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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 1,513 records · Page 84
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Initial Feasibility and Clinical Implementation of Daily MR-Guided Adaptive Head and Neck Cancer Radiation Therapy on a 1.5T MR-Linac System: Prospective R-IDEAL 2a/2b Systematic Clinical Evaluation of Technical Innovation

This prospective study is, to our knowledge, the first report of daily adaptive radiation therapy (ART) for head and neck cancer (HNC) using a 1.5T magnetic resonance imaging-linear accelerator (MR-linac) with particular focus on safety and feasibility and dosimetric results of an online rigid registration-based adapt to position (ATP) workflow.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2015

Long-lived nanosecond spin relaxation and spin coherence of electrons in monolayer MoS 2 and WS 2

The recently discovered monolayer transition metal dichalcogenides (TMDCs) provide a fertile playground to explore new coupled spin–valley physics. Although robust spin and valley degrees of freedom are inferred from polarized photoluminescence (PL) experiments PL timescales are necessarily constrained by short-lived (3–100 ps) electron–hole recombination9, 10. Direct probes of spin/valley polarization dynamics of resident carriers in electron (or hole)-doped TMDCs, which may persist long after recombination ceases, are at an early stage. Here we directly measure the coupled spin–valley dynamics in electron-doped MoS 2 and WS 2 monolayers using optical Kerr spectroscopy, and reveal very long electron spin lifetimes, exceeding 3 ns at 5 K (2-3 orders of magnitude longer than typical exciton recombination times). In contrast with conventional III–V or II–VI semiconductors, spin relaxation accelerates rapidly in small transverse magnetic fields. Supported by a model of coupled spin–valley dynamics, these results indicate a novel mechanism of itinerant electron spin dephasing in the rapidly fluctuating internal spin–orbit field in TMDCs, driven by fast inter-valley scattering. Additionally, a long-lived spin coherence is observed at lower energies, commensurate with localized states. These studies provide insight into the physics underpinning spin and valley dynamics of resident electrons in atomically thin TMDCs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Ultranarrow electroluminescence from magnetic excitons in the van der Waals antiferromagnetic semiconductor NiPS 3

Electrically driven light emission from two-dimensional (2D) semiconducting materials has enabled numerous optoelectronic technologies, including light-emitting diodes, solid-state lasers, and single-photon sources for quantum communication. Here we report ultranarrow electroluminescence from the magnetic excitonic state of the van der Waals antiferromagnetic semiconductor NiPS 3 . This electroluminescence is enabled by the fabrication of gate-tunable NiPS 3 devices that remain electrically conductive below the antiferromagnetic ordering temperature of 155 K, ultimately allowing field-effect mobilities of 1.3 cm 2 V –1 s –1 and 4.5 cm 2 V –1 s –1 to be directly measured at room temperature and 7 K, respectively. By applying a high-frequency square wave voltage to the gate electrode of the resulting field-effect transistors, electroluminescence is capacitively induced from the magnetic excitons of NiPS 3 . Due to the coupling of these excitons with the underlying NiPS 3 antiferromagnetic order, the electroluminescence has an ultranarrow linewidth of 1 meV and a high degree of linear polarization (ρ = 0.78). In addition to facilitating fundamental studies of the coupling between spin states and excitons in van der Waals magnetic semiconductors, this work will accelerate the development of emerging 2D opto-spintronic applications.

two-dimensional materials↗
DOE OSTI2015

Investigation of compression of puffing neon by deuterium current and plasma sheath in plasma focus discharge

This paper presents the results of the research of the influence of compressed neon, injected by the gas-puff nozzle in front of the anode axis by the deuterium current and plasma sheath on the evolution of the pinch, and neutron production at the current of 2 MA. The intense soft X-ray emission shows the presence of neon in the central region of the pinch. During the implosion and stopping of the plasma sheath, the deuterium plasma penetrates into the internal neon layer. The total neutron yield of 10{sup 10}–10{sup 11} has a similar level as in the pure deuterium shots. The neutron and hard X-ray pulses from fusion D-D reaction are as well emitted both in the phase of the stopping implosion and during the evolution of instabilities at the transformation of plasmoidal structures and constrictions composed in this configuration from both gases. The fast deuterons can be accelerated at the decay of magnetic field of the current filaments in these structures.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Fluid theory and simulations of instabilities, turbulent transport and coherent structures in partially-magnetized plasmas of E x B discharges

Partially-magnetized plasmas with magnetized electrons and non-magnetized ions are common in Hall thrusters for electric propulsion and magnetron material processing devices. These plasmas are usually in strongly non-equilibrium state due to presence of crossed electric and magnetic fields, inhomogeneities of plasma density, temperature, magnetic field and beams of accelerated ions. Free energy from these sources make such plasmas prone to various instabilities resulting in turbulence, anomalous transport, and appearance of coherent structures as found in experiments. This work provides an overview of instabilities that exist in such plasmas. A nonlinear fluid model has been developed for description of the Simon-Hoh, lower-hybrid and ion-sound instabilities. The model also incorporates electron gyroviscosity describing the effects of finite electron temperature. The nonlinear fluid model has been implemented in the BOUT++ framework. The results of nonlinear simulations are presented demonstrating turbulence, anomalous current and tendency toward the formation of coherent structures.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

GRB 110721A: PHOTOSPHERE 'DEATH LINE' AND THE PHYSICAL ORIGIN OF THE GRB BAND FUNCTION

The prompt emission spectra of gamma-ray bursts (GRBs) usually have a dominant component that is well described by a phenomenological Band function. The physical origin of this spectral component is debated. Although the traditional interpretation is synchrotron radiation of non-thermal electrons accelerated in internal shocks or magnetic dissipation regions, the growing trend within the community is to interpret this component as modified thermal emission from a dissipative photosphere of a GRB fireball. We analyze the time-dependent spectrum of GRB 110721A detected by the Fermi Gamma-Ray Burst Monitor and Large Area Telescope, and pay special attention to the rapid evolution of the peak energy E{sub p} . We define a 'death line' of thermally dominated dissipative photospheric emission in the E{sub p} -L plane, and show that E{sub p} of GRB 110721A at the earliest epoch has a very high E{sub p} {approx} 15 MeV that is beyond the 'death line'. Together with the finding that an additional 'shoulder' component exists in this burst that is consistent with a photospheric origin, we suggest that at least for some bursts, the Band component is not from a dissipative photosphere, but must invoke a non-thermal origin (e.g., synchrotron or inverse Compton) in the optically thin region of a GRB outflow. We also suggest that the rapid hard-to-soft spectral evolution is consistent with the quick discharge of magnetic energy in a magnetically dominated outflow in the optically thin region.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Conference Grant Proposal for ICOPS 2016

ICOPS (International Conference on Plasma Science) is an internationally renowned, well-attended annual conference that involves topics of direct interest to the Office of Fusion Energy Sciences of Department of Energy. In particular, ICOPS 2016 emphasized both the traditional areas of plasma science and the new areas of growth that include but are not limited to Fusion (Inertial, Magnetic and Alternate Concepts), Particle Acceleration with Laser and Beams, High Energy Density Matter, Laser Produced Plasma, Fast Z-pinches, Computational Plasma Physics, Plasma Diagnostics, and such frontiers as studying Warm Dense Matter using the X-ray free electron lasers. The travel support of the students at ICOPS comes usually from sponsor organizations. Increasing the participation of outstanding students at ICOPS 2016 who are the first authors of the abstracts and are selected to receive the travel support based on the scientific merit of the submitted abstracts is crucial for the creation of the new generation of the plasma physicists.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Development of Superconducting RF Cavity in Traveling-Wave Regime at Fermilab

Niobium Superconducting RF (SRF) cavities have a theoretical peak magnetic field which limits the accelerating field to 50-60 MV/m. Presently, all SRF cavities operate in a Standing Wave (SW) resonance field in which particles experience an accelerating force alternating from zero to peak. In contrast, a resonance field in Traveling Wave (TW) mode propagates along with a structure, so particles in such field can experience a constant acceleration force and could have higher energy gain than that of SW mode. This phenomenon is defined by the cavity’s transit time factor, T. A TW structure proposed in an early study achieves T ~0.9, suggesting an increase in acceleration per structure by more than 20% compared to a SW structure (T ~0.7). The early stages of developments had been funded by several SBIR grants to Euclid Techlabs and completed in collaboration with Fermilab through a 1-cell prototype and a proof-of-principle 3-cell TW cavity. It demonstrated the TW resonance excitation at room temperature in the “as-fabricated” 3-cell structure. Here we report recent progresses and the first cryogenic testing of the 3-cell TW cavity in 2 K liquid helium at Fermilab.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Relativistic Shear Boundary Layer and the Gamma-Ray Emission of GW170817

We present an analysis of the radiation characteristics of kinetic shear boundary layers created by relativistic plasma jets. Using a model of electromagnetic field data based on particle-in-cell simulations of an electron–ion plasma, we solve the motion of individual test electrons and compute their instantaneous radiated power and peak frequency. By analyzing a large number of test electrons in this manner, we find two distinct electron populations present around the shear boundary layer. The most highly radiative electrons execute looping motion due to crossed electric and magnetic fields as they are accelerated along the bulk flow of the jet, and eventually cross the shear boundary interface at steep angles. Electrons that never cross the shear boundary interface radiate much less energy as a group. Summing over all of the highly radiative electrons, we compute the distribution of the total radiated energy as a function of the angle relative to the bulk flow. Furthermore, this result has important potential implications for the observed radiation output of short gamma-ray bursts viewed at large angles from the jet axis, such as the neutron star merger event GW170817/GRB 170817A.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

The solar wind near the sun - The solar envelope.

Recent observational data and models for the structure of the solar envelope are discussed, with emphasis on those physical processes in the envelope which are decisive in determining solar wind states at 1 AU. Diffusion of relativistic solar flare protons, collisionless damping of a variable flux of hydromagnetic waves, heat transfer from electrons to protons, acceleration by Alfven waves, and magnetic field effects are examined from the viewpoint of possible contributions to solar wind behavior.

Burlaga, L. F.↗
NASA NTRS1973

SSS-A spacecraft and experiment description.

The scientific objectives of the Explorer-45 mission are discussed. The primary objective is the study of the ring current responsible for the main phase of magnetic storms. Closely associated with this objective is the determination of the relationship between magnetic storms, substorms, and the acceleration of charged particles in the magnetosphere. Further objectives are the measurement of a wide range of proton, electron and alpha-particle energies, and studies of wave-particle interactions responsible for particle transport and loss in the inner magnetosphere. The orbital parameters, the spacecraft itself, and some of its unique features, such as the data handling system, which is programmable from the ground, are described.

Longanecker, G. W.↗
NASA NTRS1977

Rocket measurements of electrons in a system of multiple auroral arcs

A Nike-Tomahawk rocket was launched into a system of auroral arcs northward of Poker Flat Research Range, Fairbanks, Alaska. The pitch-angle distribution of electrons was measured at 2.5, 5, and 10 keV and also at 10 keV on a separating forward section of the payload. The auroral activity appeared to be the extension of substorm activity centered to the east. The rocket crossed a westward-propagating fold in the brightest band. The electron spectrum was relatively hard through most of the flight, showing a peak in the range from 2.5 to 10 keV in the weaker aurora and below 5 keV in the brightest arc. The detailed structure of the pitch-angle distribution suggested that, at times, a very selective process was accelerating some electrons in the magnetic field direction, so that a narrow field-aligned component appeared superimposed on a more isotropic distribution. It is concluded that this process could not be a near-ionosphere field-aligned potential drop, although the more isotropic component may have been produced by a parallel electric field extending several thousand kilometers along the field line above the ionosphere.

Boyd, J. S.↗
NASA NTRS1977

Grain disruption in interstellar hydromagnetic shocks

The observed abundance variations of Ca, Fe, Si, and Ti in intermediate-velocity interstellar gas suggest that grains have been disrupted in clouds with velocities as low as 20 to 50 km/s. A simplified hydromagnetic shock model for such clouds is described; the dynamical equations for charged grains in the postshock region are derived, including the collisional drag and 'betatron-acceleration' effect of a magnetic-field gradient; and the fraction of grains destroyed in evaporative collisions with other grains is calculated. It is found that for shocks of 20 to 50 km/s in which the fractional H-ionization remains low, 3-10% of the grain material may be destroyed - sufficient to explain in part the heavy-element depletion pattern in intermediate velocity clouds and the well-known correlation of N(Ca II)/N(Na I) with cloud velocity.

Shull, J. M.↗
NASA NTRS1978

Mechanisms and sites for astrophysical gamma ray line production

The production of gamma ray lines and estimates of line fluxes resulting from nuclear deexcitations, positron annihilation, and electron capture at various astrophysical sites are discussed. Supernova and nova explosions synthesize long-lived radioactive isotopes and eject them into space where they produce observable gamma ray lines by decaying into excited levels of daughter nuclei or by emitting positrons. Energetic charged particles in the interstellar medium, in supernova remants, in solar or stellar flares, and possibly in the vicinity of compact objects, produce gamma-ray lines by inelastic collisions which either excite nuclear levels or produce positrons and neutrons. Energetic particles can result from acceleration in time-varying magnetic fields (solar flares) or from gravitational accretion onto neutron stars and black holes. Electromagnetic processes in the strong magnetic fields of pulsars can produce positron-electron pairs, with line emission resulting from positron annihilation. Deexcitations of quantized states in strong magnetic fields can also produce lines.

Ramaty, R.↗
NASA NTRS1981

Preacceleration in collapsing magnetic neutral sheets and anomalous abundances of solar flare particles

Levine's (1974) concept of a collapsing magnetic neutral sheet which can accelerate ambient protons to several times the mean thermal speed, provided that the collapse time scale is shorter than the proton Coulomb loss time, is applied to heavier elements in an investigation of the composition of the particles which can be accelerated by such a sheet. Tables of ionization equilibrium are combined with the thermal structure of a constant pressure loop in order to calculate phi sub x (the fraction of an element x which is accelerated) of 18 heavy elements, from carbon to nickel which satisfy a maximum-ionization criterion. Normalizing phi sub x to oxygen, it is found that relative to the composition of the ambient materials, C and N can be depleted by factors of up to 2-10, while other heavy elements, along with hydrogen, are enhanced. The numerical results obtained are qualitatively similar to anomalous abundances reported among solar flare particles.

Mullan, D. J.↗
NASA NTRS1981

Recent developments in mass drivers

In the past eight months a new mass driver concept has been explored through calculation and inductance model verification. It retains the linear synchronous principle and freedom from arcs, plasmas or physical contact between the accelerated bucket and accelerator. However, it discards passive magnetic flight and obtains transverse focussing from strong off-axis restoring forces produced by drive coils operating in a pull-only mode. This paper gives the reasoning on which the new concept is based, and applies the concept to a lunar catapult design.

Oneill, G. K.↗
NASA NTRS1985

Three-dimensional calculation of shuttle charging in polar orbit

The charged particles environment in polar orbit can be of sufficient intensity to cause spacecraft charging. In order to gain a quantitative understanding of such effects, the Air Force is developing POLAR, a computer code which simulates in three dimensions the electrical interaction of large space vehicles with the polar ionospheric plasma. It models the physical processes of wake generation, ambient ion collection, precipitating auroral electron fluxes, and surface interactions, including secondary electron generation and backscattering, which lead to vehicle charging. These processes may be followed dynamically on a subsecond timescale so that the rapid passage through intense auroral arcs can be simulated. POLAR models the ambient plasma as isotropic Maxwellian electrons and ions (0+, H+), and allows for simultaneous precipitation of power-law, energetic Maxwellian, and accelerated Gaussian distributions of electrons. Magnetic field effects will be modeled in POLAR but are currently ignored.

Cooke, D. L.↗
NASA NTRS1986

Microwave emission and scattering from Earth surface and atmosphere

Nonlinear Electromagnetic (EM) wave interactions with the upper atmosphere were investigated during the period 15 December 1985 to 15 June 1986. Topics discussed include: the simultaneous excitation of ionospheric density irregularities and Earth's magnetic field fluctuations; the electron acceleration by Langmuir wave turbulence; and the occurrence of artificial spread F. The role of thermal effects in generating ionospheric irregularities by Whistler waves, intense Quasi-DC electric fields, atmospheric gravity waves, and electrojets was investigated. A model was developed to explain the discrete spectrum of the resonant ultralow frequency (ULF) waves that are commonly observed in the magnetosphere.

Kong, J. A.↗