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Accepted: 2018-11-03, 資料番号: SA1180234000

We report the result from a search for charged-current coherent pion production induced by muon neutrinos with a mean energy of 1.3 GeV. The data are collected with a fully active scintillator detector in the K2K long-baseline neutrino oscillation experiment. No evidence for coherent pion production is observed and an upper limit of $0.60 \times 10^{-2}$ is set on the cross section ratio of coherent pion production to the total charged-current interaction at 90% confidence level. This is the first experimental limit for coherent charged pion production in the energy region of a few GeV., 5 pages, 4 figures

The generation mechanism of counter-streaming plasmas, by irradiating an inner-surface of the 1st-plane of double-plane target, is investigated experimentally using optical imaging and collective Thomson scattering (CTS) ion-term measurement. The CTS measurement reveals that counter-streaming plasmas exist at the same time at the same position, which is the evidence for the collisionless interaction between the counter-streaming plasmas. The generation of the 2nd-plane plasma is in two steps: First, the 2nd-plane is ablated nearly at the laser timing by radiation from the 1st-plane plasma. Then ≃ 5 − 7 ns later, when the plasma from the 1st-plane reaches the 2ns-plane, it induces higher-density plasma generation.

A preliminary numerical experiment is conducted for laboratory experiments on the generation of magnetized collisionless shocks with high-power lasers by using one-dimensional particle-in-cell simulation. The present study deals with the interaction between a moving aluminum plasma and a nitrogen plasma at rest. In the numerical experiment, the nitrogen plasma is unmagnetized or magnetized by a weak external magnetic field. Since the previous study suggested the generation of a spontaneous magnetic field in the piston (aluminum) plasma due to the Biermann battery, the effect of the magnetic field is of interest. Sharp jumps of the electron density and magnetic field are observed around the interface between the two plasmas as long as one of the two plasmas is magnetized, which indicates the formation of tangential electron-magneto-hydro-dynamic discontinuity. When the aluminum plasma is magnetized, strong compression of both the density and the magnetic field takes place in the pure aluminum plasma during the gyration of nitrogen ions in the aluminum plasma region. The formation of a shock downstream is obtained from the shock jump condition. The results suggest that the spontaneous magnetic field in the piston (aluminum) plasma plays an essential role in the formation of a perpendicular collisionless shock., ファイル公開：2020-03-05

We developed an experimental platform for studying magnetic reconnection in an external magnetic field with simultaneous measurements of plasma imaging, flow velocity, and magnetic-field variation. Here, we investigate the stagnation and acceleration in counter-streaming plasmas generated by high-power laser beams. A plasma flow perpendicular to the initial flow directions is measured with laser Thomson scattering. The flow is, interestingly, accelerated toward the high-density region, which is opposite to the direction of the acceleration by pressure gradients. This acceleration is possibly interpreted by the interaction of two magnetic field loops initially generated by Biermann battery effect, resulting in a magnetic reconnection forming a single field loop and additional acceleration by a magnetic tension force., Comment: 6 pages, 4 figures, Physics of Plasmas, in press

Collisionless shock is ubiquitous in space and astrophysical plasmas, and is believed to be a source of cosmic rays. In this review article, historical achievements of three different types of coll...

A high-sensitivity sensor to measure titanium atom density based on time-resolved cavity ring-down spectroscopy (CRDS) was developed to monitor the wall erosion and predict the lifetime of Hall thrusters. The minimum detection limit for the sensor was dependent on the discharge current oscillation in the Hall thruster. A Volterra engine management system was employed for time-resolved measurements to develop the time-resolved CRDS system, which was synchronized to the discharge current oscillation. The results confirmed that the path-integrated number density of sputtered titanium atoms was synchronized with the discharge current oscillation. The minimum detection limit was decreased by approximately 30% from 2×10^12 to 6×10^11 m^-2.

We have implemented laser Thomson scattering for local plasma measurement of electron and ion temperatures, electron density, flow velocity, and charge state. The electron density increases by two times in the interaction of two plasma flows, indicating collisionless interaction. The density and velocity show fluctuations only at t = 40 ns, and the density suddenly decreases, indicating the plasma ejection from the interaction region, which can be explained by a magnetic reconnection. The electron temperature in the double-flow is larger than that in the single flow. This may be explained by the energy transfer from the plasma kinetic energy to thermal energy. The ion temperature is much larger than electron temperature in the double-flow, and this may be explained by collisional effects between two plasmas, and/or possibly interpreted as a thermalization due to magnetic reconnection.

Astrophysical shocks are commonly revealed by the non-thermal emission of energetic electrons accelerated in situ1–3. Strong shocks are expected to accelerate particles to very high energies4–6; however, they require a source of particles with velocities fast enough to permit multiple shock crossings. While the resulting diffusive shock acceleration 4 process can account for observations, the kinetic physics regulating the continuous injection of non-thermal particles is not well understood. Indeed, this injection problem is particularly acute for electrons, which rely on high-frequency plasma fluctuations to raise them above the thermal pool7,8. Here we show, using laboratory laser-produced shock experiments, that, in the presence of a strong magnetic field, significant electron pre-heating is achieved. We demonstrate that the key mechanism in producing these energetic electrons is through the generation of lower-hybrid turbulence via shock-reflected ions. Our experimental results are analogous to many astrophysical systems, including the interaction of a comet with the solar wind 9 , a setting where electron acceleration via lower-hybrid waves is possible. Electrons can be accelerated by astrophysical shocks if they are sufficiently fast to start with. As laboratory laser-produced shock experiments reveal, this can be achieved by lower-hybrid waves generated by a shock-reflected ion instability.

The first demonstration of target acceleration by a strong magnetic field is conducted. Two miniature targets, a gold ball and a neodymium permanent magnet, are accelerated by a magnetic field. A magnetic-field strength as high as 1 kT is produced using a capacitor-coil in which two copper disks are connected by a single-turn coil, and the capacitor was driven by two beams from the GEKKO-XII high-power laser. The targets are accelerated by the interaction force between two magnetic moments: the target and the capacitor-coil. The velocity of the accelerated gold ball was estimated as 10 m/s from shadowgraphy images. In the case of the magnet, although the image could not be observed clearly due to the plasma generated around the magnet, it was confirmed that the magnet interacts with the magnetic field of the capacitor-coil.

Recent progress in the generation in the laboratory of a strong ($g100$-T) magnetic field enables us to investigate experimentally unexplored magnetohydrodynamics phenomena of a high-energy-density plasma, which an external magnetic field of 200--300 T notably affects due to anisotropic thermal conduction, even when the magnetic field pressure is much lower than the plasma pressure. The external magnetic field reduces electron thermal conduction across the external magnetic field lines because the Larmor radius of the thermal electrons in the external magnetic field is much shorter than the mean free path of the thermal electrons. The velocity of a thin polystyrene foil driven by intense laser beams in the strong external magnetic field is faster than that in the absence of the external magnetic field. Growth of sinusoidal corrugation imposed initially on the laser-driven polystyrene surface is enhanced by the external magnetic field because the plasma pressure distribution becomes nonuniform due to the external magnetic-field structure modulated by the perturbed plasma flow ablated from the corrugated surface.

The interaction between two counter-streaming laser-produced plasmas is studied with shadowgraph and interferometry on the Shenguang II (SG-II) laser facility. Shockwaves and filaments are observed at different timing. The simulation and theoretical analysis indicate that these structures are probably induced by collisionless mechanisms.

Long time evolution of collisionless shocks in laser-produced plasmas is discussed. By irradiating a double plane target a high Mach number collisionless shock has been observed in laser produced counterstreaming plasmas [Kuramitsu et al., Phys. Rev. Lett., 106, 175002 (2011)]. While in early time we observe the shock in front of one plane, which is irradiated with the laser, we observe another shock in front of the other plane in much later time than the first shock formation. These two shocks coexist and collide or merge with each other as time passes. This means that the upstream plasmas for the first and second shocks have to be provided from the second and first shock sides, respectively, i.e., both the first and second shock have to be collisionless. There are two major candidates to account for the long time evolution of the collisionless shocks. One is that the secondary plasmas at the planes can be continuously created by the plasmas from the other planes. Another is that the actual shock thickness is much thiner than the detection limit, as indicated by numerical simulations.

A study of the transition from collisional to collisionless plasma flows has been carried out at the National Ignition Facility using high Mach number (M>4) counterstreaming plasmas. In these experiments, CD-CD and CD-CH planar foils separated by 6-10 mm are irradiated with laser energies of 250 kJ per foil, generating ∼1000 km/s plasma flows. Varying the foil separation distance scales the ion density and average bulk velocity and, therefore, the ion-ion Coulomb mean free path, at the interaction region at the midplane. The characteristics of the flow interaction have been inferred from the neutrons and protons generated by deuteron-deuteron interactions and by x-ray emission from the hot, interpenetrating, and interacting plasmas. A localized burst of neutrons and bright x-ray emission near the midpoint of the counterstreaming flows was observed, suggesting strong heating and the initial stages of shock formation. As the separation of the CD-CH foils increases we observe enhanced neutron production compared to particle-in-cell simulations that include Coulomb collisions, but do not include collective collisionless plasma instabilities. The observed plasma heating and enhanced neutron production is consistent with the initial stages of collisionless shock formation, mediated by the Weibel filamentation instability.

One of the important and interesting problems in astrophysics and plasma physics is collimation of plasma jets. The collimation mechanism, which causes a plasma flow to propagate a long distance, has not been understood in detail. We have been investigating a model experiment to simulate astrophysical plasma jets with an external magnetic field [Nishio et al., EPJ. Web of Conferences 59, 15005 (2013)]. The experiment was performed by using Gekko XII HIPER laser system at Institute of Laser Engineering, Osaka University. We shot CH plane targets (3 mm × 3 mm × 10 μm) and observed rear-side plasma flows. A collimated plasma flow or plasma jet was generated by separating focal spots of laser beams. In this report, we measured plasma jet structure without an external magnetic field with shadowgraphy, and simultaneously measured the local parameters of the plasma jet, i.e., electron density, electron and ion temperatures, charge state, and drift velocity, with collective Thomson scattering.

A model experiment of magnetic field amplification (MFA) via the Richtmyer-Meshkov instability (RMI) in supernova remnants (SNRs) was performed using a high-power laser. In order to account for very-fast acceleration of cosmic rays observed in SNRs, it is considered that the magnetic field has to be amplified by orders of magnitude from its background level. A possible mechanism for the MFA in SNRs is stretching and mixing of the magnetic field via the RMI when shock waves pass through dense molecular clouds in interstellar media. In order to model the astrophysical phenomenon in laboratories, there are three necessary factors for the RMI to be operative: a shock wave, an external magnetic field, and density inhomogeneity. By irradiating a double-foil target with several laser beams with focal spot displacement under influence of an external magnetic field, shock waves were excited and passed through the density inhomogeneity. Radiative hydrodynamic simulations show that the RMI evolves as the density inhomogeneity is shocked, resulting in higher MFA.

International audience; In this paper, recent results obtained on highly radiative shocks generated in a xenon filled gas cell using the GEKKO XII laser facility are presented. Data show extremely high shock velocity (>=150 km/s) never achieved before in gas. Preliminary analyses based on theoretical dimensionless numbers and numerical simulations suggest that these radiative shocks reach a new radiative regime where the radiative pressure plays a role in the dynamics and structure of the shock. A major effect observed is a strong anisotropic emission in the downstream gas. This unexpected feature is discussed and compared to available 2D radiation hydrodynamic simulations.

Density inhomogeneities for a test bed of magnetic field amplification in supernova remnants (SNRs) were created in laser produced plasmas. The density inhomogeneity is considered to be essential to the large magnetic field amplification to account for the very fast cosmic ray acceleration. In order to model the density variations about an order of magnitude in an interstellar medium, we performed three types of experiments using a high-power laser system: (1) irradiating a plastic (CH) plane with a single focal spot beams, (2) the same target with spatial separation of laser focal spots, and (3) irradiating a striped target of thin and thick CH plane. By irradiating a CH plane target with a single focal spot laser beams, a plasma plume was produced with the large density range. On the other hand, when the several laser beams with displacements of the focal spots, bumpy structures of electron density were produced. Making thin stripes on a CH plane target, density and velocity inhomogeneities were produced by irradiating the striped target with the laser beams. In the all methods the density variations were very large, which can be used for a model experiment of the magnetic field amplification.

We have successfully developed a portable pulsed magnetic field generation system incorporating a number of techniques to avoid the effects of noise, including shielding, a self-power capability, and a high-capability semiconductor switch. The system fits into a cubical box less than 0.5 m in linear dimensions and can easily be installed in experimental facilities, including noisy environments such as high-power laser facilities. The system can generate a magnetic field of several tesla sustainable for several tens of microseconds over a spatial scale of several centimeters. In a high-power laser experiment with Gekko-XII, the system operated stably despite being subjected to a high level of electrical noise from laser shots of 600 J.

A magnetic thrust chamber is an important system of a laser fusion rocket, in which the plasma kinetic energy is converted into vehicle thrust by a magnetic field. To investigate the plasma extraction from the system, the ions in a plasma are diagnosed outside the system by charge collectors. The results clearly show that the ion extraction does not strongly depend on the magnetic field strength when the energy ratio of magnetic field to plasma is greater than 4.3, and the magnetic field pushes back the plasma to generate a thrust, as previously suggested by numerical simulation and experiments.

Plasma jet formation was observed in counterstreaming plasmas in a laboratory experiment. In order to model an ambient plasma of astrophysical jets, the counterstreaming plasmas were created by irradiating a double CH-plane target with a high-power laser system. Since the mean free paths of the ions in terms of the counterstreaming motion were larger than the scale length of the experiment, the two-stream interaction of the plasmas was essentially collisionless. The time evolution of the jet collimation was obtained over several shots with different timing by shadowgraphy. When a single CH-plane target was irradiated, no jet collimation was observed. The counterstreaming plasma as an ambient plasma is essential for the jet plasma to collimate.

In this paper, we present the recent results obtained regarding highly radiative shocks (RSs) generated in a low-density gas filled cell on the GEKKO XII laser facility. The RS was generated by using an ablator-pusher two-layer target (CH/Sn) and a propagation medium (Xe). High velocity RSs have been generated (100–140 km/s), while limiting as much as possible the preheating produced by the corona emission. Both self-emission and visible probe diagnostics highlighted a strong emission in the shock and an electron density in the downstream gas. The RS characteristics that depend on the initial conditions are described here as well as its precursor interaction with an aluminium foil used as an obstacle. The obtained results are discussed which show a strong extension of the radiative precursor (1 mm) leading to an expansion velocity of the obstacle up to ≈30 km/s compatible to a 20 eV temperature.

We present a method for evaluating the properties of electrostatic shock in laser-produced plasmas by using optical diagnostics. A shock is formed by a collimated jet in counter-streaming plasmas in nearly collisionless condition, showing the steepening of the transition width in time. In the present experiment, a streaked optical pyrometry was applied to evaluate the electron density and temperatures in the upstream and downstream regions of the shock so that the shock conditions are satisfied, by assuming thermal bremsstrahlung emission in optically thin plasmas. The derived electron densities are nearly consistent with those estimated from interferometry.

We demonstrate a magnetic thrust chamber system, in which an expanding plasma is controlled by an external magnetic field to produce a thrust. The plasma structure and energy dependences are discussed in terms of the drive laser energy and magnetic field strength. The density distribution from two different experiments show identical structure despite the laser energy is different by two order of magnitude when the ratio of magnetic field to plasma energy is more or less same. The experimental results indicate that this ratio is one of the essential factors to extrapolate the plasma dynamics for much larger energy such as inertial confinement fusion plasmas.

Collisionless shocks mediated by Weibel instability are attracting attention for their relevance to experimental demonstrations of astrophysical shocks in high-intensity laser facilities. The three dimensional structure of Weibel-mediated shocks is investigated through a fully kinetic particle-in-cell simulation. The structures obtained are characterized by the following features: (i) helical magnetic field lines elongated in the direction upstream of the shock region, (ii) high and low density filaments inside the helical field lines. These structures originate from the interaction between counter-streaming plasma flow and magnetic vortexes caused by Weibel instability, and potentially affect the shock formation mechanism.

A set of experiments has been performed exploring unique characteristics of pair jets and plasmas at several energetic short-pulse laser facilities including Titan at Livermore and OMEGA EP in Rochester, as well as the Osaka LFEX and AWE Orion lasers. New results are summarized, including positron beam emittance, scaling of pair production vs. laser energy, and initial results on the pair jet collimation using electromagnetic fields.

We have developed a two-dimensional magneto-radiation hydrodynamics code with an induction equation solver to analyze laboratory-astrophysics experiments relevant to a supernova remnant (SNR) environment. The computed magnetic field was amplified by a order of magnitude from the background level as expected for accelerating fields of cosmic rays in the SNR context. A part of the amplification of the magnetic field is caused by Richtmyer-Meshkov instability (RMI) that stretches a contact discontinuity. However, the maximum magnetic field is smaller than the theoretically predicted limit since the non-linear RMI cannot sufficiently grow and the field is amplified by the compression effect rather than the stretching in the present experimental condition.

We have built a sputter erosion sensor using Cavity Ring-Down Spectroscopy (CRDS) for validating the numerical analysis tool called ``JIEDI tool''. In this paper, we measured the velocity distribution function of the aluminum atoms sputtered from an aluminum acceleration grid of the ion thruster. The experimentally obtained aluminum velocity distribution have been found to be compatible with those calculated by the numerical analysis method.

A temporally and spatially resolved optical pyrometer system has been fielded on Gekko XII experiments. The system is based on the self-emission measurements with a gated optical imager (GOI) and a streaked optical pyrometer (SOP). Both detectors measure the intensity of the self-emission from laser-produced plasmas at the wavelength of 450 nm with a bandpass filter with a width of ∼10 nm in FWHM. The measurements were calibrated with different methods, and both results agreed with each other within 30% as previously reported [T. Morita et al., Astrophys. Space Sci. 336, 283 (2011)]10.1007/s10509-010-0525-5. As a tool for measuring the properties of low-density plasmas, the system is applicable for the measurements of the electron temperature and density in collisionless shock experiments [Y. Kuramitsu et al., Phys. Rev. Lett. 106, 175002 (2011)]10.1103/PhysRevLett.106.175002.

We report the experimental results of a turbulent electric field driven by Kelvin-Helmholtz instability associated with laser produced collisionless shock waves. By irradiating an aluminum double plane target with a high-power laser, counterstreaming plasma flows are generated. As the consequence of the two plasma interactions, two shock waves and the contact surface are excited. The shock electric field and transverse modulation of the contact surface are observed by proton radiography. Performing hydrodynamic simulations, we reproduce the time evolutions of the reverse shocks and the transverse modulation driven by Kelvin-Helmholtz instability.

Approximately 90% of wheat consumed in Japan is imported from such countries as the United States (the U.S.) and Australia. Therefore, Australia plays an important role to ensure food security of Japan. This study attempted to make development of growth monitoring application of Australian wheat using both ground measurement hyperspectral data (FieldSpec) and airborne hyperspectral data (HyMap). As a result, estimation of head moisture and leaf area index (LAI) of the wheat was achieved by using the hyperspectral data acquired in the late grain filling period. This attempt also made it possible to visualize a growth situation of the wheat over a wide area. In this study, the multiple regression analysis achieved the most suitable estimation results among various indices.

We investigated the time evolution of a strong collisionless shock in counterstreaming plasmas produced using a high-power laser pulse. The counterstreaming plasmas were generated by irradiating a CH double-plane target with the laser. In self-emission streaked optical pyrometry data, steepening of the self-emission profile as the two-plasma interaction evolved indicated shock formation. The shock thickness was less than the mean free path of the counterstreaming ions. Two-dimensional snapshots of the self-emission and shadowgrams also showed very thin shock structures. The Mach numbers estimated from the flow velocity and the brightness temperatures are very high.

Astrophysical collisionless shocks are common in the universe, occurring in supernova remnants, gamma ray bursts, and protostellar jets. They appear in colliding plasma flows when the mean free path for ion-ion collisions is much larger than the system size. It is believed that such shocks could be mediated via the electromagnetic Weibel instability in astrophysical environments without pre-existing magnetic fields. Here, we present laboratory experiments using high-power lasers and investigate the dynamics of high-Mach-number collisionless shock formation in two interpenetrating plasma streams. Our recent proton-probe experiments on Omega show the characteristic filamentary structures of the Weibel instability that are electromagnetic in nature with an inferred magnetization level as high as ∼1% [C. M. Huntington et al., “Observation of magnetic field generation via the weibel instability in interpenetrating plasma flows,” Nat. Phys. 11, 173–176 (2015)]. These results imply that electromagnetic instabilities are significant in the interaction of astrophysical conditions.

In this paper, we present experimental results obtained on the LULI2000 laser facility regarding structure and dynamics of astrophysical jets propagating in interstellar medium. The jets, generated by using a cone-shaped target, propagate in a nitrogen gas that mimics the interstellar medium. X-ray radiography as well as optical diagnostics were used to probe both high and low density regions. In this paper, we show how collimation of the jets evolves with the gas density.

We performed an improved search for {nu}{sub {mu}}{yields}{nu}{sub e} oscillation with the KEK to Kamioka (K2K) long-baseline neutrino oscillation experiment, using the full data sample of 9.2x10{sup 19} protons on target. No evidence for a {nu}{sub e} appearance signal was found, and we set bounds on the {nu}{sub {mu}}{yields}{nu}{sub e} oscillation parameters. At {delta}m{sup 2}=2.8x10{sup -3} eV{sup 2}, the best-fit value of the K2K {nu}{sub {mu}} disappearance analysis, we set an upper limit of sin{sup 2}2{theta}{sub {mu}}{sub e}

We present a combined analysis of fully-contained, partially-contained and upward-going muon atmospheric neutrino data from a 1489 day exposure of the Super--Kamiokande detector. The data samples span roughly five decades in neutrino energy, from 100 MeV to 10 TeV. A detailed Monte Carlo comparison is described and presented. The data is fit to the Monte Carlo expectation, and is found to be consistent with neutrino oscillations of $\nu_\mu \leftrightarrow \nu_\tau$ with $\sin^22\theta > 0.92$ and $1.5\times 10^{-3} < \Delta m^2 < 3.4\times 10^{-3}{\rm eV}^2$ at 90% confidence level., Comment: 32 pages, 44 figures. As accepted for publication in Physical Review D

We present a search for electron neutrino appearance from accelerator produced muon neutrinos in the K2K long baseline neutrino experiment. One candidate event is found in the data corresponding to an exposure of 4.8*10^19 protons on target. The expected background in the absence of neutrino oscillations is estimated to be 2.4+-0.6 events and is dominated by mis-identification of events from neutral current pi^0 production. We exclude the \nu_\mu to \nu_e oscillations at 90% C.L. for the effective mixing angle in 2-flavor approximation of sin^2(2theta_\mu_e) (~= 1/2 sin^2 2 th_13) > 0.15 at Delta m^2_\mu_e = 2.8*10^{-3} eV^2, the best fit value of the \nu_\mu disappearance analysis in K2K. The most stringent limit of sin^2(2theta_\mu_e) < 0.09 is obtained at Delta m^2_\mu_e = 6*10^{-3} eV^2., Comment: 5 pages with 2 figures embeded in two column revtex4 style. Accepted to be published in Phys. Rev. Lett

A search for a non-zero neutrino magnetic moment has been conducted using 1496 live days of solar neutrino data from {\SK}. Specifically, we searched for distortions to the energy spectrum of recoil electrons arising from magnetic scattering due to a non-zero neutrino magnetic moment. In the absence of clear signal, we found $\mu_{\nu} \leq 3.6 \times 10^{-10}$ $\mu_{B}$ at 90% C.L. by fitting to the Super-Kamiokande day/night spectra. The fitting took into account the effect of neutrino oscillation on the shapes of energy spectra. With additional information from other solar neutrino and KamLAND experiments constraining the oscillation region, a limit of $\mu_{\nu} \leq 1.1 \times 10^{-10}$ $\mu_{B}$ at 90% C.L. was obtained., Comment: 5 pages, 4 figures