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On the basis of two-dimensional coupled relativistic Fokker–Planck and fluid simulations, the core heating by fast electron beam in subignition cone-guiding fast ignition targets is investigated. It was found that the magnetic field due to the thermoelectric current reduces the beam pinching due to the resistive magnetic field. Compared with the carbon-deuterium fuel used in the early phase of Fast Ignition Realization Experiment, phase-I, the core heating of deuterium-tritium (DT) fuel used in the later phase is less effective. One reason is the slower temperature relaxation between bulk electron and ion for the DT case, which is previously reported by Johzaki et al. [Phys. Plasmas 15, 062702 (2008)]. In addition, in the lower-Z DT case, the pinching effects by the resistive magnetic field are weaker due to smaller resistivity; the optical thickness for fast electron beam is smaller, which leads to further reduction in core heating efficiency. It was also found that the fast electron beam quality deteriorates during the transport in the only 10 μm thickness high-Z (e.g., Au and Cu) cone tip because of the collisional scattering and drag. As the result, the core heating efficiency considerably goes down compared to the case neglecting the transport in the cone tip. [ABSTRACT FROM AUTHOR]

In the fast ignition realization experiment project phase-I (FIREX-I) [H. Azechi and the FIREX Project, Plasma Phys. Control. Fusion 48, B267 (2006)], core heating up to an ion temperature of 5 keV is expected for subignition-class carbon-deuterium (CD) and deuterium-tritium (DT) fuels. The dependence of the achieved ion temperature on heating pulse parameters, and core density is investigated using two-dimensional simulations. Since the core size in FIREX-I is insufficient for self-ignition, and the confinement time is comparable to the heating duration (∼10 ps), the temperature relaxation between the bulk electrons and ions is important for efficient ion heating. High compression (a core density of ρ>200 g/cm3) is required for pure DT fuel to shorten the relaxation time. In this case, a heating energy of Eh>2 kJ and a duration of τh<10 ps are required to achieve an ion temperature of 5 keV. For CD and DT+foam fuels, the requirement for implosion is relaxed since the relaxation is fast, due to a large effective charge; 5 keV ion heating is achieved at ρ>=50 g/cm3 with Eh>2 kJ and τh∼10 ps. [ABSTRACT FROM AUTHOR]

In the fast ignition (FI) scheme, at first, high-density fuel core plasma is assembled by implosion laser, and is then heated by petawatt laser to achieve a fusion burning condition. The formation of high-density fuel core plasma is one of the key issues for FI. A typical target for FI is a shell fitted with a reentrant gold cone to make a pass for heating laser. The ablated plasma of gold cone interferes with the implosion dynamics, which is quite different from that of the conventional central-hot-spot approach. Therefore, the dynamics of a nonspherical implosion must be controlled to assemble high density and high areal density. Numerical simulations are performed to study radiation hydrodynamics of cone-guided implosions. In the results, the effect of the cone on implosion dynamics is clarified. The cone surface is irradiated by the radiation and ablated plasma affects the imploding shell. Coating on the cone, which tamps the gold plasma, is effective to improve the implosion performance, although the result does not satisfy the condition of core plasma for the first stage of the Fast Ignition Realization Experiment [K. Mima et al., Proceedings of the IAEA Fusion Energy Conference, Lyon, 2002 (IAEA, Vienna, 2002), Paper No. IAEA-CN-94/IF/03]. [ABSTRACT FROM AUTHOR]

Effective cleaning of tin contamination on the collecting mirrors in extreme ultraviolet source is one of the key techniques to improve throughput and cost performance of extreme ultraviolet lithography. Hydrogen radicals produced in hydrogen plasma that is induced by wideband extreme ultraviolet radiation are expected to be utilized for in situ tin contamination cleaning in extreme ultraviolet sources. In this Letter, we clarified absolute density and cleaning ability of the hydrogen radicals produced by intense extreme ultraviolet pulse through ground state population density measurement by laser-induced fluorescence technique. The experimentally obtained radical parameters coincided well with simulation results and collisional radiative model. It was found that the extreme ultraviolet induced plasma was in quasi-steady state with abundant amount of hydrogen radicals in ground state. Further, it was found that the in situ tin contamination cleaning in extreme ultraviolet lithography source would become more practical with increase in operational parameters, such as extreme ultraviolet emission intensity, gas pressure, and radical production cross section. [ABSTRACT FROM AUTHOR]

The self-similar propagation of expanding spherical flames in lean hydrogen–air mixtures at elevated pressure was experimentally investigated using a dual-chamber apparatus with the quartz windows that enabled observing a flame radius of up to 110 mm. The flame images of the fully developed cellular structure were recorded, and the largest values of r / r c and Pe for the dual-chamber apparatus were estimated. The acceleration exponent, α , corresponding to the fractal dimension in lean hydrogen–air mixtures at elevated pressure was evaluated by plotting the experimental flame radius as a function of time. In this study, the experimental values of α (α = 1.25–1.43) occur within a wide range of r / r c (r / r c = 2–15). The observed trend of α to increase with a decrease in equivalence ratio, φ , and an increase in initial pressure, P i , is owing to the effects of the diffusional–thermal and Darrieus–Landau instabilities. The value of α increased with an increase in r / r c and saturated at α = 1.43 at r / r c > 10, and the saturated values of the hydrogen–air flame are within α = 1.4–1.5. The results demonstrated that the flame propagation in lean hydrogen–air mixtures was classified as laminar (r / r c < 1), acceleration (r / r c > 1), transition (1 < r / r c < 10), and self-similar (r / r c > 10) regimes. Consequently, the practical three-dimensional fractal dimensions for accidental hydrogen–air explosions are D 3 = 2.24–2.33. • Self-similar propagation of lean hydrogen-air flames was experimentally investigated. • The acceleration exponent and fractal excess in lean hydrogen-air flames were evaluated. • The value of exponent increased with an increase in dimensionless radius and saturated to the critical value. [ABSTRACT FROM AUTHOR]

One of the critical issues in lithography using extreme ultraviolet (EUV) light is tin contamination of the EUV collector mirrors in the tin-based LPP-EUV light source. The contamination can be removed by the reaction of tin atoms with hydrogen radicals producing stannane (SnH4), which is gaseous at the normal temperature. Hydrogen radicals can be produced from hydrogen molecules through photo-dissociation and photo-ionization induced by broadband radiation emitted from the EUV light source. In this work, an efficient production of hydrogen radical using vacuum ultraviolet (VUV) light emitted from laser-produced high-Z plasma is experimentally demonstrated. A two-dimensional radiation hydrodynamic simulation, coupled with photoionization and photo-dissociation cross sections, also shows the efficient hydrogen radical production by increased VUV light emission, as observed in the experiment. [ABSTRACT FROM AUTHOR]

Abstract In this study, Minimum Ignition Energy (MIE) and Minimum Explosible Concentration (MEC) of L-isoleucine and glycine powder with two different particle size distributions were experimentally investigated. The experimental results showed that the values of MIE and MEC of L-isoleucine powder were lower than those of glycine powder. The pyrolysis and the combustible products of glycine and L-isoleucine powders were measured. The results showed that a number of flammable and combustible materials are formed during the pyrolysis of L-isoleucine, although few flammable compounds are generated during the pyrolysis of glycine due to polymerization. This result indicated that the L-isoleucine powder has a high risk of dust explosion as compared to the glycine powder. Consequentially, an appropriate risk assessment for the L-isoleucine powder with fine particles is necessary in the powder handle processes. Graphical abstract Unlabelled Image Highlights • The values of MIE and MEC of L-isoleucine were lower than those of glycine. • A number of flammable materials during pyrolysis of L-isoleucine were formed. • Few combustible compounds during pyrolysis of glycine were generated. • L-isoleucine has a high risk of dust explosion as compared to the glycine. [ABSTRACT FROM AUTHOR]

The development of cellularity for expanding spherical hydrogen-air flames at elevated pressure, owing to hydrodynamic and diffusional-thermal instabilities was experimentally investigated using high-speed schlieren images. The critical flame radii and critical Peclet numbers at which cracks begin to propagate and for the onset of flame acceleration were obtained. The dependency of Pe cl on Ma for hydrogen-air flames was compared with that for other fuels. The results showed that cellular development depends on the intensity of the diffusional-thermal instability. The acceleration exponent and fractal excess for the acceleration regime of a hydrogen-air flame were evaluated. The values show that the transition regime and self-similar regime exist in the acceleration regime of expanding spherical hydrogen-air flames. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Electron energy characteristics generated by the irradiation of ultraintense laser pulses onto solid targets are controlled by using cone targets. Two parameters characterizing the laser-cone interaction are introduced, which are cone angle and the ratio of the laser spot size to the cone tip size. By changing these parameters, the energy absorption rate, laser irradiance at the cone tip, and electron acceleration at the cone tip and side wall are controlled. The optimum cone targets for fast ignition are 30° cone angle with double-cone geometry, and a tip size comparable to the core size, with the irradiation of a laser pulse with a spot size of about four times the cone tip size. Cone targets have the possibility to enhance the maximum energy of laser-accelerated protons by using a smaller angle cone depending on the laser f-number. [ABSTRACT FROM AUTHOR]

The effect of laser ignition on the deflagration-to-detonation transition (DDT) was experimentally investigated. Explosive gas, which was 0.87[(1/4)C 2 H 4 +(3/4)O 2 ]+0.13N 2 contained in a smooth-wall tube at 100 kPa and approximately 20 °C, was ignited by a 1064-nm 12-ns laser at either 8 or 88.8 mm from the closed tube end connected to the gas-feeding pipe, where the incident laser energy was either 40, 80, or 120 mJ. When the gas was ignited at 8 mm from the closed tube end, although laser ignition promoted DDT, the run-up distance to DDT was shortened by approximately 10% only. This was because the DDT behavior was essentially the same as the one observed typically in ordinary spark-plug ignition, in which a flame is accelerated approximately to be a choked flame before the detonation onset. When the gas was ignited at 88.8 mm from the closed tube end, two DDT scenarios were observed. The first scenario was the typical one mentioned above. In the second scenario, the detonation onset was induced by the collision between the flame and a shock wave following the flame before the flame became the choked flame. Significant DDT promotion was realized in the second scenario by the use of laser ignition with a larger energy. The shock wave inducing the detonation onset was likely created by the end-gas autoignition near the tube end or inside the gas-feeding pipe. As a result of the analysis on the shock wave, the mixing of the unburned and burned gasses induced by the vortex generation at the flame front by the baroclinic torque (Richtmyer-Meshkov instability) is the most plausible mechanism for initiating the micro-explosions that trigger the detonation onset. [ABSTRACT FROM AUTHOR]

Foot and pedestal pulses that precede the main pulse from a high-intensity laser greatly affect laser-plasma interactions. Especially in fast ignition schemes, preceding pulses generate a plasma prior to irradiation by the main pulse. This results in a too energetic and divergent electron beam being generated in the preformed plasma, which reduces the energy coupling efficiency from the heating laser to the dense fuel core. A preformed plasma with a density scale length of 40-60 μm was observed by a time- and space-resolved x-ray backlight technique using the LFEX laser system at the Institute of Laser Engineering, Osaka University. Preceding pulses (i.e., the foot and pedestal) of the LFEX were characterized by comparing observations with calculations results obtained using a two-dimension (2D) radiation-hydrodynamic simulation code. In a separate experiment, the 2D code was benchmarked with the experimentally observed hydrodynamic behavior of a gold plasma produced by a nanosecond laser pulse that mimicked foot and pedestal pulses (intensity: 1 ×1011-1×1012W/cm2). The preceding pulses were estimated to have an intensity of 1 ×1012-1013W/cm2, a duration of 2.0 ns, and a spot diameter at the target of 200-600 μm by comparing the measured hydrodynamics of the preformed plasma with that calculated by the 2D hydrodynamic simulation code. [ABSTRACT FROM AUTHOR]

Laser ignition and spark-plug ignition were experimentally compared in high-speed ethylene-oxygen mixture flows of up to approximately 100 m/s. Nd:YAG laser of 12-ns pulse duration and a semi-surface-discharge-type spark plug of 1.8-ms discharge duration were used to conduct the experiments with deposited energy of approximately 24 mJ in both cases. The self-emission was observed by a high-speed camera. The flame-spread behavior and ignition ability were examined in lean-fuel conditions. The study findings revealed that laser ignition was superior to the spark-plug ignition in the aspect of the early-stage rapid flame spread, although it showed lower probability of successful ignition than that by the spark plug near the lean-fuel ignitable limit. These findings suggest that the ignition in high-speed flows is significantly influenced by the turbulence via the enhancement of heat transport in particular. [ABSTRACT FROM AUTHOR]

For examining the ability of laser ignition, a quiescent lean-fuel propane–air mixture was ignited in a constant-volume chamber by a spark induced by a pair of electrodes and an automotive spark driver, a spark induced by focused high-power laser, or a spark induced by focused high-power laser between a pair of dummy electrodes, where the deposited energy into the gas was the same in all cases. In the experiments, the mole fraction of the fuel in the mixture was varied, and the pressure history on the inner wall of the chamber, probability of successful ignition, and the high-speed multi-frame schlieren images of an ignition process were obtained. The results showed that the ignition ability of a laser-induced spark was superior to that of a conventional electrical spark in lean-fuel conditions near the ignitable limit, and that the higher ignition ability of a laser-induced spark was due not only to the lack of heat loss to electrodes but also to a large initial flame kernel whose effective energy was augmented by rapid heat release from the combustible mixture sucked into the kernel by a non-spherical inward flow created by a laser-induced spark. In addition, reinjection of the transmitted laser light into plasma strengthened the ignition ability of the laser. This was predominantly due to the increase of the total absorbed energy. [ABSTRACT FROM AUTHOR]

The effect of particle size on the minimum ignition energy (MIE) of aluminum dust clouds is investigated using a MIKE 3 apparatus. The results show that the MIE values decrease with particle size d p. The dependence of d p 3 on the MIE for micron-sized and larger aluminum particles is in agreement with the theoretical correlation. When the particle size is approximately <10 μm, the dependence of d p 0.5 on the MIE for 30 nm < d p < 10 μm can be observed. This study confirms the observations of the dependence of d p on the MIE of aluminum dust clouds: MIE∼ d p 3 for diffusion- and MIE∼ d p 0.5 for kinetic-controlled regimes. [Display omitted] • Effect of minimum ignition energy (MIE) on particle size of aluminum dust clouds. • MIE values decrease with the particle size of aluminum dust clouds. • Dependence of d p 3 on MIE of micron-sized and larger particles agrees with theory. • MIE∼ d p 3 for the diffusion- and MIE∼ d p 0.5 for kinetic-controlled regimes. [ABSTRACT FROM AUTHOR]

The energy distribution of the produced high energy electrons in the interaction of ultraintense picosecond laser pulses with high-Z solid targets is shown to be sensitive to the preformed plasma created by the prepulse and the amplified spontaneous emission pedestal. The created preformed plasmas, which are obtained by radiation hydrodynamic simulations for the present heating laser system at ILE, Osaka University, are seen to extend up to 30–100 μm just before the arrival of the main pulse. The dependences of the coupling efficiency of the laser energy to high energy electrons, and the energy spectra of these accelerated electrons, on this preformed plasma, are studied via a two-dimensional particle-in-cell simulation code. It is found that in a small preformed plasma case, J×B heating is dominant and the produced electron temperature agrees well with Haines’ scaling law [Haines et al., Phys. Rev. Lett., 102, 045008 (2009)]. While in a large preformed plasma case, in addition to J×B heating and/or vacuum heating, other acceleration mechanisms, such as stochastic heating, can accelerate electrons to very high energies, carrying a significant fraction of input laser energy. Even after several picoseconds, the number of high energy electrons (0.5 MeV

When ultraintense laser pulses irradiate solid targets with a large incident angle, quasistatic magnetic and electric fields are induced, which confine electrons along the target surface in an electrostatic and vector potential well. In this case, electrons are resonantly accelerated along the surface by laser electric field inside the potential well. By this surface acceleration process, high energy electrons are effectively generated whose temperature well exceeds the ponderomotive energy. The optimum conditions for realizing surface acceleration and its energy scalings are given. Capillary type targets are shown to have an advantage in utilizing the surface acceleration process by increasing the interaction length. [ABSTRACT FROM AUTHOR]

Abstract: We have developed a simulation code to treat behaviors of wall ablation by X-rays, α particles and charged particles from fusion burning targets. The absorption of energy of α particles and charged particles by ablated wall plasmas was estimated using the stopping power obtained by dielectric functions of plasmas and Bethe formulas. We show that ablated wall material (Pb) moves as clump. We clarify that the screening effects of charged particles expanding from fusion burning by ablated wall material (Pb) decrease the ablation depth of liquid Pb compared with that of no screening effects. [Copyright &y& Elsevier]

Population kinetics and spectral synthesis codes have been developed to analyze Kα emission from partially ionized chlorine in hydrocarbon plasmas irradiated with high-intensity ultra-short laser pulses. The population kinetics processes are calculated using a bi-Maxwellian electron temperature distribution composed of fast and cold electrons. The fast electrons dominantly contribute to ionize the K-shell bound electrons. i.e., inner-shell ionization, while the cold electrons produce ionization from the outer-shell electrons. The resultant Kα emission provides a distinct spectral signature for each charge state. Also included in the calculation are the opacity effects in the framework of an escape probability method. It is shown that the Kα emission is saturated at a plasma thickness of more than a few microns. Further, we find that unresolved satellite lines overlap significantly with the corresponding parent Kα lines of the next charge state, and make a large contribution to composite spectrum. Finally, time-dependent properties of the Kα emission are also discussed briefly. [Copyright &y& Elsevier]

An experiment is conducted to measure an impulse for demonstrating a magnetic thrust chamber system for laser fusion rocket. The impulse is produced by the interaction between plasma and magnetic field. In the experiment, the system consists of plasma and neodymium permanent magnets. The plasma is created by a single-beam laser aiming at a polystyrene spherical target. The impulse is 1.5 to 2.2 μNs by means of a pendulum thrust stand, when the laser energy is 0.7 J. Without magnetic field, the measured impulse is found to be zero. These results indicate that the system for generating impulse is working. [ABSTRACT FROM AUTHOR]

The dependence of the mean kinetic energy of laser-accelerated relativistic electrons (REs) on the laser intensity, so-called ponderomotive scaling, explains well the experimental results to date; however, this scaling is no longer applicable to multi-picosecond (multi-ps) laser experiments. Here, the production of REs was experimentally investigated via multi-ps relativistic laser–plasma-interaction (LPI). The lower slope temperature shows little dependence on the pulse duration and is close to the ponderomotive scaling value, while the higher slope temperature appears to be affected by the pulse duration. The higher slope temperature is far beyond the ponderomotive scaling value, which indicates super-ponderomotive REs (SP-REs). Simulation and experimental evidence are provided to indicate that the SP-REs are produced by LPI in an under-critical plasma, where a large quasi-static electromagnetic field grows rapidly after a threshold timing during multi-ps LPI. When a material is irradiated with a high-intensity laser pulse, its surface ionization generates electrons that are accelerated close to the speed of light by the ponderomotive force of the laser light, yet how electrons' kinetic energy scales with laser power is still unclear. The authors experimentally clarify this relation and by modelling individual electron trajectories using numerical simulations identify two acceleration mechanisms for the generation of relativistic electrons related to the dependency of the electric and magnetic fields. [ABSTRACT FROM AUTHOR]