Engineering topics
Shimada, T.
Publications and source records attributed to Shimada, T..
MeV bremsstrahlung X rays from intense laser interaction with solid foils
Laser-based compact MeV X-ray sources are useful for a variety of applications such as radiography and active interrogation of nuclear materials. MeV X rays are typically generated by impinging the intense laser onto ~mm-thick high-Z foil. Here, we have characterized such a MeV X-ray source from 120 TW (80 J, 650 fs) laser interaction with a 1 mm-thick tantalum foil. Our measurements show X-ray temperature of 2.5 MeV, flux of 3 × 10 12 photons/sr/shot, beam divergence of ~0.1 sr, conversion efficiency of ~1%, that is, ~1 J of MeV X rays out of 80 J incident laser, and source size of 80 m. Our measurement also shows that MeV X-ray yield and temperature is largely insensitive to nanosecond laser contrasts up to 10 -5 . Also, preliminary measurements of similar MeV X-ray source using a double-foil scheme, where the laser-driven hot electrons from a thin foil undergoing relativistic transparency impinging onto a second high-Z converter foil separated by 50–400 m, show MeV X-ray yield more than an order of magnitude lower compared with the single-foil results.
Laser-driven micro-Coulomb charge movement and energy conversion to relativistic electrons
Development of robust instrumentation has shown evidence for a multi-μC expulsion of relativistic electrons from a sub-μm-thick foil, laser illuminated with 60–70 J on target at 2 × 10 20 W/cm 2 . From previous work and with electron spectroscopy, it is seen that an exponential electron energy distribution is accurate enough to calculate the emitted electron charge and energy content. The 5–10- μ C charge for the >100-TW Trident Laser represents the first active measurement of the >50% laser-light-to-electron conversion efficiency. By shorting out the TV/m electric field usually associated with accelerating multi-MeV ions from such targets, one finds that this charge is representative of a multi-MA current of relativistic electrons for diverse applications from electron fast ignition to advanced radiography concepts. Also included with the details of the discoveries of this research, shortcomings of the diagnostics and means of improving their fidelity are discussed.
Surface segregation and oxidation of Pt 3 Ni(1 1 1) alloys under oxygen environment
Utilizing ambient pressure X-ray photoelectron spectroscopy (AP-XPS), the surface segregation and oxidation of Pt 3 Ni(1 1 1) alloys are investigated as a function of temperature and oxygen pressure. The in situ AP-XPS measurements of oxygen oxidation process show that the Pt “skin” surface is not stable under the exposure of oxygen pressure of 100 mTorr at room temperature. As the temperature and pressure are elevated, the formations of Ni 2 O 3 , NiO x , and NiO are observed on surface while Pt atom starts to reduce its adsorbed oxygen, which is a clear sign of surface segregation of Ni to surface. Upon the evacuation of oxygen gas, i.e. ultrahigh vacuum condition, both of NiO x and NiO oxide get reduced and Ni 2 O 3 remains on the surface. Finally, the DFT calculation is employed to explain the formation of surface oxides under oxidation condition.
High-resolution measurements of shock behavior across frictional Be/Cu interfaces
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Laser-to-hot-electron conversion limitations in relativistic laser matter interactions due to multi-picosecond dynamics
High-energy short-pulse lasers are pushing the limits of plasma-based particle acceleration, x-ray generation, and high-harmonic generation by creating strong electromagnetic fields at the laser focus where electrons are being accelerated to relativistic velocities. Understanding the relativistic electron dynamics is key for an accurate interpretation of measurements. We present a unified and self-consistent modeling approach in quantitative agreement with measurements and differing trends across multiple target types acquired from two separate laser systems, which differ only in their nanosecond to picosecond-scale rising edge. Insights from high-fidelity modeling of laser-plasma interaction demonstrate that the ps-scale, orders of magnitude weaker rising edge of the main pulse measurably alters target evolution and relativistic electron generation compared to idealized pulse shapes. This can lead for instance to the experimentally observed difference between 45 MeV and 75 MeV maximum energy protons for two nominally identical laser shots, due to ps-scale prepulse variations. Our results indicate that the realistic inclusion of temporal laser pulse profiles in modeling efforts is required if predictive capability and extrapolation are sought for future target and laser designs or for other relativistic laser ion acceleration schemes.
Quasi-steady carbon plasma source for neutral beam injector
Carbon plasma is successfully sustained during 1000 s without any carrier gas in the bucket type ionization chamber with cusp magnetic field. Every several seconds, seed plasmas having ∼3 ms duration time are injected into the ionization chamber by a shunting arch plasma gun. The weakly ionized carbon plasma ejected from the shunting arch is also ionized by 2.45 GHz microwave at the electron cyclotron resonance surface and the plasma can be sustained even in the interval of gun discharges. Control of the gun discharge interval allows to keep high pressure and to sustain the plasma for long duration.
Laser-driven 1 GeV carbon ions from preheated diamond targets in the break-out afterburner regime
Experimental data are presented for laser-driven carbon C{sup 6+} ion-acceleration, verifying 2D-PIC studies for multi-species targets in the Break-Out Afterburner regime. With Trident's ultra-high contrast at relativistic intensities of 5 × 10{sup 20} W/cm{sup 2} and nm-scale diamond targets, acceleration of carbon ions has been optimized by using target laser-preheating for removal of surface proton contaminants. Using a high-resolution wide angle spectrometer, carbon C{sup 6+} ion energies exceeding 1 GeV or 83 MeV/amu have been measured, which is a 40% increase in maximum ion energy over uncleaned targets. These results are consistent with kinetic plasma modeling and analytic theory.
Investigation of ruthenium dioxide formation mechanisms in containment glass synthesized by liquid feed
The presenting paper focuses on the structural configuration of ruthenium in vitreous matrices with the objective of obtaining more insight into ruthenium incorporation and solubilization mechanisms in borosilicate glasses. To determine the structural effect of an increasing RuO{sub 2} in a borosilicate glass, a series of glass samples were selected from a benchmark composition. {sup 11}B NMR shows that the borate network is influenced by the presence of RuO{sub 2} in the glass: the addition of 2% RuO{sub 2} in a borosilicate glass led to a significant rise of BO{sub 4}. RuO{sub 2} precipitated in the glass does not seem to be the cause of this modification because when RuO{sub 2} increase there is no further change of BO{sub 4} fraction. This effect can be therefore attributed to RuO{sub 2} dissolved in the glass. RuO{sub 2} is known to have a very low solubility in borosilicate glasses (50 to 2000 ppm depending on the temperature, sodium concentration and conditions of synthesis). The link between the ruthenium and the borated network is not yet clearly identified, however, assumptions can be made such as the formation of Ru-O-B when a sufficient quantity of Ru is add (> 400 ppm). Ruthenium could play the role of silicon in increasing the possibility of bridging bond formation with boron coordination number IV.
Characterization of a novel, short pulse laser-driven neutron source
We present a full characterization of a short pulse laser-driven neutron source. Neutrons are produced by nuclear reactions of laser-driven ions deposited in a secondary target. The emission of neutrons is a superposition of an isotropic component into 4π and a forward directed, jet-like contribution, with energies ranging up to 80 MeV. A maximum flux of 4.4 × 10{sup 9} neutrons/sr has been observed and used for fast neutron radiography. On-shot characterization of the ion driver and neutron beam has been done with a variety of different diagnostics, including particle detectors, nuclear reaction, and time-of-flight methods. The results are of great value for future optimization of this novel technique and implementation in advanced applications.
Bright Laser-Driven Neutron Source Based on the Relativistic Transparency of Solids
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