Satoshi Jinno

Characterization of submicron-sized CO2 clusters formed with a supersonic expansion of a mixed-gas using a three-staged nozzle

The size of CO2 clusters, produced in a supersonic expansion of a mixed-gas of CO2/He or CO2/H2 through a three-staged conical nozzle designed based on the Boldarevs model, has been evaluated by measuring the angular distribution of light scattered from the clusters. The data are analyzed utilizing the Mie scattering theory, and the sizes of CO2 clusters are estimated as 0.22 μm and 0.25 μm for the cases of CO2/He and CO2/H2 gas mixtures, respectively. The results confirm that the Boldarevs model is reliable enough for the production of micron-sized clusters.

Mie scattering from submicron-sized CO 2 clusters formed in a supersonic expansion of a gas mixture

A detailed mathematical model is presented for a submicron-sized cluster formation in a binary gas mixture flowing through a three-staged conical nozzle. By measuring the angular distribution of light scattered from the clusters, the size of CO(2) clusters, produced in a supersonic expansion of the mixture gas of CO(2)(30%)/H(2)(70%) or CO(2)(10%)/He(90%), has been evaluated using the Mie scattering method. The mean sizes of CO(2) clusters are estimated to be 0.28 ± 0.03 μm for CO(2)/H(2) and 0.26 ± 0.04 μm for CO(2)/He, respectively. In addition, total gas density profiles in radial direction of the gas jet, measuring the phase shift of the light passing through the target by utilizing an interferometer, are found to be agreed with the numerical modeling within a factor of two. The dryness (= monomer/(monomer + cluster) ratio) in the targets is found to support the numerical modeling. The apparatus developed to evaluate the cluster-gas targets proved that our mathematical model of cluster formation is reliable enough for the binary gas mixture.

Insertable pulse cleaning module with a saturable absorber pair and a compensating amplifier for high-intensity ultrashort-pulse lasers

We demonstrate the performance of an efficient insertable pulse cleaning module (IPCM) that uses a saturable absorber (SA) pair with a compensating multi-pass amplifier. IPCM consists of a first SA, a grating compressor, a second SA, a stretcher and a compensating Ti:sapphire amplifier. It is implemented with a conventional chirped pulse amplification (CPA) Ti:sapphire laser system, resulting in a double CPA system architecture, and suppresses the amplified spontaneous emission (ASE) level of the pulse pedestal by about three orders of magnitude while preserving the output pulse energy and repetition-rate of the overall laser system. The duration of recompressed cleaned pulses is comparable to that obtained without the cleaning module. The effectiveness of the cleaning module is confirmed in laser-driven proton acceleration experiments. At the 10(9) W/cm2 pedestal level, the surface structure and electrical resistivity of an insulator target (100 nm silicon nitride) are preserved prior to the arrival of the intense ultrashort pulse.

The precise energy spectra measurement of laser-accelerated MeV/n-class high-Z ions and protons using CR-39 detectors

The diagnosis method, using a combination of a permanent magnet and CR-39 track detectors, has been developed to separately measure the energy spectrum of the laser-accelerated MeV/n-class high-Z ions and that of MeV protons. The main role of magnet is separating between high-Z ions and protons, not for the usual energy spectrometer, while ion energy was precisely determined from careful analysis of the etch pit shapes and the etch pit growth behaviors in the CR-39. The method was applied to laser-driven ion acceleration experiments using CO2 clusters embedded in a background H2 gas. Ion energy spectra with uncertainty ΔE = 0.1 MeV n−1 for protons and carbon/oxygen ions were simultaneously obtained separately. The maximum energies of carbon/oxygen ions and protons were determined as 1.1 ± 0.1 MeV and 1.6 ± 0.1 MeV n−1, respectively. The sharp decrease around 1 MeV n−1 observed in the energy spectrum of carbon/oxygen ions could be due to a trace of the ambipolar hydrodynamic expansion of CO2 clusters. Thanks to the combination of the magnet and the CR-39, the method is robust against electromagnetic pulse (EMP).

Two-plasmon decay instability’s signature in spectral lines and spectroscopic measurements of charge exchange rate in a femtosecond laser-driven cluster-based plasma

We present the first study of two kinds of dips (L-dips and X-dips) in spectral lines from femtosecond laser-driven cluster-based plasma. We found that the observed L-dips are caused by Langmuir waves resulting from the two-plasmon decay instability and our experiment constitutes the first observation of the signature of this instability in spectral line profiles. We also observed an X-dip caused by charge exchange and used it for the experimental determination of the rate of charge exchange between the hydrogenic oxygen and fully-stripped helium—an important fundamental reference data virtually inaccessible by other experimental methods.

Generation of 50-MeV/u He ions in laser-driven ion acceleration with cluster-gas targets

We demonstrate a new ion diagnosis method for high energy ions by utilizing a combination of a single CR-39 detector and plastic plates, which enables to detect high energy ions beyond the detection threshold limit of the CR-39. This detection method coupled with a magnetic spectrometer is applied to identify high energy ions of 50 MeV per nucleon in laser-driven ion acceleration experiments using cluster-gas targets.

Transition from nonlocal electron transport to radiative regime in an expanding blast wave

We have investigated the formation, evolution, and late-time propagation of a laser-generated cylindrical blast wave (BW). The whole blast wave evolution over timescales of several nanoseconds was reconstructed experimentally (via temporally resolved interferometric measurements) and via hydrodynamic simulations that included modeling of nonlocal electron transport and radiation diffusion. Comparison between the experimental results and the simulations indicates that the early expansion phase is characterised by nonlocal electron heat transport causing energy spread on times shorter than the typical timescales for hydrodynamic expansion. Nonlocal electron transport ionizes the gas ahead of the plasma front and gives rise to a smooth radial density gradient. At later times, once the shock is launched and the BW is formed, radiation results in reduced shock velocity compared to the adiabatic case. These investigations provide a suitable and effective platform to benchmark the inclusion of kinetic and radiat...

Characterization of micron-size hydrogen clusters using Mie scattering

Hydrogen clusters with diameters of a few micrometer range, composed of 108-10 hydrogen molecules, have been produced for the first time in an expansion of supercooled, high-pressure hydrogen gas into a vacuum through a conical nozzle connected to a cryogenic pulsed solenoid valve. The size distribution of the clusters has been evaluated by measuring the angular distribution of laser light scattered from the clusters. The data were analyzed based on the Mie scattering theory combined with the Tikhonov regularization method including the instrumental functions, the validity of which was assessed by performing a calibration study using a reference target consisting of standard micro-particles with two different sizes. The size distribution of the clusters was found discrete peaked at 0.33 ± 0.03, 0.65 ± 0.05, 0.81 ± 0.06, 1.40 ± 0.06 and 2.00 ± 0.13 µm in diameter. The highly reproducible and impurity-free nature of the micron-size hydrogen clusters can be a promising target for laser-driven multi-MeV proton sources with the currently available high power lasers.

10-TW high-contrast double CPA laser system for ion acceleration

We demonstrate an insertable pulse cleaning module (IPCM) that is connected to a commercial CPA Ti:sapphire laser system and reduces the background of amplified spontaneous emission (ASE), when the output energy and repetition rate of the original laser system is completely preserved. The ASE temporal contrast is suppressed by three orders of magnitude on the benefit of double saturable absorbers involved in the module. Our method can reform old conventional lasers into high-contrast advanced systems.

Development of an Apparatus for Characterization of Cluster-Gas Targets for Laser-Driven Particle Accelerations

CO\(_2\) clusters formed in supersonic expansion of a mixed-gas of CO\(_2\)/H\(_2\) or CO\(_2\)/He through a three-staged conical nozzle have been verified by measuring the angular distribution of the light scattered from cluster target. The angular distribution is fitted by convolving a lognormal size distribution with the scattering coefficients calculated based on the Mie theory. The reliability of the size measurement is verified to be within an experimental error of 10 % using standard particles. The mean sizes of CO\(_2\) clusters at the target center for the cases of CO\(_2\)/H\(_2\) and CO\(_2\)/He gas mixtures are estimated to be 0.26 and 0.22 \(\upmu \)m, respectively. For the CO\(_2\)/H\(_2\) mixed-gas target, the variation of the mean cluster size inside the gas jet is constant within the experimental error. Furthermore, the cluster density is estimated to be \(5.5 \times 10^8\) clusters/cm\(^2\) by measuring the attenuation of the laser beam intensity. In addition, total gas density profiles in radial direction are obtained via the Abel inversion from the phase shift of the light passing through the target by utilizing an interferometer. The variation of the cluster mass fraction along the radial direction of the target is almost constant, which is consistent with a Boldarev’s model.