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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 1,603 records · Page 89
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Upgrade of the neutral particle analyzers for the TJ-II stellarator

The TJ-II stellarator, a magnetically confined plasma device, is equipped with a broad range of diagnostics for plasma characterization. These include 4 neutral particle analyzers (NPAs), consisting of two Acord-12's, to perform poloidal measurements, plus a compact NPA, and an Acord-24, these in tangential viewing positions. The Acord-12's were originally equipped with two rows of 6 channels each, one for hydrogen neutrals and the other for deuterium neutrals but were changed to a single row of 12 detectors for hydrogen, the principal working gas in TJ-II. With this upgrade the resultant improved energy resolution spectrum has allowed more reliable ion temperature estimates to be obtained. Here we present the upgrades undertaken and present results to demonstrate the improved performance of this diagnostic.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Theory of ITG turbulent saturation in stellarators: identifying mechanisms to reduce turbulent transport

A three-field fluid model that allows for general three-dimensional equilibrium geometry is developed to describe ion temperature gradient turbulent saturation processes in stellarators. The theory relies on the paradigm of nonlinear transfer of energy from unstable to damped modes at comparable wavelength as the dominant saturation mechanism. The unstable-to-damped mode interaction is enabled by a third mode that for dominant energy transfer channels primarily serves as a regulator of the nonlinear energy transfer rate. The identity of the third wave in the interaction defines different scenarios for turbulent saturation with the dominant scenario depending upon the properties of the 3D geometry. The nonlinear energy transfer physics is quantified by the product of a turbulent correlation lifetime and a geometric coupling coefficient. The turbulent correlation time is determined by a three-wave frequency mismatch, which at long wavelength can be calculated from the sum of the linear eigenfrequencies of the three modes. Larger turbulent correlation times denote larger levels of nonlinear energy transfer and hence smaller turbulent transport. The theory provides an analytic prediction for how 3D shaping can be tuned to lower turbulent transport through saturation processes.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

DESC: A stellarator equilibrium solver

In this paper, the new code DESC is presented to solve for fixed-boundary ideal magnetohydrodynamic equilibria in stellarators. The approach directly solves the equilibrium force balance as a system of nonlinear equations in the form f(x) = 0. The independent variables x represent nested magnetic flux surfaces expressed in the inverse representation with toroidal flux coordinates, and the equations f(x) quantify equilibrium force balance errors at discrete points in real space. Discretizing with global Fourier–Zernike basis functions properly treats the magnetic axis and minimizes the number of coefficients needed to describe the flux surfaces. The pseudospectral method provides great flexibility in where the errors are evaluated, and the system of equations is efficiently solved with a Newton–Raphson iteration. Equilibria are computed and compared against VMEC for both axisymmetric and non-axisymmetric examples. The results show fast convergence rates and solutions with low errors throughout the plasma volume.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Solving the problem of overdetermination of quasisymmetric equilibrium solutions by near-axis expansions. II. Circular axis stellarator solutions

Here we apply the near-axis expansion method for quasisymmetric magnetic fields with anisotropic pressure to construct numerical solutions to circular axis stellarators. The solutions are found to second order in the distance from the axis, not possible in the standard Garren–Boozer construction, which assumes magnetostatic equilibria with isotropic pressure. In the limit of zero anisotropy, it is shown that a subset of coefficients can be chosen to avoid the overdetermination problem.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Global gyrokinetic simulation of neoclassical ambipolar electric field and its effects on microturbulence in W7-X stellarator

Global neoclassical simulations of a model equilibrium of the W7-X stellarator find an ambipolar electric field with either an ion root or an electron root solution depending on the temperature ratio between electrons and ions. The ambipolar electric field is then used as an equilibrium field in the turbulence simulations of ion temperature gradient (ITG) instability. The shear of the ambipolar electric field has modest effects on the ITG linear instability, nonlinear saturation amplitude, and turbulent transport in the ion root case. However, in the electron root case, the ambipolar electric field significantly reduces the linear ITG growth rate, turbulence intensity, and radial correlation length, resulting in an ion heat conductivity comparable to the neoclassical transport level in the strong shear region.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Poor confinement in stellarators at high energy

High-energy particle resonances can modify particle distributions and even cause significant particle loss. Resonances can be present in any toroidal confinement device and can easily be found numerically. Many stellarators have weak magnetic shear so that large islands and large chaotic regions can be produced by resonant perturbations with small amplitudes. Additionally, while the choice of the field line helicity profile in the plasma can limit the presence of resonances at low particle energy, the resonance location is energy-dependent, and they can move into the plasma at higher energy. If resonances match the toroidal variation of the equilibrium, they can produce wide islands in the phase space of orbits even in the absence of perturbations due to instabilities. These islands increase in size with particle energy and can seriously affect the confinement of high-energy ions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Assessment of radial transport induced by Alfvénic resonances in tokamaks and stellarators

In a symmetric plasma confinement device, a perturbation such as an Alfvén mode produces a resonance island which flattens the local energy gradient due to induced rotation about the elliptic points of the resonance. The location of the resonance is mode frequency and particle energy dependent, and particles away from the resonance are not affected. Here, a projection onto the Poincaré subspace allows the evaluation of resonance widths and internal rotation rates. In a stellarator without symmetry because of the toroidal dependence of the equilibrium magnetic field, a mode instead produces chaos in all orbits passing where the amplitude is large and is, thus, much more effective in producing radial transport and loss. The mode growth rate and saturation are also affected by the local chaos.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Conceptual design of a heavy ion beam probe diagnostic for the Wendelstein 7-X Stellarator

This article describes the current state of the design of the heavy ion beam probe (HIBP) for Wendelstein 7-X (W7-X). It will be the first HIBP diagnostic on an optimized stellarator and is designed to study electric fields and ion scale turbulence in all W7-X reference magnetic configurations. The use of an existing 2 MV accelerator, located outside of the torus hall, results in the need for a circuitous primary beamline. This increases the complexity of the ion optics design to deliver a focused beam to the plasma. To access most of the magnetic configuration space of W7-X, the secondary beamline and an energy analyzer are designed to pivot, thereby redirecting a wider range of secondary beam trajectories. Signal level estimates indicate that the equilibrium potential can be measured at all radii and that the radial coverage for potential and density fluctuations measurements depends on the plasma density.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Multi-delay coherence imaging spectroscopy optimized for ion temperature measurements in the divertor plasma of the Wendelstein 7-X stellarator

A new coherence imaging spectroscopy (CIS) diagnostic optimized to measure the C 2+ impurity ion temperature T i spatial distribution in the divertor plasma of the W7-X stellarator is designed, tested, and validated. Using CIS to obtain T i in the edge of magnetically confined plasmas has historically been challenging because Doppler broadening and Zeeman splitting have comparable effects on the shape of spectral emission lines. To distinguish between these two mechanisms, a novel approach to birefringent crystal design is employed to minimize the diagnostic’s sensitivity to Zeeman splitting. The recently developed pixelated multi-delay CIS approach is also used to obtain four times as much spectral information as traditional CIS approaches. The T i -optimized CIS diagnostic is validated in a long-pulse W7-X plasma by comparison with a high-resolution spectrometer whose sightlines overlap with the CIS field of view. The CIS and spectrometer T i profiles have the same shape and agree to within 10% on average and 25% in the worst case. Images of the T i distribution near the divertor show toroidally elongated bands aligned with the magnetic field, with T i ranging between 10 and 40 eV.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Poor confinement in stellarators at high energy II

This work discusses particle resonances in stellarators. There are two important effects. First, resonances provide locations for Alfvén modes, which can cause significant particle loss. Second, if a resonance matches the period of the modulation of the equilibrium field magnitude it produces islands in particle orbits increasing in size with particle energy, also possibly causing loss. A previous publication suffered from a restriction to the consideration of deeply passing particle orbits, clearly showing the resonance islands. Large particle magnetic moment increases the effect of particle cross field drift, essential for this phenomenon, so passing particles with larger values of magnetic moment give larger islands. It is possible to avoid designs exhibiting such resonances.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

SPECTRAL TYPING OF LATE-TYPE STELLAR COMPANIONS TO YOUNG STARS FROM LOW-DISPERSION NEAR-INFRARED INTEGRAL FIELD UNIT DATA

We used the Project 1640 near-infrared coronagraph and integral field spectrograph to observe 19 young solar-type stars. Five of these stars are known binary stars and we detected the late-type secondaries and were able to measure their JH spectra with a resolution of R {approx} 30. The reduced, extracted, and calibrated spectra were compared to template spectra from the IRTF spectral library. With this comparison, we test the accuracy and consistency of spectral-type determination with the low-resolution near-infrared spectra from P1640. Additionally, we determine effective temperature and surface gravity of the companions by fitting synthetic spectra calculated with the PHOENIX model atmosphere code. We also present several new epochs of astrometry of each of the systems. Together, these data increase our knowledge and understanding of the stellar make up of these systems. In addition to the astronomical results, the analysis presented helps validate the Project 1640 data reduction and spectral extraction processes and the utility of low-resolution, near-infrared spectra for characterizing late-type companions in multiple systems.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

EXPLORING THE EFFECTS OF STELLAR ROTATION AND WIND CLEARING: DEBRIS DISKS AROUND F STARS

We have conducted a study of debris disks around F stars in order to explore correlations between rotation, stellar winds, and circumstellar disks. We obtained new 24 {mu}m photometry from the Multiband Imaging Photometer for Spitzer (MIPS) camera for a sample of 188 relatively nearby F dwarfs with various rotation rates and optical colors, and combined it with archival MIPS data for 66 more F stars, as well as Wide-field Infrared Survey Explorer data for the entire sample, plus 9 more F stars. Based on the objects' K{sub s} - [24] and [3.4] - [22] colors, we identify 22 stars in our sample as having 22 and/or 24 {mu}m excesses above our detection limit, 13 of which are new discoveries. Our overall disk detection rate is 22/263, or 8%, consistent with previous determinations of disk fractions in the solar neighborhood. While fast-rotating stars are expected to have strong winds capable of efficiently removing dust, we find no correlation between rotational velocity and infrared excess. Similarly, we find no significant difference in excess detection rate between late-type F stars, which have convective surfaces, and early-type F stars, which have fully radiative envelopes. However, the essentially unknown range of ages in this sample may be washing out any effects relating rotation, winds, and disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The optical and infrared photometric evolution of the recent stellar merger, V1309 Sco

Nova Sco 2008 was recently shown to have resulted from the merger of the two stars in the contact binary V1309 Sco. This is the first stellar merger ever observed between two convective stars. We present archival data, new infrared photometry, and Hubble Space Telescope WFC3 imaging of V1309 Sco. Spitzer observations show that it had a large infrared excess in the 3.6 μm to 8 μm range more than a year before the merger. Standard color diagnostics of the pre-merger infrared colors place V1309 Sco in the same region where evolved stars with chemically complex mass loss are located. Since the nova outburst subsided in optical bandpasses in 2008, the merger remnant's brightness in optical bandpasses, near-IR bandpasses, and the Spitzer 3.6 μm and 4.5 μm channels has varied by several magnitudes and in complex ways. A temporary, strong increase in the reddening during 2010 suggests the occurrence of a dust formation event. We point out several peculiarities in the relative fluxes and time behavior of the optical and near-IR magnitudes, which could be explained if some of the photometric bandpasses in the 1-5 μm range are strongly affected by emission lines.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

New H-BAND stellar spectral libraries for the SDSS-III/APOGEE Survey

The Sloan Digital Sky Survey-III (SDSS-III) Apache Point Observatory Galactic Evolution Experiment (APOGEE) has obtained high-resolution (R ∼ 22,500), high signal-to-noise ratio (>100) spectra in the H-band (∼1.5–1.7 μm) for about 146,000 stars in the Milky Way galaxy. We have computed spectral libraries with effective temperature (T{sub eff}) ranging from 3500 to 8000 K for the automated chemical analysis of the survey data. The libraries, used to derive stellar parameters and abundances from the APOGEE spectra in the SDSS-III data release 12 (DR12), are based on ATLAS9 model atmospheres and the ASSϵT spectral synthesis code. We present a second set of libraries based on MARCS model atmospheres and the spectral synthesis code Turbospectrum. The ATLAS9/ASSϵT (T{sub eff} = 3500–8000 K) and MARCS/Turbospectrum (T{sub eff} = 3500–5500 K) grids cover a wide range of metallicity (−2.5 ⩽ [M/H] ⩽ +0.5 dex), surface gravity (0 ≤ log g ⩽ 5 dex), microturbulence (0.5 ⩽ ξ ⩽ 8 km s{sup −1}), carbon (−1 ⩽ [C/M] ⩽ +1 dex), nitrogen (−1 ⩽ [N/M] ⩽ +1 dex), and α-element (−1 ⩽ [α/M] ⩽ +1 dex) variations, having thus seven dimensions. We compare the ATLAS9/ASSϵT and MARCS/Turbospectrum libraries and apply both of them to the analysis of the observed H-band spectra of the Sun and the K2 giant Arcturus, as well as to a selected sample of well-known giant stars observed at very high resolution. The new APOGEE libraries are publicly available and can be employed for chemical studies in the H-band using other high-resolution spectrographs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

EVIDENCE FOR CLOUD-CLOUD COLLISION AND PARSEC-SCALE STELLAR FEEDBACK WITHIN THE L1641-N REGION

We present high spatial resolution {sup 12}CO (J = 1-0) images taken by the Nobeyama 45 m telescope toward a 48' Multiplication-Sign 48' area, including the L1641-N cluster. The effective spatial resolution of the maps is 21'', corresponding to 0.04 pc at a distance of 400 pc. A recent 1.1 mm dust continuum map reveals that the dense gas is concentrated in several thin filaments. We find that a few dust filaments are located at the parts where {sup 12}CO (J = 1-0) emission drops sharply. Furthermore, the filaments have two components with different velocities. The velocity difference between the two components is about 3 km s{sup -1}, corresponding to a Mach number of 10, significantly larger than the local turbulent velocity in the cloud. These facts imply that the collision of the two components (hereafter, the cloud-cloud collision) possibly contributed to the formation of these filaments. Since the two components appear to overlap toward the filaments on the plane of the sky, the collision may have occurred almost along the line of sight. Star formation in the L1641-N cluster was probably triggered by such a collision. We also find several parsec-scale CO shells whose centers are close to either the L1641-N cluster or the V 380 Ori cluster. We propose that these shells were created by multiple winds and/or outflows from cluster young stellar objects, i.e., 'protocluster winds'. One exceptional dust filament located at the western cloud edge lies along a shell; it is presumably part of the expanding shell. Both the cloud-cloud collision and protocluster winds are likely to influence the cloud structure and kinematics in this region.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SEARCH FOR IONIZED JETS TOWARD HIGH-MASS YOUNG STELLAR OBJECTS

We are carrying out multi-frequency radio continuum observations, using the Australia Telescope Compact Array, to systematically search for collimated ionized jets toward high-mass young stellar objects (HMYSOs). Here we report observations at 1.4, 2.4, 4.8, and 8.6 GHz, made with angular resolutions of about 7'', 4'', 2'', and 1'', respectively, toward six objects of a sample of 33 southern HMYSOs thought to be in very early stages of evolution. The objects in the sample were selected from radio and infrared catalogs by having positive radio spectral indices and being luminous (L{sub bol} > 2 Multiplication-Sign 10{sup 4} L{sub Sun }), but underluminous in radio emission compared with that expected from its bolometric luminosity. This criterion makes the radio sources good candidates for being ionized jets. As part of this systematic search, two ionized jets have been discovered: one previously published and the other reported here. The rest of the observed candidates correspond to three hypercompact H II regions and two ultracompact H II regions. The two jets discovered are associated with two of the most luminous (7 Multiplication-Sign 10{sup 4} and 1.0 Multiplication-Sign 10{sup 5} L{sub Sun }) HMYSOs known to harbor this type of object, showing that the phenomena of collimated ionized winds appear in the formation process of stars at least up to masses of {approx}20 M{sub Sun} and provide strong evidence for a disk-mediated accretion scenario for the formation of high-mass stars. From the incidence of jets in our sample, we estimate that the jet phase in high-mass protostars lasts for {approx}4 Multiplication-Sign 10{sup 4} yr.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

HOW SIGNIFICANT IS RADIATION PRESSURE IN THE DYNAMICS OF THE GAS AROUND YOUNG STELLAR CLUSTERS?

The impact of radiation pressure on the dynamics of the gas in the vicinity of young stellar clusters is thoroughly discussed. The radiation over the thermal/ram pressure ratio time evolution is calculated explicitly and the crucial roles of the cluster mechanical power, the strong time evolution of the ionizing photon flux, and the bolometric luminosity of the exciting cluster are stressed. It is shown that radiation has only a narrow window of opportunity to dominate the wind-driven shell dynamics. This may occur only at early stages of the bubble evolution and if the shell expands into a dusty and/or a very dense proto-cluster medium. The impact of radiation pressure on the wind-driven shell always becomes negligible after about 3 Myr. Finally, the wind-driven model results allow one to compare the model predictions with the distribution of thermal pressure derived from X-ray observations. The shape of the thermal pressure profile then allows us to distinguish between the energy and the momentum-dominated regimes of expansion and thus conclude whether radiative losses of energy or the leakage of hot gas from the bubble interior have been significant during bubble evolution.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ROTATION AND STABILITY OF THE TOROIDAL MAGNETIC FIELD IN STELLAR RADIATION ZONES

The stability of the magnetic field in radiation zones is of crucial importance for mixing and angular momentum transport in the stellar interior. We consider the stability properties of stars containing a predominant toroidal field in spherical geometry by means of a linear stability in the Boussinesq approximation taking into account the effect of thermal conductivity. We calculate the growth rate of instability and analyze in detail the effects of stable stratification and heat transport. We argue that the stabilizing influence of gravity can never entirely suppress the instability caused by electric currents in radiation zones. However, the stable stratification can essentially decrease the growth rate of instability.

79 ASTRONOMY AND ASTROPHYSICS↗