma inside the cone target to be 27–47 microns between the critical and 1/10 of the critical density, based on the two-dimensional radiation hydrodynamic simulations. In order to reduce the generation of pre-formed plasma, we investigated a thin CH foil pre-pulse absorber, and proposed using a cone target with a pointed tip. We simulated CH plasma expansion to show that the CH foils works as a pre-pulse absorber. We also show the aluminum pointed tip of the cone target can delay the shock arrival time by 20 ps, much longer than the delay for the 10 micron thickness gold tip used in the typical implosion of GXII at Osaka University.
Journal Article