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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Accelerated oxidation of epoxy thermosets with increased O 2 pressure

Polymer oxidation is usually accelerated with temperature, which is therefore applied in nearly every experimental approach dealing with predictive materials aging. Because of this simple approach, we may tend to neglect that the effective concentration of oxygen also acts as a rate multiplier for oxidation. Increasing the oxygen partial pressure in an aging environment accelerates oxidation, and while there is often a near proportional increase initially, the effect of additional oxygen usually transitions to a saturation level at some elevated pressure. This has been theoretically described in the general autoxidation scheme and is well recognized. However, for many materials the exact rate behavior under moderately increased oxygen concentration remains to be established. We therefore review epoxy oxidation and offer a broader overview of its behavior under increased O 2 partial pressure. Experimental data are given for a few thermoset materials demonstrating their rate behavior under O 2 partial pressure up to 4 atm, meaning approximately 20 times more than under standard atmospheric conditions. Confirmative evidence suggests that epoxy materials will reach saturation oxidation rates only at significantly higher O 2 partial pressure. In such a high-pressure regime it is theoretically possible to not only accelerate oxidation, but to transition into a condition where O 2 diffusion can be increased without further accelerating the oxidation rate. Finally, this can reduce diffusion limited oxidation effects under specific accelerated aging conditions as a combination of temperature and O 2 partial pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Interpreting accelerated tests on perovskite modules using photooxidation of MAPbI 3 as an example

Solar panels (modules) based on metal halide perovskites are following a fast track to commercialization. Unlike more established solar cell materials, there are not yet decades-long field observations to increase consumer confidence. The "physics and chemistry of failure" approach is used in other industries and estimates product degradation based on laboratory accelerated tests integrated with an understanding of degradation mechanisms. This work uses that approach to quantify the relationship between accelerated tests and projected product behavior for metal halide perovskite modules. Degradation involving photooxidation of methylammonium lead iodide is used to illustrate the method. Acceleration factors in common accelerated tests are found to be low. Conclusions emphasize that the accelerated tests on photovoltaics should not be interpreted as equivalent across module types or as a green light for commercialization unless supported by the appropriate field data or physics and chemistry of failure analysis.

14 SOLAR ENERGY↗
DOE OSTI2016

Demonstration of a positron beam-driven hollow channel plasma wakefield accelerator

Plasma wakefield accelerators have been used to accelerate electron and positron particle beams with gradients that are orders of magnitude larger than those achieved in conventional accelerators. In addition to being accelerated by the plasma wakefield, the beam particles also experience strong transverse forces that may disrupt the beam quality. Hollow plasma channels have been proposed as a technique for generating accelerating fields without transverse forces. In this study, we demonstrate a method for creating an extended hollow plasma channel and measure the wakefields created by an ultrarelativistic positron beam as it propagates through the channel. The plasma channel is created by directing a high-intensity laser pulse with a spatially modulated profile into lithium vapour, which results in an annular region of ionization. A peak decelerating field of 230 MeV m -1 is inferred from changes in the beam energy spectrum, in good agreement with theory and particle-in-cell simulations.

43 PARTICLE ACCELERATORS↗
DOE OSTI2016

High-field plasma acceleration in a high-ionization-potential gas

Plasma accelerators driven by particle beams are a very promising future accelerator technology as they can sustain high accelerating fields over long distances with high energy efficiency. They rely on the excitation of a plasma wave in the wake of a drive beam. To generate the plasma, a neutral gas can be field-ionized by the head of the drive beam, in which case the distance of acceleration and energy gain can be strongly limited by head erosion. In our research, we overcome this limit and demonstrate that electrons in the tail of a drive beam can be accelerated by up to 27 GeV in a high-ionization-potential gas (argon), boosting their initial 20.35 GeV energy by 130%. Particle-in-cell simulations show that the argon plasma is sustaining very high electric fields, of ~150 GV m -1 , over ~20 cm. Lastly, the results open new possibilities for the design of particle beam drivers and plasma sources.

43 PARTICLE ACCELERATORS↗
DOE OSTI2021

Turn-key constrained parameter space exploration for particle accelerators using Bayesian active learning

Abstract Particle accelerators are invaluable discovery engines in the chemical, biological and physical sciences. Characterization of the accelerated beam response to accelerator input parameters is often the first step when conducting accelerator-based experiments. Currently used techniques for characterization, such as grid-like parameter sampling scans, become impractical when extended to higher dimensional input spaces, when complicated measurement constraints are present, or prior information known about the beam response is scarce. Here in this work, we describe an adaptation of the popular Bayesian optimization algorithm, which enables a turn-key exploration of input parameter spaces. Our algorithm replaces the need for parameter scans while minimizing prior information needed about the measurement’s behavior and associated measurement constraints. We experimentally demonstrate that our algorithm autonomously conducts an adaptive, multi-parameter exploration of input parameter space, potentially orders of magnitude faster than conventional grid-like parameter scans, while making highly constrained, single-shot beam phase-space measurements and accounts for costs associated with changing input parameters. In addition to applications in accelerator-based scientific experiments, this algorithm addresses challenges shared by many scientific disciplines, and is thus applicable to autonomously conducting experiments over a broad range of research topics.

43 PARTICLE ACCELERATORS↗
DOE OSTI2019

Ionization injection of highly-charged copper ions for laser driven acceleration from ultra-thin foils

Laser-driven ion acceleration is often analyzed assuming that ionization reaches a steady state early in the interaction of the laser pulse with the target. This assumption breaks down for materials of high atomic number for which the ionization occurs concurrently with the acceleration process. Using particle-in-cell simulations, we have examined acceleration and simultaneous field ionization of copper ions in ultra-thin targets (20-150 nm thick) irradiated by a laser pulse with intensity 1 × 10 21 W/cm 2 . At this intensity, the laser pulse drives strong electric fields at the rear side of the target that can ionize Cu to charge states with valence L-shell or full K-shell. The highly-charged ions are produced only in a very localized region due to a significant gap between the M- and L-shells' ionization potentials and can be accelerated by strong, forward-directed sections of the field. Such an "ionization injection" leads to well-pronounced bunches of energetic, highly-charged ions. We also find that for the thinnest target (20 nm) a push by the laser further increases the ion energy gain. Thus, the field ionization, concurrent with the acceleration, offers a promising mechanism for the production of energetic, high-charge ion bunches.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Direct laser acceleration of electron by an ultra intense and short-pulsed laser in under-dense plasma

Direct laser acceleration (DLA) of electron by an ultra intense and short-pulsed laser interacting with under-dense plasma is investigated based on 2.5-dimensional particle-in-cell simulation. A high-density electron beam is generated by the laser longitudinal ponderomotive force. Although the total number of DLA electrons is significantly smaller than the number of electrons trapped in the bubble, the total charge of high-energy DLA electrons (E>800MeV) reaches 67 pC/{mu}m. It is found that the electron beam occurs in a two-stage acceleration, i.e., accelerated in vacuum by the laser directly soon after a DLA process in plasma. The beam is accelerated violently with effective acceleration gradient in 100 GeV/cm. The energy spectrum of electrons presents a Maxwellian distribution with the highest energy of about 3.1 GeV. The dependence of maximum electron energy and electric quantity with laser intensity, laser width, pulse duration, and initial plasma density are also studied.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

A 2 MV Van de Graaff accelerator as a tool for planetary and impact physics research

Investigating the dynamical and physical properties of cosmic dust can reveal a great deal of information about both the dust and its many sources. Over recent years, several spacecraft (e.g., Cassini, Stardust, Galileo, and Ulysses) have successfully characterised interstellar, interplanetary, and circumplanetary dust using a variety of techniques, including in situ analyses and sample return. Charge, mass, and velocity measurements of the dust are performed either directly (induced charge signals) or indirectly (mass and velocity from impact ionisation signals or crater morphology) and constrain the dynamical parameters of the dust grains. Dust compositional information may be obtained via either time-of-flight mass spectrometry of the impact plasma or direct sample return. The accurate and reliable interpretation of collected spacecraft data requires a comprehensive programme of terrestrial instrument calibration. This process involves accelerating suitable solar system analogue dust particles to hypervelocity speeds in the laboratory, an activity performed at the Max Planck Institut fuer Kernphysik in Heidelberg, Germany. Here, a 2 MV Van de Graaff accelerator electrostatically accelerates charged micron and submicron-sized dust particles to speeds up to 80 km s{sup -1}. Recent advances in dust production and processing have allowed solar system analogue dust particles (silicates and other minerals) to be coated with a thin conductive shell, enabling them to be charged and accelerated. Refinements and upgrades to the beam line instrumentation and electronics now allow for the reliable selection of particles at velocities of 1-80 km s{sup -1} and with diameters of between 0.05 {mu}m and 5 {mu}m. This ability to select particles for subsequent impact studies based on their charges, masses, or velocities is provided by a particle selection unit (PSU). The PSU contains a field programmable gate array, capable of monitoring in real time the particles' speeds and charges, and is controlled remotely by a custom, platform independent, software package. The new control instrumentation and electronics, together with the wide range of accelerable particle types, allow the controlled investigation of hypervelocity impact phenomena across a hitherto unobtainable range of impact parameters.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2012

3 MV hypervelocity dust accelerator at the Colorado Center for Lunar Dust and Atmospheric Studies

A hypervelocity dust accelerator for studying micrometeorite impacts has been constructed at the Colorado Center for Lunar Dust and Atmospheric Studies (CCLDAS) at the University of Colorado. Based on the Max-Planck-Instituet fuer Kernphysik (MPI-K) accelerator, this accelerator is capable of emitting single particles of a specific mass and velocity selected by the user. The accelerator consists of a 3 MV Pelletron generator with a dust source, four image charge pickup detectors, and two interchangeable target chambers: a large high-vacuum test bed and an ultra-high vacuum impact study chamber. The large test bed is a 1.2 m diameter, 1.5 m long cylindrical vacuum chamber capable of pressures as low as 10{sup -7} torr while the ultra-high vacuum chamber is a 0.75 m diameter, 1.1 m long chamber capable of pressures as low as 10{sup -10} torr. Using iron dust of up to 2 microns in diameter, final velocities have been measured up to 52 km/s. The spread of the dust particles and the effect of electrostatic focusing have been measured using a long exposure CCD and a quartz target. Furthermore, a new technique of particle selection is being developed using real time digital filtering techniques. Signals are digitized and then cross-correlated with a shaped filter, resulting in a suppressed noise floor. Improvements over the MPI-K design, which include a higher operating voltage and digital filtering for detection, increase the available parameter space of dust emitted by the accelerator. The CCLDAS dust facility is a user facility open to the scientific community to assist with instrument calibrations and experiments.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2012

Energy enhancement of proton acceleration in combinational radiation pressure and bubble by optimizing plasma density

The combinational laser radiation pressure and plasma bubble fields to accelerate protons are researched through theoretical analysis and numerical simulations. The dephasing length of the accelerated protons bunch in the front of the bubble and the density gradient effect of background plasma on the accelerating phase are analyzed in detail theoretically. The radiation damping effect on the accelerated protons energy is also considered. And it is demonstrated by two-dimensional particle-in-cell simulations that the protons bunch energy can be increased by using the background plasma with negative density gradient. However, radiation damping makes the maximal energy of the accelerated protons a little reduction.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Hybrid proton acceleration scheme using relativistic intense laser light

Ion acceleration phenomena at relativistic intense laser interaction with thin foil targets are studied to find an efficient laser-target interaction concept at the conditions, where neither the ponderomotive pressure of the laser light nor the hot electron pressure is negligible. Particle in cell simulations and the analytical model are allowing to predict optimum laser-target parameters and suggesting a significant increase of proton energy if a hybrid proton acceleration scheme is used. In the proposed scenario, the laser polarisation is changed during the acceleration process: First with circularly polarised laser light the target is accelerated as a whole by the ponderamotive pressure, and then with linearly polarised laser light the electrons are heated which additionally increases the accelerating field. The calculations are in good agreement with experimental findings.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

High contrast ion acceleration at intensities exceeding 10{sup 21} Wcm{sup −2}

Ion acceleration from short pulse laser interactions at intensities of 2×10{sup 21}Wcm{sup −2} was studied experimentally under a wide variety of parameters, including laser contrast, incidence angle, and target thickness. Trends in maximum proton energy were observed, as well as evidence of improvement in the acceleration gradients by using dual plasma mirrors over traditional pulse cleaning techniques. Extremely high efficiency acceleration gradients were produced, accelerating both the contaminant layer and high charge state ions from the bulk of the target. Two dimensional particle-in-cell simulations enabled the study of the influence of scale length on submicron targets, where hydrodynamic expansion affects the rear surface as well as the front. Experimental evidence of larger electric fields for sharp density plasmas is observed in simulation results as well for such targets, where target ions are accelerated without the need for contaminant removal.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Laser ion acceleration by using the dynamic motion of a target

Proton acceleration by using a 620 TW, 18 J laser pulse of peak intensity of 5×10{sup 21} W/cm{sup 2} irradiating a disk target is examined using three-dimensional particle-in-cell simulations. It is shown that protons are accelerated efficiently to high energy for a “light” material in the first layer of a double-layer target, because a strongly inhomogeneous expansion of the first layer occurs by a Coulomb explosion within such a material. Moreover, a large movement of the first layer for the accelerated protons is produced by radiation-pressure-dominant acceleration. A time-varying electric potential produced by this expanding and moving ion cloud accelerates protons effectively. In addition, using the best material for the target, one can generate a proton beam with an energy of 200 MeV and an energy spread of 2%.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

The beat in laser-accelerated ion beams

Regular modulation in the ion velocity distribution becomes detectable if intense femtosecond laser pulses with very high temporal contrast are used for target normal sheath acceleration of ions. Analytical and numerical analysis of the experimental observation associates the modulation with the half-cycle of the driving laser field period. In processes like ion acceleration, the collective and laser-frequency determined electron dynamics creates strong fields in plasma to accelerate the ions. Even the oscillatory motion of electrons and its influence on the acceleration field can dominate over smoothing effects in plasma if a high temporal contrast of the driving laser pulse is given. Acceleration parameters can be directly concluded out of the experimentally observed modulation period in ion velocity spectra. The appearance of the phenomenon at a temporal contrast of ten orders between the intensity of the pulse peak and the spontaneous amplified emission background as well as remaining intensity wings at picosecond time-scale might trigger further parameter studies with even higher contrast.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Stable long range proton acceleration driven by intense laser pulse with underdense plasmas

Proton acceleration is investigated by 2.5-dimensional particle-in-cell simulations in an interaction of an ultra intense laser with a near-critical-density plasma. It was found that multi acceleration mechanisms contribute together to a 1.67 GeV collimated proton beam generation. The W-BOA (breakout afterburner based on electrons accelerated by a wakefield) acceleration mechanism plays an important role for the proton energy enhancement in the area far from the target. The stable and continuous acceleration maintains for a long distance and period at least several pico-seconds. Furthermore, the energy scalings are also discussed about the target density and the laser intensity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

On the mechanism of acceleration behavior of plasma bullet

Two special experiments are designed to study the mechanism of the acceleration behavior of a plasma bullet when it exits a nozzle. First, a T-shape device is used to simulate the air diffusion when a plasma plume exits the nozzle. It is found that adding just 1% of N{sub 2}, O{sub 2}, or air to the main working gas He results in the acceleration of the plasma bullet. Second, materials of different permittivity are added to the left part of the outside of the tube. The experimental results show that the plasma bullet accelerates at the moment when it enters into the right part of the tube where there is no extra material on the outside of the tube. These two experiments confirm that the acceleration behavior of the plasma bullet when it exits the nozzle is due to the air diffusion, hence Penning ionization, and the permittivity change when the bullet exits the nozzle, for example, from a tube with high permittivity to air with low permittivity. Besides, electric field measurements show that the electric field in the bullet head increases when the plasma bullet accelerates. This confirms the electric field driven nature of the plasma bullet propagation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Role of stochastic heating in wakefield acceleration when optical injection is used

The dynamics of an electron in two counterpropagating waves is investigated. Conditions for stochastic acceleration are derived. The possibility of stochastic heating is confirmed when two waves interact with low density plasma by performing PIC (Particle In Cell) code simulations. It is shown that stochastic heating can play an important role in laser wakefield acceleration. When considering low density plasma interacting with a high intensity wave perturbed by a low intensity counterpropagating wave, stochastic heating can provide electrons with the right momentum for trapping in the wakefield. The influence of stochastic acceleration on the trapping of electrons is compared to the one of the beatwave force which is responsible for cold injection. To do so, several polarizations for the colliding pulses are considered. For some value of the plasma density and pulse duration, a transition from an injection due to stochastic acceleration to a cold injection dominated regime—regarding the trapped charge—has been observed from 2D and 3D PIC code simulations. This transition is ruled by the ratio of the interaction length of the pulses to the longitudinal size of the bubble. When the interaction length of the laser pulses reaches the radius of the accelerating cavity stochastic heating becomes dominant, and might be necessary to get electrons trapped into the wakefield, when wakefield inhibition grows with plasma density.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

The mechanisms of electron heating and acceleration during magnetic reconnection

The heating of electrons in collisionless magnetic reconnection is explored in particle-in-cell simulations with non-zero guide fields so that electrons remain magnetized. In this regime, electric fields parallel to B accelerate particles directly, while those perpendicular to B do so through gradient-B and curvature drifts. The curvature drift drives parallel heating through Fermi reflection, while the gradient B drift changes the perpendicular energy through betatron acceleration. We present simulations in which we evaluate each of these mechanisms in space and time in order to quantify their role in electron heating. For a case with a small guide field (20% of the magnitude of the reconnecting component), the curvature drift is the dominant source of electron heating. However, for a larger guide field (equal to the magnitude of the reconnecting component) electron acceleration by the curvature drift is comparable to that of the parallel electric field. In both cases, the heating by the gradient B drift is negligible in magnitude. It produces net cooling because the conservation of the magnetic moment and the drop of B during reconnection produce a decrease in the perpendicular electron energy. Heating by the curvature drift dominates in the outflow exhausts where bent field lines expand to relax their tension and is therefore distributed over a large area. In contrast, the parallel electric field is localized near X-lines. This suggests that acceleration by parallel electric fields may play a smaller role in large systems where the X-line occupies a vanishing fraction of the system. The curvature drift and the parallel electric field dominate the dynamics and drive parallel heating. A consequence is that the electron energy spectrum becomes extremely anisotropic at late time, which has important implications for quantifying the limits of electron acceleration due to synchrotron emission. An upper limit on electron energy gain that is substantially higher than earlier estimates is obtained by balancing reconnection drive with radiative loss.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗