High Flux Electron Beam Research Articles

Cross-Filament Stochastic Acceleration of Electrons in Kilojoule Picosecond Laser Interactions with Near-Critical-Density Plasmas

Understanding the interaction of a kilojoule picosecond laser pulse with long-scale-length preplasma or homogeneous near-critical-density (NCD) plasma is crucial for guiding experiments at national short-pulse laser facilities. Using full three-dimensional particle-in-cell simulations, we demonstrate that in this regime, cross-filament stochastic acceleration is an important mechanism that contributes to the production of superponderomotive high-flux electron beams. Since the laser power significantly exceeds the threshold of the relativistic self-focusing, multiple filaments are generated and can propagate independently over a long distance. Electrons jump across the filaments during the acceleration and their motion becomes stochastic. We find that the effective temperature of electrons increases with the total interaction time following a scaling like ${T}_{\mathrm{eff}}\ensuremath{\propto}{\ensuremath{\tau}}_{i}^{0.65}$. By irradiating a submillimeter-thick NCD target, the space charge of electrons with energy above 2.5 MeV reaches tens of microcoulombs. Such high-flux electrons with superponderomotive energies significantly facilitate applications in high-energy-density science, nuclear science, secondary sources, and diagnostic techniques.

Near-ultraviolet continuum modeling of the 1985 April 12 great flare of AD Leo

White-light stellar flares are now reported by the thousands in long-baseline, high-precision, broad-band photometry from missions like Kepler, K2, and TESS. These observations are crucial inputs for assessments of biosignatures in exoplanetary atmospheres and surface ultraviolet radiation dosages for habitable-zone planets around low-mass stars. A limitation of these assessments, however, is the lack of near-ultraviolet spectral observations of stellar flares. To motivate further empirical investigation, we use a grid of radiative-hydrodynamic simulations with an updated treatment of the pressure broadening of hydrogen lines to predict the λ ≈ 1800 − 3300 Å continuum flux during the rise and peak phases of a well-studied superflare from the dM3e star AD Leo. These predictions are based on semi-empirical superpositions of radiative flux spectra consisting of a high-flux electron beam simulation with a large, low-energy cutoff (≳ 85 keV) and a lower-flux electron beam simulation with a smaller, low-energy cutoff (≲ 40 keV). The two-component models comprehensively explain the hydrogen Balmer line broadening, the optical continuum color temperature, the Balmer jump strength, and the far-ultraviolet continuum strength and shape in the rise/peak phase of this flare. We use spatially resolved analyses of solar flare data from the Interface Region Imaging Spectrograph, combined with the results of previous radiative-hydrodynamic modeling of the 2014 March 29 X1 solar flare (SOL20140329T17:48), to interpret the two-component electron beam model as representing the spatial superposition of bright kernels and fainter ribbons over a larger area.

Electron irradiation induced modifications of Ti(1-x)AlxN coatings and related buffer layers on steel substrates

Ti(1-x)AlxN hard coatings were prepared by reactive magnetron sputtering onto steel substrates (51CrV4 – 1.8159) and subsequently exposed for a short-time (~1 s) to high-flux electron beam (EB) treatment. Morphology, composition and the structure of as-deposited and EB treated coatings were investigated using transmission electron microscopy (TEM), secondary ion mass spectroscopy (SIMS) and X-ray diffraction (XRD). It was found that the EB treatment had only a minor influence on the morphology and crystallinity of the Ti(1-x)AlxN phase, however, the stress-free lattice parameter and partly the compressive stress in the coatings were clearly reduced by the treatment. On the other hand, major changes of composition profiles and structure were observed in the metallic buffer layer between substrate and Ti(1-x)AlxN. The observed modifications in the coating-substrate system are explained by rapid heat up and radiation damage due to the fast electron exposure.

High-flux electron beams from laser wakefield accelerators driven by petawatt lasers

Laser wakefield accelerators (LWFAs) are considered to be one of the most competitive next-generation accelerator candidates. In this paper, we will study the potential high-flux electron beam production of an LWFA driven by petawatt-level laser pulses. In our three-dimensional particle-in-cell simulations, an optimal set of parameters gives of charge with laser power, thus of instantaneous current if we assume the electron beam duration is 100 fs. This high flux and its secondary radiation are widely applicable in nuclear and QED physics, industrial imaging, medical and biological studies.

A Monte Carlo simulation code for calculating damage and particle transport in solids: The case for electron-bombarded solids for electron energies up to 900 MeV

Current popular Monte Carlo simulation codes for simulating electron bombardment in solids focus primarily on electron trajectories, instead of electron-induced displacements. Here we report a Monte Carol simulation code, DEEPER (damage creation and particle transport in matter), developed for calculating 3-D distributions of displacements produced by electrons of incident energies up to 900 MeV. Electron elastic scattering is calculated by using full-Mott cross sections for high accuracy, and primary-knock-on-atoms (PKAs)-induced damage cascades are modeled using ZBL potential. We compare and show large differences in 3-D distributions of displacements and electrons in electron-irradiated Fe. The distributions of total displacements are similar to that of PKAs at low electron energies. But they are substantially different for higher energy electrons due to the shifting of PKA energy spectra towards higher energies. The study is important to evaluate electron-induced radiation damage, for the applications using high flux electron beams to intentionally introduce defects and using an electron analysis beam for microstructural characterization of nuclear materials.

Beam collimation and transport of quasineutral laser-accelerated protons by a solenoid field

This article reports about controlling laser-accelerated proton beams with respect to beam divergence and energy. The particles are captured by a pulsed high field solenoid with a magnetic field strength of 8.6 T directly behind a flat target foil that is irradiated by a high intensity laser pulse. Proton beams with energies around 2.3 MeV and particle numbers of 1012 could be collimated and transported over a distance of more than 300 mm. In contrast to the protons the comoving electrons are strongly deflected by the solenoid field. They propagate at a submillimeter gyroradius around the solenoid’s axis which could be experimentally verified. The originated high flux electron beam produces a high space charge resulting in a stronger focusing of the proton beam than expected by tracking results. Leadoff particle-in-cell simulations show qualitatively that this effect is caused by space charge attraction due to the comoving electrons. The collimation and transport of laser-accelerated protons is the first step to provide these unique beams for further applications such as postacceleration by conventional accelerator structures.

Thermally Induced Transfiguration of Polymer Nanowires under Irradiation of Electron Beams

Polymer nanowires were found to respond to external perturbations and to undergo transfiguration upon exposure to the irradiation of high-flux electron beams. The phenomenon is general, observable in the nanowires of conjugated and nonconjugated polymers with linear and hyperbranched structures. The transfiguration ceased to occur after the nanowires had been annealed at temperatures higher than the glass transition temperatures of the polymers. The phenomenon is rationalized to be associated with the residual internal stress in the nanowires that is relieved through the electron-beam irradiation and/or thermal annealing. This work thus offers cautionary advice that a proper annealing treatment should be exercised, if one wishes to fabricate polymer nanostructures and miniature devices with reliable stability and durable performance.

Highly bright X-ray generator using heat of fusion with a specially designed rotating anticathode

A new type of rotating anticathode X-ray generator has been developed, in which the electron beam irradiates the inner surface of a U-shaped anticathode (Cu). A high-flux electron beam is focused on the inner surface by optimizing the shape of the bending magnet. The power of the electron beam can be increased to the point at which the irradiated part of the inner surface is melted, because a strong centrifugal force fixes the melted part on the inner surface. When the irradiated part is melted, a large amount of energy is stored as the heat of fusion, resulting in emission of X-rays 4.3 times more brilliant than can be attained by a conventional rotating anticathode. Oscillating translation of the irradiated position on the inner surface during use is expected to be very advantageous for extending the target life. A carbon film coating on the inner surface is considered to suppress evaporation of the target metal and will be an important technique in further realization of highly bright X-ray generation.

Phase-transition properties of VO 2 thin films changed by high flux electron beam radiation

VO 2 is a typical material able to make a phase-transition by heating. At approximately Tc=68 °C, VO 2 can be suddenly transferred from the phase of anatase to rutile, accompanying the rapid change of resistance and transmissivity. It is quite interesting to study this material. In this work, VO 2 thin films have been prepared by the method of vacuum evaporation combined with vacuum deoxidation and irradiated by electron beam with fluence of 6.3×10 16 cm 2 and energy of 1.7 MeV, and then have been annealed at different temperature after irradiation. Both phase transition properties and crystalline structures are characterized by X-ray diffraction, UV–VIS transmittance and resistance analysis. The results show that the annealing process after electron irradiation modifies the phase-transition temperature and the width of phase-transition hysteresis.

Electron Beam Induced Carbon Depositionand Etching

ABSTRACTElectron induced carbon deposition and etching was investigated by Auger electron spectroscopy in a custom designed vacuum system. The Auger electron spectrometer was used to provide a high flux electron beam to induce reactions and to monitor surface composition. During the e-beam induced deposition or etching, the gas phase pressure was 10-4 to 10-5 Torr. Several carbon precursors: benzene, cyclohexane and propane were used for deposition. The deposition rate depended on the precursor sticking coefficient and bonding structure. Among the three precursors tested, the deposition rate of carbon was cyclohexane > benzene > propane. The e-beam induced etching of carbon films was carried out in 1 × 10-4 torr oxygen ambient and the carbon film was prepared by reactive physical vapor deposition. The etching process can be divided into three stages: bulk film, interface, and substrate. For the bulk carbon film, the decrease of film thickness varies linearly with the e-beam flux, while at the interface, the film thickness shows an exponential decay with the electron flux. For the C in the Si substrate, a very slow etching rate was observed. The etching rate for bulk carbon film was ∼ 0.1 nm/min under the experimental conditions, which is equivalent tp 2.4 × 10-27 cm3/electron. At the interfacial region, the cross section of carbon removal by electrons was ∼ 4.6 × 10-21 cm2. Based on the change of the carbon line shape at the interface, we concluded that the etching rate is related to the chemical nature of the carbon species.

Diamond growth by hollow cathode arc chemical vapor deposition at low pressure range of 0.02–2 mbar

Abstract An attempt was made to synthesize diamond films on (001) silicon substrates by means of a graphite or tungsten hollow cathode arc chemical vapor deposition at a lower pressure range of 0.02–2 mbar. The hollow cathode arc provides the advantage of the generation of a large area, high-flux electron beam, a very high-density plasma, and the high kinetic reaction species due to relatively low pressure operation. Diamond films have been characterized by scanning electron microscopy and Raman spectroscopy. The quality of diamond films deposited using the graphite hollow cathode was better than that using the tungsten hollow cathode at 2 mbar pressure. With further decreasing the deposition pressure, the evaporation and sputtering of the graphite hollow cathode are increased and the film quality was deteriorated. The growth rate of diamond films decreased and the nucleation density increased with decreasing deposition pressure.

Nanoscale Modification Of Amorphous Diamond-Like Carbon Film Surfaces

ABSTRACTScanning Tunneling and Atomic Force Microscopy (STM, AFM) of amorphous tetrahedral carbon films (a-tC) film surfaces grown by pulsed laser deposition indicate extreme flatness and uniform electronic properties.1 This is consistent with the observed predominance of sp3 (diamond-like) bonding in these materials.2 Potential applications of a-tC films may be enhanced with the ability to modify their surfaces on the nanoscale. Exploiting the metastable nature of the sp3 hybridized state of carbon, the high flux electron beam from an STM tip was used to modify ˜100 nm regions; processing in air or in vacuum produces similar results. STM and AFM maps of the modified regions indicate stable morphologic and electronic structures consistent with a local transformation to sp2 (graphitic) hybridization. Surface potentiometric mapping of these regions indicates a correlation between electron dose and a lowering of the surface potential. In addition, spatially-resolved electron energy loss spectroscopy of the modified areas performed with a Scanning Auger Microscope shows plasmon energy shifts that confirm an elevated sp2 content in the modified regions.3

Plasma structures in front of a floated emissive electrode

A particle simulation with plasma source is carried out on plasma structures generated by an electron emissive electrode floated in a collisionless plasma. When low-temperature, high-density thermal electrons are emitted, there appears a negative potential dip in front of the electrode, which is always accompanied by a low-frequency oscillation. On the other hand, three regimes of plasma structures appear for an electron beam injection. When a high-flux electron beam is injected, an electron sheath is generated in front of the electrode. The sheath reflects ions flowing to the electrode, providing an increase in the plasma density. When a low-flux electron beam is injected, no electron sheath is generated. When an intermediate-flux beam is injected, the electron sheath structure appears periodically in time. The lifetime of the sheath is proportional to the system length. These results of beam injection are almost consistent with those of a Q-machine experiment.