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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Femtosecond exciton dynamics in WSe 2 optical waveguides

Van-der Waals (vdW) atomically layered crystals can act as optical waveguides over a broad range of the electromagnetic spectrum ranging from Terahertz to visible. Unlike common Si-based waveguides, vdW semiconductors host strong excitonic resonances that may be controlled using non-thermal stimuli including electrostatic gating and photoexcitation. Here, we utilize waveguide modes to examine photo-induced changes of excitons in the prototypical vdW semiconductor, WSe 2 , prompted by femtosecond light pulses. Using time-resolved scanning near-field optical microscopy we visualize the electric field profiles of waveguide modes in real space and time and extract the temporal evolution of the optical constants following femtosecond photoexcitation. By monitoring the phase velocity of the waveguide modes, we detect incoherent A-exciton bleaching along with a coherent optical Stark shift in WSe 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Parametric excitation of fast upper hybrid waves by non-resonant beating of counter-propagating X-mode lasers in a magnetized plasma

Generation of fast and slow upper hybrid waves by two plasmon decay of non-resonant beating mode of two counter-propagating X-mode lasers is modelled in magnetized plasma. Two counter-propagating lasers having frequencies and wave-vectors (ω{sub 1},k{sub 1}) and (ω{sub 2},k{sub 2}), respectively, generate a non resonant beat wave at frequency difference ω{sub 0}≈ω{sub 1}∼ω{sub 2} and wave number k{sup →}{sub 0}≈k{sup →}{sub 1}+k{sup →}{sub 2} which parametrically excites a pair of copropagating fast and slow upper hybrid waves at ω{sub 0}≈2ω{sub h}+(3k{sub 1}{sup 2}v{sub th}{sup 2}/ω{sub h}) (1−ω{sub h}/ω{sub 1}) where ω{sub h} and v{sub th} are the upper hybrid frequency and electron thermal speed, respectively. The fast upper hybrid wave can be utilized for electron acceleration because its phase velocity is close to c. The growth rate of decay process is Γ∼ω{sub p}/10 at scattering angle θ{sub s}∼5π/6 and magnetic field ∼90 T, which is one order higher as compared to the growth rate of Raman process. The growth rate can be further enhanced (∼20%) by increasing the magnetic field ∼450 T.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Numerical study on the stability of weakly collisional plasma in E×B fields

Plasma stability in weakly collisional plasmas in the presence of E×B fields is studied with numerical simulations. Different types of ion-neutral collisions are considered in a fully magnetized regime. We study the influence of ion-neutral collisions and the role of collision types on the stability of plasma. It is found that the stability of plasma depends on the type of ion-neutral collisions, with the plasma being unstable for charge exchange collisions, and stable for the elastic scattering. The analysis focuses on the temporal evolution of the velocity phase space, RMS values of the potential fluctuations, and coherent structures in potential densities. For the unstable case, we observe growth and propagation of electrostatic waves. Simulations are performed with a three-dimensional electrostatic particle in cell code.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Intrinsic left-handed electromagnetic properties in anisotropic superconductors

Left-handed materials usually are realized in artificial subwavelength structures. In this, we show that some anisotropic superconductors such as Bi 2 Sr 2 CaCu 2 O 8+δ , YBa 2 Cu x O y , and La 2-x Sr x CuO 4 , are intrinsic left-handed materials. The condition is that the plasma frequency in the c axis, ω c , and in the ab plane, ω ab , and the operating angular frequency, ω , satisfy ω c < ω < ω ab . In addition, ω should be smaller than the superconducting energy gap to sustain superconductivity. We study the reflection and transmission of electromagnetic waves and reveal negative refraction and the backward wave with the phase velocity opposite to the direction of energy flux propagation. We also discuss possible approaches for improvement, making these properties feasible for experimental validation. Being intrinsic left-hand materials, the anisotropic superconductors are promising for applications in functional electromagnetic devices in the terahertz frequency band.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Wavefront modulation and controlling for Lamb waves using surface bonded slice lenses

We present that there is a growing interest in controlling the propagation of Lamb waves to achieve better performance for structural health monitoring and nondestructive evaluation. In this study, we present new methods of controlling Lamb waves using simple slice lenses through wavefront modulation. The lenses are made of metal slices with designed shapes. Once bonded on plates, the lenses modify the effective plate thickness, and therefore change the Lamb wave characteristics such as the wavenumber and phase velocity, providing convenient ways of modulating wavefront and controlling Lamb waves in plates. Numerically and experimentally, we demonstrated that a straight wavefront A 0 mode can be focused on the desired focal point using a plano-concave aspherical lens. We also demonstrated that the A 0 mode can be steered with different frequency components steered to different directions using a simple prism made of metal slices. The work shows that such simple slice lenses provide easy ways for controlling Lamb waves, and have a great potential for Lamb wave based applications.

42 ENGINEERING↗
DOE OSTI2017

Radial properties of the geodesic acoustic mode

In magnetized toroidal plasmas, neoclassical effects and turbulent drift waves can induce the geodesic acoustic mode (GAM). We simulate the GAM using the gyro-kinetic code GTS for typical tokamak parameters and investigate its properties, especially its frequency continuum, evolution of its radial wave number, and propagation characteristics. The simulation findings are compared with those of the relevant theory and experiment. It is discovered that the radial phase velocity of the GAM is roughly proportional to the ion thermal speed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Nonlinear co-existence of beta-induced Alfvén eigenmodes and beta-induced Alfvén-acoustic eigenmodes

The nonlinear co-existence of β–induced Alfvén eigenmode (BAE) and β–induced Alfvén-acoustic eigenmode (BAAE) is found in simulations using the gyrokinetic toroidal code, which provides a new mechanism responsible for BAAE excitation in tokamaks. In this work, the normalized pressure β is the ratio between plasma thermal pressure and magnetic pressure. The nonlinear simulation findings show that the BAAE branch emerges after the BAE branch is saturated. The mode structure's evolution shows that existence of BAAE will change the original BAE mode structure. The perturbed distribution functions in the velocity phase space show that a new resonance region manifesting the wave-particle resonance in the BAAE branch appears during the nonlinear co-existence stage.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

A neoclassical drift-magnetohydrodynamical fluid model of the interaction of a magnetic island chain with a resonant error-field in a high temperature tokamak plasma

A two-fluid, neoclassical theory of the interaction of a single magnetic island chain with a resonant error-field in a quasi-cylindrical, low-β, tokamak plasma is presented. The plasmas typically found in large hot tokamaks lie in the so-called weak neoclassical flow-damping regime in which the neoclassical ion stress tensor is not the dominant term in the ion parallel equation of motion. Nevertheless, flow-damping in such plasmas dominates ion perpendicular viscosity, and is largely responsible for determining the phase velocity of a freely rotating island chain (which is in the ion diamagnetic direction relative to the local E × B frame at the rational surface). The critical vacuum island width required to lock the island chain is mostly determined by the ion neoclassical poloidal flow damping rate at the rational surface. The stabilizing effect of the average field-line curvature, as well as the destabilizing effect of the perturbed bootstrap current, is the same for a freely rotating, a non-uniformly rotating, and a locked island chain. The destabilizing effect of the error-field averages to zero when the chain is rotating and only manifests itself when the chain locks. The perturbed ion polarization current has a small destabilizing effect on a freely rotating island chain, but a large destabilizing effect on both a non-uniformly rotating and a locked island chain. Finally, this behavior may account for the experimentally observed fact that locked island chains are much more unstable than corresponding freely rotating chains.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Harnessing the relativistic Buneman instability for laser-ion acceleration in the transparency regime

Here we examine the relativistic Buneman instability in systems relevant to high-intensity laser-plasma interactions under conditions of relativistically-induced transparency, as this instability can generate large-amplitude electrostatic waves at low frequencies that are pertinent to ion dynamics in these systems. Ion flows are shown to significantly alter the range of unstable wave numbers and to increase the phase velocities of the unstable modes; we particularly highlight the relativistic effects from both the ion and electron (with transverse motion) populations. These findings are related to the mode structure seen in particle-in-cell simulation results of a short-pulse laser breaking through an initially opaque target with the onset of relativistic transparency. Additionally, driving mechanisms from free energy present in density and velocity gradients are shown to be capable of significantly enhancing the growth rates, and these instabilities furthermore extend the breadth of the unstable wave number range. We discuss how the transverse self-generated magnetic fields characteristic of short-pulse interactions can potentially constrain the unstable wave numbers in a non-trivial manner.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

A detailed examination of laser-ion acceleration mechanisms in the relativistic transparency regime using tracers

Here we present a particle-in-cell study of linearly polarized laser-ion acceleration systems, in which we use both two-dimensional (2D) and three-dimensional (3D) simulations to characterize the ion acceleration mechanisms in targets which become transparent to the laser pulse during irradiation. First, we perform a target length scan to optimize the peak ion energies in both 2D and 3D, and the predictive capabilities of 2D simulations are discussed. Tracer analysis allows us to isolate the acceleration into stages of target normal sheath acceleration (TNSA), hole boring (HB), and break-out afterburner (BOA) acceleration, which vary in effectiveness based on the simulation parameters. The thinnest targets reveal that enhanced TNSA is responsible for accelerating the most energetic ions, whereas the thickest targets have ions undergoing successive phases of HB and TNSA (in 2D) or BOA and TNSA (in 3D); HB is not observed to be a dominant acceleration mechanism in the 3D simulations. It is in the intermediate optimal regime, both when the laser breaks through the target with appreciable amplitude and when there is enough plasma to form a sustained high density flow, that BOA is most effective and is responsible for the most energetic ions. Eliminating the transverse laser spot size effects by performing a plane wave simulation, we can isolate with greater confidence the underlying physics behind the ion dynamics we observe. Specifically, supplemented by wavelet and FFT analyses, we match the post-transparency BOA acceleration with a wave-particle resonance with a high-amplitude low-frequency electrostatic wave of increasing phase velocity, consistent with that predicted by the Buneman instability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Extracting the turbulent flow-field from beam emission spectroscopy images using velocimetry

The 2D turbulent E × B flow-field is inferred from density fluctuation images obtained with the beam emission spectroscopy diagnostic on DIII-D using the orthogonal dynamic programming velocimetry algorithm. A synthetic turbulence model is used to test the algorithm and optimize it for measuring zonal flows. Zonal flow measurements are found to require a signal-to-noise ratio above ~10 and a zonal flow wavelength longer than ~2 cm. Comparison between the velocimetry-estimated flow-field and the E × B flow-field using a nonlinear gyrokinetic GENE simulation finds that the flow-fields have identical spatial structure and differ only by the mean turbulence phase velocity, which is spatially uniform in this flux tube simulation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Angular streaking of betatron X-rays in a transverse density gradient laser-wakefield accelerator

In a plasma with a transverse density gradient, laser wavefront tilt develops gradually due to phase velocity differences in different plasma densities. The wavefront tilt leads to a parabolic trajectory of the plasma wakefield and hence the accelerated electron beam, which leads to an angular streaking of the emitted betatron radiation. In this way, the temporal evolution of the betatron X-ray spectra will be converted into angular “streak,” i.e., having a critical energy-angle correlation. An analytical model for the curved trajectory of a laser pulse in a transverse density gradient is presented. This gives the deflection angle of the electron beam and the betatron X-rays as a function of the plasma and laser parameters, and it was verified by particle-in-cell simulations. This angular streaking could be used as a single-shot diagnostic technique to reveal the temporal evolution of betatron X-ray spectra and hence the electron acceleration itself.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Measuring heat flux from collective Thomson scattering with non-Maxwellian distribution functions

Here, heat flux was measured in coronal plasmas using collective Thomson scattering from electron-plasma waves. A laser-produced plasma from a planar aluminum target created a temperature gradient along the target normal. Thomson scattering probed electron-plasma waves in the direction of the temperature gradient with phase velocities relevant to heat flux. The heat-flux measurements were reduced from classical values inferred from the measured plasma conditions in regions with large temperature gradients and agreed with classical values for weak gradients. In regions where classical theory was invalid, the heat flux was determined by reproducing the measured Thomson-scattering spectra using electron distribution functions consistent with nonlocal thermal transport. Lastly, full-scale hydrodynamic simulations using flux-limited thermal transport ( FLASH ) and the multigroup nonlocal Schurtz, Nicolai, and Busquet (SNB) models both underestimated the heat flux at all locations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Linear response of a Hall magnetic drift wave for verification of Hall MHD algorithms

Numerical implementations of Hall magnetohydrodynamics (Hall MHD) can be challenging due to the nonlinear multidimensional nature of the Hall term. Here, a model problem is presented that couples the hydrodynamic motion of the plasma to Hall MHD evolution of the magnetic field. The Hall MHD equations are linearized about unperturbed solutions in both cylindrical and Cartesian coordinates in two dimensions. The magnetic field is assumed to lie in the ignorable direction, and the linear response about the unperturbed solution is considered. The resulting ordinary differential equation is used to numerically compute the eigenfunctions and eigenfrequencies of the mode. The resulting eigenfunctions do not make the local wave approximation but are instead global solutions that depend on the spatial dependence of the unperturbed Alfvén speed. Hall MHD simulations are then performed in the Ares multiphysics code and shown to agree with the predicted phase velocities of the wave, and the simulated solutions are shown to numerically converge to the semianalytic modes. By varying the background density of the plasma (and correspondingly, the ion inertial length), the importance of Hall physics can be varied. This allows the test problem to transition from the classical MHD limit to the extreme Hall MHD limit. Furthermore, this problem is a useful tool for the verification of Hall MHD routines implemented in various codes, and the robustness of a routine can be tested in regimes in which Hall physics is dominant.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Instability of nonlinear Trivelpiece-Gould waves. II. Weakly trapped particles

This paper discusses a novel parametric instability mechanism caused by particles that are weakly trapped in the potential wells of a nonlinear “pump” wave. The pump wave is unstable to the growth of daughter waves with longer wavelength and nearly the same phase velocity as the pump. This induces adjacent potential peaks in the wave to slowly approach one-another, receding from other pairs of peaks. Particles that are weakly trapped between approaching peaks, with kinetic energies just below the potential maxima, are heated by compression and escape the well, and then become retrapped on the other side of the approaching peaks, where they amplify the compression by pushing the peaks together. The mechanism applies to low-collisionality plasmas supporting waves with near-acoustic dispersion relations such as ion sound waves, magnetized Langmuir waves, or Alfven waves. The theory is compared to particle in cell simulations of TrivelpieceGould (TG) traveling waves, as well as to experiments on pure ion plasmas that observe parametric instability in TG standing waves.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Laser-plasma acceleration beyond wave breaking

Laser wakefield accelerators rely on the extremely high electric fields of nonlinear plasma waves to trap and accelerate electrons to relativistic energies over short distances. When driven strongly enough, plasma waves break, trapping a large population of the background electrons that support their motion. Aside from limiting the maximum electric field, this trapping can lead to accelerated electron bunches with large energy spreads. Here we introduce a novel regime of plasma wave excitation and wakefield acceleration that allows for arbitrarily high electric fields while avoiding the deleterious effects of unwanted trapping. Here, the regime, enabled by spatiotemporal shaping of laser pulses, exploits the property that nonlinear plasma waves with superluminal phase velocities cannot trap charged particles and are therefore immune to wave breaking.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Filamentation and focusing of electron beams due to interactions with plasma waves

Results of numerical modeling of the interaction of an electron beam propagating across relativistic plasma waves indicate that electron beam filamentation and focusing may occur under certain conditions. The model is based on solving the relativistic equation of motion in three dimensions for the individual electrons in a tenuous Gaussian beam, as they pass through a relativistic plasma wave. Several electron beam and plasma wave parameters were varied, and the results are summarized. One of the results is that the spacing of the electron beam filaments correlates with the wavelength of the plasma wave. The electron beam filaments appear as vertical slabs after the beam exits the plasma waves. The electron beam also compresses to a focus after it exits the plasma, and the focal distance depends on several parameters including the electron beam energy and phase velocity of the relativistic plasma wave. It is suggested that these focusing and filamentation phenomena may be the basis for diagnostics schemes for laser plasma interactions. The parameters used in the model were electron beam energies in the 5–50 keV range and plasma wave properties typical for the beat-wave produced by CO2 lasers, which correspond to the facilities available in our laboratory. The limitations of these results to lower energy density beam and plasma regimes and to higher energy density regimes will be discussed.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

A multi-sheath model for highly nonlinear plasma wakefields

An improved description for nonlinear plasma wakefields with phase velocities near the speed of light is presented and compared against fully kinetic particle-in-cell simulations. These wakefields are excited by intense particle beams or lasers pushing plasma electrons radially outward, creating an ion bubble surrounded by a sheath of electrons characterized by the source term S≡−1enp(ρ−Jz/c), where ρ and Jz are the charge and axial current densities, respectively. Previously, the sheath source term was described phenomenologically with a positive-definite function, resulting in a positive definite wake potential. In reality, the wake potential is negative at the rear of the ion column which is important for self-injection and accurate beam loading models. To account for this, we introduce a multi-sheath model in which the source term, S, of the plasma wake can be negative in regions outside the ion bubble. Using this model, we obtain a new expression for the wake potential and a modified differential equation for the bubble radius. Numerical results obtained from these equations are validated against particle-in-cell simulations for unloaded and loaded wakes. The new model provides accurate predictions of the shape and duration of trailing bunch current profiles that flatten plasma wakefields. It is also used to design a trailing bunch for a desired longitudinally varying loaded wakefield. We present beam loading results for laser wakefields and discuss how the model can be improved for laser drivers in future work. Finally, we discuss differences between the predictions of the multi- and single-sheath models for beam loading.

Dalichaouch, T. N. (ORCID:0000000247350150)↗