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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 1,045 records · Page 58

All sources NASA NTRS DOE OSTI

DOE OSTI2016

A plasma photonic crystal bandgap device

A fully tunable plasma photonic crystal is used to control the propagation of free space electromagnetic waves in the S to X bands of the microwave spectrum. An array of discharge plasma tubes forms a simple square crystal structure with the individual plasma dielectric constant tuned through variation in the plasma density. We show, through simulations and experiments, that transverse electric mode bandgaps exist, arising from the positive and negative dielectric constant regimes of the plasma, and that the respective bandgap frequencies can be shifted through changing the dielectric constant by varying discharge current density.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2016

Waveguiding and bending modes in a plasma photonic crystal bandgap device

Waveguiding and bending modes are investigated in a fully tunable plasma photonic crystal. The plasma device actively controls the propagation of free space electromagnetic waves in the S to X band of the microwave spectrum. An array of discharge plasma tubes form a square crystal lattice exhibiting a well-defined bandgap, with individual active switching of the plasma elements to allow for waveguiding and bending modes to be generated dynamically. We show, through simulations and experiments, the existence of transverse electric (TE) mode waveguiding and bending modes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Radiation from laser-microplasma-waveguide interactions in the ultra-intense regime

When a high-contrast ultra-relativistic (>10{sup 20} W/cm{sup 2}) laser beam enters a micro-sized plasma waveguide, the pulse energy is coupled into waveguide modes, which significantly modifies the interaction between the electrons and electromagnetic wave. Electrons pulled out from the walls of the waveguide form a dense helical bunch inside the channel and are efficiently accelerated by the transverse magnetic modes to hundreds of MeV. The asymmetry in the transverse electric and magnetic fields drives strong oscillations, which lead to the emission of bright, well-collimated, hard X-rays. In this paper, we present our study on the underlying physics in the aforementioned process using 3D particle-in-cell simulations. The mechanism of electron acceleration and the dependence of radiation properties on different laser plasma parameters are addressed. An analytic model and basic scalings for X-ray emission are also presented by considering the lowest optical modes in the waveguide, which is adequate to describe the basic phenomenon. In addition, the effects of high-order modes as well as laser polarization are also qualitatively discussed. The considered X-ray source has promising features, potentially making it a competitive candidate for a future tabletop synchrotron source.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2016

Broadband microwave characteristics of a novel coaxial gridded hollow cathode argon plasma

The interaction between microwave and large area plasma is crucially important for space communication. Gas pressure, input power, and plasma volume are critical to both the microwave electromagnetic wave phase shift and electron density. This paper presents a novel type of large coaxial gridded hollow cathode plasma having a 50 cm diameter and a 40 cm thickness. Microwave characteristics are studied using a microwave measurement system that includes two broadband antennae in the range from 2 GHz to 18 GHz. The phase shift under varying gas pressure and input power is shown. In addition, the electron density n{sub e}, which varies from 1.2 × 10{sup 16} m{sup −3} to 8.7 × 10{sup 16} m{sup −3} under different discharge conditions, is diagnosed by the microwave system. The measured results accord well with those acquired by Langmuir Probe measurement and show that the microwave properties in the large volume hollow cathode discharge significantly depend on the input power and gas pressure.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2017

O–X mode conversion in a non-symmetric torus at electron cyclotron frequencies

Previous work on this topic, [Weitzner, Phys. Plasmas 11, 866 (2004)], applicable in a system with toroidal symmetry, is here extended to the case of a non-symmetric background equilibrium state. Maxwell's equations with the cold plasma dielectric tensor are used to represent the plasma-electromagnetic wave interaction. Away from the mode conversion region, geometrical optics adequately characterizes the wave propagation. A new, simpler derivation of the wave equations in the mode conversion region is given. Aside from one very special case in which a general plasma equilibrium behaves like a stratified medium, the previous results apply and highly effective mode conversion is found. The matching of the mode conversion solution to the geometrical optics solution, not previously examined, is discussed. A relatively weak condition on the perpendicular wave number near the resonance layer is found. Provided that the perpendicular wave number is small, it tends to zero at the mode conversion layer and the solutions match effectively.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2017

Microscale magnetic compasses

Microscale magnetic compasses have been synthesized with high yield. These ferromagnetic iron carbide nano-particles, which are encapsulated in a pair of parallel carbon needles, change their orientation in response to an external magnetic field. Electron holography reveals magnetic fields confined to the vicinity of the bicone-shaped particles, which are composed of only a few ferromagnetic domains. Aligned magnetically and encapsulated in an acrylate polymer matrix, these micro-compasses exhibit anisotropic bulk magnetic permeability with an easy axis normal to the needle direction that can be understood as a result of the anisotropic demagnetizing field of a non-spherical single-domain particle. Finally, this novel type of material with orthogonal magnetic and structural axes could be highly useful as magnetic components in electromagnetic wave absorbent materials and magnetorheological fluids.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2017

Nonlinear fluid simulation study of stimulated Raman and Brillouin scatterings in shock ignition

Here, we developed a new nonlinear fluid laser-plasma-instability code ( FLAME ) using a multi-fluid plasma model combined with full electromagnetic wave equations. The completed one-dimensional version of FLAME was used to study laser-plasma instabilities in shock ignition. The simulations results showed that absolute stimulated Raman scattering (SRS) modes growing near the quarter-critical surface were saturated by Langmuir-wave Decay Instabilities (LDI) and pump depletion. The ion-acoustic waves from LDI acted as seeds of Stimulated Brillouin Scattering, which displayed a bursting pattern and caused strong pump depletion. Re-scattering of SRS at the 1/16th-critical surface was also observed in a high temperature case. These results largely agreed with the corresponding Particle-in-Cell simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2018

A conservative scheme for electromagnetic simulation of magnetized plasmas with kinetic electrons

A conservative scheme has been formulated and confirmed for gyrokinetic particle simulations of electromagnetic waves and instabilities in magnetized plasmas. An electron continuity equation derived from the drift kinetic equation is used to time advance the electron density perturbation by using the perturbed mechanical flow calculated from the parallel vector potential, and the parallel vector potential is solved by using the perturbed canonical flow from the perturbed distribution function. In gyrokinetic particle simulations using this new scheme, the shear Alfvén wave dispersion relation in the shearless slab and continuum damping in the sheared cylinder have been recovered. The new scheme overcomes the stringent requirement in the conventional perturbative simulation method that perpendicular grid size needs to be as small as electron collisionless skin depth even for the long wavelength Alfvén waves. The new scheme also circumvents the problem in the typical approach that an unphysically large parallel electric field arises due to the inconsistency between electrostatic potential calculated from the perturbed density and vector potential calculated from the perturbed canonical flow. Lastly, the gyrokinetic particle simulations of the Alfvén waves in sheared cylinder have superior numerical properties compared with the fluid simulations, which suffer from numerical difficulties associated with singular mode structures.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2019

Real and complex valued geometrical optics inverse ray-tracing for inline field calculations

In this work, a 3-D ray based model for computing laser fields in dissipative and amplifying media is presented. The eikonal equation is solved using inverse ray-tracing on a dedicated nonstructured 3-D mesh. Inverse ray-tracing opens the possibility of using Complex Geometrical Optics (CGO), for which we propose a propagation formalism in a finite element mesh. Divergent fields at caustics are corrected using an etalon integral method for fold-type caustics. This method is successfully applied in dissipative media by modifying the ray-ordering and root selection rules, thereby allowing one to reconstruct the field in the entire caustic region. In addition, we demonstrate how caustics in the CGO framework can disappear entirely for sufficiently dissipative media, making the complex ray approach valid in the entire medium. CGO is shown to offer a more precise modeling of laser refraction and absorption in a dissipative medium when compared to Geometrical Optics (GO). In the framework of Inertial Confinement Fusion (ICF), this occurs mostly at intermediate temperatures or at high temperatures close to the critical density. Additionally, GO is invalid at low temperatures if an approximated expression of the permittivity is used. The inverse ray-tracing algorithm for GO and CGO is implemented in the IFRIIT code, in the framework of a dielectric permittivity described in 3-D using a piecewise linear approximation in tetrahedrons. Fields computed using GO and CGO are compared to results from the electromagnetic wave solver LPSE. Excellent agreement is obtained in 1-D linear and nonlinear permittivity profiles. Good agreement is also obtained for ICF-like Gaussian density profiles in 2-D. Finally, we demonstrate how the model reproduces Gaussian beam diffraction using CGO. The IFRIIT code will be interfaced inline to 3-D radiative hydrodynamic codes to describe the nonlinear laser plasma interaction in ICF and high-energy-density plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2019

THz-frequency magnon-phonon-polaritons in the collective strong-coupling regime

Strong coupling between light and matter occurs when the two interact such that new hybrid modes, the so-called polaritons, are formed. Here, we report on the strong coupling of both the electric and the magnetic degrees of freedom to an ultrafast terahertz (THz) frequency electromagnetic wave. In our system, optical phonons in a slab of ferroelectric lithium niobate are strongly coupled to a THz electric field to form phonon-polaritons, which are simultaneously strongly coupled to magnons in an adjacent slab of canted antiferromagnetic erbium orthoferrite via the magnetic-field component of the same THz pulse. We juxtapose experimental results of bare slabs consisting of the two materials with a photonic crystal cavity, consisting of a two-dimensional array of air holes cut into the hybrid slab. In both cases, the strong coupling leads to the formation of new magnon-phonon-polariton modes, which we experimentally observe in the time domain as a normal-mode beating and which corresponds in the frequency domain to an avoided crossing. Finally, our simple yet versatile waveguide platform provides a promising avenue through which to explore ultrafast THz spintronics, quantum electrodynamics, sensing, and spectroscopic applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2020

Filament-assisted mode conversion in magnetized plasmas

At density filaments in magnetized plasmas, electromagnetic waves are guided along the filament ("wave-filament bound states"). Several wave-filament bound states exist beyond those predicted. Here, the new bound states occur under experimentally relevant conditions, and are especially common under typical NSTX conditions, where they are a plausible mechanism for the experimentally observed power losses in the edge plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2021

Observation of a highly conductive warm dense state of water with ultrafast pump–probe free-electron-laser measurements

The electrical conductivity of water under extreme temperatures and densities plays a central role in modeling planetary magnetic fields. Experimental data are vital to test theories of high-energy-density water and assess the possible development and presence of extraterrestrial life. These states are also important in biology and chemistry studies when specimens in water are confined and excited using ultrafast optical or free-electron lasers (FELs). Here we utilize femtosecond optical lasers to measure the transient reflection and transmission of ultrathin water sheet samples uniformly heated by a 13.6 nm FEL approaching a highly conducting state at electron temperatures exceeding 20 000 K. The experiment probes the trajectory of water through the high-energy-density phase space and provides insights into changes in the index of refraction, charge carrier densities, and AC electrical conductivity at optical frequencies. At excitation energy densities exceeding 10 MJ/kg, the index of refraction falls to n = 0.7, and the thermally excited free-carrier density reaches ne = 5 × 1027 m−3, which is over an order of magnitude higher than that of the electron carriers produced by direct photoionization. Significant specular reflection is observed owing to critical electron density shielding of electromagnetic waves. The measured optical conductivity reaches 2 × 104 S/m, a value that is one to two orders of magnitude lower than those of simple metals in a liquid state. At electron temperatures below 15 000 K, the experimental results agree well with the theoretical calculations using density-functional theory/molecular-dynamics simulations. With increasing temperature, the electron density increases and the system approaches a Fermi distribution. In this regime, the conductivities agree better with predictions from the Ziman theory of liquid metals.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2023

Resolving the mystery of electron perpendicular temperature spike in the plasma sheath

A large family of plasmas has collisional mean-free-path much longer than the non-neutral sheath width, which scales with the plasma Debye length. The plasmas, particularly the electrons, assume strong temperature anisotropy in the sheath. The temperature in the sheath flow direction (Te∥) is lower and drops toward the wall as a result of the decompressional cooling by the accelerating sheath flow. The electron temperature in the transverse direction of the flow field (Te⊥) not only is higher but also spikes up in the sheath. This abnormal behavior of Te⊥ spike is found to be the result of a negative gradient of the parallel heat flux of transverse degrees of freedom (qes) in the sheath. The non-zero heat flux qes is induced by pitch-angle scattering of electrons via either their interaction with self-excited electromagnetic waves in a nearly collisionless plasma or Coulomb collision in a collisional plasma, or both in the intermediate regime of plasma collisionality.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2023

Variable magnetic field electron spectrometer to measure hot electrons in the range of 50–460 keV

Resonance absorption (RA) occurs when a p-polarized electromagnetic wave, obliquely incident on an inhomogeneous plasma, tunnels past its turning point and resonantly excites an electron plasma wave (EPW) at the critical density. This phenomenon is important, for instance, in the direct drive approach to inertial fusion energy and is a particular example of a wider phenomenon in plasma physics known as mode conversion, which is crucial for heating magnetic fusion devices, such as tokamaks, via RF heating. Direct measurement of these RA-generated EPW accelerated hot electrons, with energy in the range of a few tens to a few hundreds of keV, is a challenging task due to the relatively low deflecting magnetic fields needed. The solution described here is a magnetic electron spectrometer (MES) with a continually changing magnetic field, lower at the entrance of the MES and gradually increasing toward the end, that enables the measurement of a wide spectral range of electrons with energies between 50 and 460 keV. Electron spectra taken in a LaserNetUS RA experiment were acquired from plasmas generated by irradiating polymer targets with the combination of an ~300 ps pulse followed by a series of ten high intensity 50–200 fs duration laser pulses from the ALEPH laser at Colorado State University. In conclusion, the high intensity beam is designed as spike trains of uneven duration and delay pulses in order to modify the RA phenomenon.

47 OTHER INSTRUMENTATION↗

DOE OSTI2023

Ultrafast laser-induced spin–lattice dynamics in the van der Waals antiferromagnet CoPS 3

CoPS 3 stands out in the family of the van der Waals antiferromagnets XPS 3 (X = Mn, Ni, Fe, and Co) due to the unquenched orbital momentum of the magnetic Co 2+ ions, which is known to facilitate the coupling of spins to both electromagnetic waves and lattice vibrations. Here, using a time-resolved magneto-optical pump–probe technique, we experimentally study the ultrafast laser-induced dynamics of mutually correlated spins and lattice. It is shown that a femtosecond laser pulse acts as an ultrafast heater and, thus, results in the melting of the antiferromagnetic order. At the same time, the resonant pumping of the 4 T 1g → 4 T 2g electronic transition in Co 2+ ions effectively changes their orbital momentum, giving rise to a mechanical force that moves the ions in the direction parallel to the orientation of their spins, thus generating a coherent B g phonon mode at the frequency of about 4.7 THz.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Nanofilm thickness measurement by resonant frequencies

We report a theoretical investigation of monochromatic laser light – thin metal film interaction. The dependences of transmission, reflection and absorption coefficients of an electromagnetic wave on the incidence angle, layer thickness and effective electron collision frequency are obtained. The above coefficients are analysed in the region of resonant frequencies. The resulting formula for the transmission, reflection and absorption coefficients are found to be valid for any angles of incidence. The case of mirror boundary conditions is considered. A formula is derived for contactless measurement of the film thickness by the observed resonant frequencies. (laser applications and other topics in quantum electronics)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Extremely tight focusing of light at the nanoapex of a metal microtip

We study the focusing of an optical electromagnetic wave into a nanoscale spatial region in the vicinity of the nanoapex of a metal microtip, arising due to a convergent surface plasmon wave. The metal boundary near the nanoapex is approximated by a paraboloid of revolution. It is proved that an increase in the metal absorption in approaching the nanoapex, associated with an increase in the frequency of collisions of electrons with the surface, is an essential but not limiting factor for this method of light nanofocusing. It is shown that the minimum possible size of the focusing region can be about of 1 nm. (nanooptics)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2019

Spectral dynamics of shift current in ferroelectric semiconductor SbSI

Photoexcitation in solids brings about transitions of electrons/holes between different electronic bands. If the solid lacks an inversion symmetry, these electronic transitions support spontaneous photocurrent due to the geometric phase of the constituting electronic bands: the Berry connection. This photocurrent, termed shift current, is expected to emerge on the timescale of primary photoexcitation process. We observe ultrafast evolution of the shift current in a prototypical ferroelectric semiconductor antimony sulfur iodide (SbSI) by detecting emitted terahertz electromagnetic waves. By sweeping the excitation photon energy across the bandgap, ultrafast electron dynamics as a source of terahertz emission abruptly changes its nature, reflecting a contribution of Berry connection on interband optical transition. The shift excitation carries a net charge flow and is followed by a swing over of the electron cloud on a subpicosecond timescale. Understanding these substantive characters of the shift current with the help of first-principles calculation will pave the way for its application to ultrafast sensors and solar cells.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗