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Exceptional Pt-like electrocatalytic activity was achieved in a sandwich-like catalyst of CoP/Ni 5 P 4 /CoP microsheet arrays for pH-universal hydrogen evolution through simply wrapping Ni 5 P 4 nanosheet arrays in CoP nanoparticles.
The relationship between noble metal phase particles and fission gas bubble production in used nuclear fuel is described. The majority of Te atoms within noble metal phase undergo radioactive decay to form stable Xe within a few hours after particle formation. This results in the production of clusters of Xe atoms contained within the solid metal matrix exhibiting an equivalent gas bubble pressure approaching 1 GPa. These high pressure bubbles are stabilized by the UO2 within the bulk of the fuel. However, when these bubbles form near the fuel / cladding interface, in combination with local and temporal damage caused by fission recoil, they are capable of overcoming the fracture strength of the UO2 and rupturing catastrophically. The force of the resulting bubble rupture is sufficient to eject noble metal phase particles several microns into the cladding. This proposed mechanism explains the observance of noble metal phase in cladding and is consistent with a host of morphological features found near the fuel / cladding interface.
A cobalt complex supported by the 2-(diisopropylphosphaneyl)benzenethiol ligand was synthesized and its electronic structure and reactivity were explored.
Two methods using a laser-diode tapered amplifier to produce high-power, high-efficiency optical frequency sidebands over a wide tunable frequency range are studied and compared. For a total output of 500 mW at 811 nm, 20% of the power can be placed in each of the first-order sidebands. Functionality and characterization are presented within the sideband frequency region of 0.8–2.3 GHz, and it is shown that both methods can be applied beyond this frequency range. These methods provide a versatile and effective tool for atomic physics experiments.
The synthesis of metal nanoparticles by ultrafast laser ablation of nanometers-thick metal films has been studied experimentally and computationally. Near-threshold backside laser ablation of 2–20 nm-thick Pt films deposited on fused silica substrates was found to produce nanoparticles with size distributions that were bimodal for the thicker films, but collapsed into a single mode distribution for the thinnest film. Time-resolved imaging of blackbody emission from the Pt nanoparticles was used to reveal the nanoparticle propagation dynamics and estimate their temperatures. The observed nanoparticle plume was compact and highly forward-directed with a well-defined collective velocity that permitted multiple rebounds with substrates to be revealed. Large-scale molecular dynamics simulations were used to understand the evolution of compressive and tensile stresses in the thicker melted liquid films that lead to their breakup and ejection of two groups of nanoparticles with different velocity and size distributions. Ultrafast laser irradiation of ultrathin (few nm) metal films avoids the splitting of the film and appears to be a method well-suited to cleanly synthesize and deposit nanoparticles from semitransparent thin film targets in highly directed beams.
In past studies, we have observed copious emissions of ionic and atomic Zn from single-crystal ZnO accompanying irradiation of single-crystal ZnO with 193-nm excimer laser irradiation at fluences below the onset of optical breakdown. The Zn{sup +} and ground state Zn° are studied using time-of-flight techniques and are mass selected using a quadrupole mass spectrometer. Simultaneously, we have observed emitted particles that are detectable with a Channeltron electron multiplier but cannot be mass selected. It is a reasonable hypothesis that these particles correspond to a neutral atom or molecule in highly excited long lived states. We provide strong evidence that they correspond to high lying Rydberg states of atomic Zn. We propose a production mechanism involving laser excitation via a two photon resonance excitation of Zn°.
Fluid Reynolds stress from turbulence has usually been considered to be responsible for the anomalous toroidal momentum transport in tokamak plasma. Experiment by Müller et al. [Phys. Rev. Lett. 106, 115001 (2011)], however, reported that neither the observed edge rotation profile nor the inward momentum transport phenomenon at the edge region of an H-mode plasma could be explained by the fluid Reynolds stress measured with reciprocating Langmuir-probe. The full-function gyrokinetic code XGC1 is used to explain, for the first time, Müller et al.'s experimental observations. It is discovered that, unlike in the plasma core, the fluid Reynolds stress from turbulence is not sufficient for momentum transport physics in plasma edge. The “turbulent neoclassical” physics arising from the interaction between kinetic neoclassical orbit dynamics and plasma turbulence is key in the tokamak edge region across the plasma pedestal into core.
The frequency noise spectrum of a magnetic tunnel junction based spin torque oscillator is examined where multiple modes and mode-hopping events are observed. The frequency noise spectrum is found to consist of both white noise and 1/f frequency noise. We find a systematic and similar dependence of both white noise and 1/f frequency noise on bias current and the relative angle between the reference and free layers, which changes the effective damping and hence the mode-hopping behavior in this system. The frequency at which the 1/f frequency noise changes to white noise increases as the free layer is aligned away from the anti-parallel orientation w.r.t the reference layer. These results indicate that the origin of 1/f frequency noise is related to mode-hopping, which produces both white noise as well as 1/f frequency noise similar to the case of ring lasers.
We present results of three-dimensional electromagnetic particle-in-cell simulations of the lower hybrid ion ring instability, similar to our earlier results [D. Winske and W. Daughton, Phys. Plasma 19, 072109 (2012)], but at higher electron beta (β{sub e} = ratio of electron thermal pressure to magnetic pressure = 0.06, rather than at 0.006) with T{sub i} = T{sub e}. At higher electron beta, the level of lower hybrid waves at saturation normalized to the ion thermal energy (β{sub i} = 0.06 also) is only slightly smaller, but the corresponding magnetic fluctuations are about an order of magnitude larger, consistent with linear theory. After saturation, the waves evolve into whistler waves, through a number of possible mechanisms, with an average growth rate considerably smaller than the linear growth rate of the lower hybrid waves, to a peak fluctuation level that is about 20% above the lower hybrid wave saturation level. The ratio of the peak magnetic fluctuations associated with the whistler waves relative to those of the saturated lower hybrid waves, the ratio of the nonlinear growth rate of whistlers relative to the linear growth rate of lower hybrid waves, the amount of energy extracted from the ring, and the amount of heating of the background ions and electrons are comparable to those in the lower electron beta 3D simulation. This suggests that even at higher electron beta, the linear and nonlinear physics of the lower hybrid ion ring instability is dominated by electrostatic, wave-particle rather than wave-wave interactions.
Recent experiments on the Laboratory for Laser Energetics' OMEGA laser have been carried out to produce strong shocks in solid spherical targets with direct laser illumination. The shocks are launched at pressures of several hundred Mbars and reach Gbar upon convergence. The results are relevant to the validation of the shock-ignition scheme and to the development of an OMEGA experimental platform to study material properties at Gbar pressures. The experiments investigate the strength of the ablation pressure and the hot-electron production at incident laser intensities of ∼2 to 6 × 10{sup 15 }W/cm{sup 2} and demonstrate ablation pressures exceeding 300 Mbar, which is crucial to developing a shock-ignition target design for the National Ignition Facility. The timing of the x-ray flash from shock convergence in the center of the solid plastic target is used to infer the ablation and shock pressures. Laser–plasma instabilities produce hot-electrons with a moderate temperature (<100 keV). The instantaneous conversion efficiencies of laser power into hot-electron power reached up to ∼15% in the intensity spike. The large amount of hot electrons is correlated with an earlier x-ray flash and a strong increase in its magnitude. This suggests that hot electrons contribute to the augmentation of the shock strength.
This study presents experimental scalings of the electrons and positrons produced by intense laser-target interactions at relativistic laser intensities (10 18 –10 20 W cm -2 ). The data were acquired from three short-pulse laser facilities with laser energies ranging from 80 to 1500 J. We found a non-linear (≈E L 2 ) scaling of positron yield [Chen et al., Phys. Rev. Lett. 114, 215001 (2015)] and a linear scaling of electron yield with the laser energy. These scalings are explained by theoretical and numerical analyses. Positron acceleration by the target sheath field is confirmed by the positron energy spectrum, which has a pronounced peak at energies near the sheath potential, as determined by the observed maximum energies of accelerated protons. The parameters of laser-produced electron-positron jets are summarized together with the theoretical energy scaling. Finally, the measured energy-squared scaling of relativistic electron-positron jets indicates the possibility to create an astrophysically relevant experimental platform with such jets using multi-kilojoule high intensity lasers currently under construction.
The magnetized shock problem is studied in the context where supersonic plasma flows past a solid obstacle. This problem exhibits interesting and important phenomena such as a bow shock, magnetotail formation, reconnection, and plasmoid formation. This study is carried out using a discontinuous Galerkin method to solve an extended magneto-hydrodynamic model (XMHD). The main goals of this paper are to present a reasonably complete picture of the properties of this interaction using the MHD model and then to compare the results to the XMHD model. The inflow parameters, such as the magnetosonic Mach number M{sub f} and the ratio of thermal pressure to magnetic pressure β, can significantly affect the physical structures of the flow-obstacle interaction. The Hall effect can also significantly influence the results in the regime in which the ion inertial length is numerically resolved. Most of the results presented are for the two-dimensional case; however, two three-dimensional simulations are presented to make a connection to the important case in which the solar wind interacts with a solid body and to explore the possibility of performing scaled laboratory experiments.
Experimental results demonstrate the ability of a surface electrode pattern to produce sufficient in-plane strain in a PbZr0.52Ti0.48O3 (PZT) thin film clamped by a Si substrate to control magnetism in a 1000 nm diameter Ni ring. The electrode pattern and the Ni ring/PZT thin film heterostructure were designed using a finite element based micromagnetics code. The magnetoelectric heterostructures were fabricated on the PZT film using e-beam lithography and characterized using magnetic force microscopy. Application of voltage to the electrodes moved one of the “onion” state domain walls. This method enables the development of complex architectures incorporating strain-mediated multiferroic devices.
This article addresses the modeling of non-equilibrium phenomena in inductively coupled plasma discharges. In the proposed computational model, the electromagnetic induction equation is solved together with the set of Navier-Stokes equations in order to compute the electromagnetic and flow fields, accounting for their mutual interaction. Semi-classical statistical thermodynamics is used to determine the plasma thermodynamic properties, while transport properties are obtained from kinetic principles, with the method of Chapman and Enskog. Particle ambipolar diffusive fluxes are found by solving the Stefan-Maxwell equations with a simple iterative method. Two physico-mathematical formulations are used to model the chemical reaction processes: (1) A Local Thermodynamics Equilibrium (LTE) formulation and (2) a thermo-chemical non-equilibrium (TCNEQ) formulation. In the TCNEQ model, thermal non-equilibrium between the translational energy mode of the gas and the vibrational energy mode of individual molecules is accounted for. The electronic states of the chemical species are assumed in equilibrium with the vibrational temperature, whereas the rotational energy mode is assumed to be equilibrated with translation. Three different physical models are used to account for the coupling of chemistry and energy transfer processes. Numerical simulations obtained with the LTE and TCNEQ formulations are used to characterize the extent of non-equilibrium of the flow inside the Plasmatron facility at the von Karman Institute. Each model was tested using different kinetic mechanisms to assess the sensitivity of the results to variations in the reaction parameters. A comparison of temperatures and composition profiles at the outlet of the torch demonstrates that the flow is in non-equilibrium for operating conditions characterized by pressures below 30 000 Pa, frequency 0.37 MHz, input power 80 kW, and mass flow 8 g/s.
Soft x-ray emission from laser irradiated gold foils was measured at the Omega-60 laser system using the Dante photodiode array. The foils were heated with 2 kJ, 6 ns laser pulses and foil thicknesses were varied between 0.5, 1.0, and 2.0 μm. Initial Dante analysis indicates peak emission temperatures of roughly 100 eV and 80 eV for the 0.5 μm and 1.0 μm thick foils, respectively, with little measurable emission from the 2.0 μm foils.
Abstract not provided
A horn shaped Langevin ultrasonic transducer used in a single axis levitator was characterized to better understand the role of the acoustic profile in establishing stable traps. The method of characterization included acoustic beam profiling performed by raster scanning an ultrasonic microphone as well as finite element analysis of the horn and its interface with the surrounding air volume. The results of the model are in good agreement with measurements and demonstrate the validity of the approach for both near and far field analyses. Our results show that this style of transducer produces a strong acoustic beam with a total divergence angle of 10 degree, a near-field point close to the transducer surface and a virtual sound source. These are desirable characteristics for a sound source used for acoustic trapping experiments