Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “stellar”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Design of an arrangement of cubic magnets for a quasi-axisymmetric stellarator experiment

The usage of permanent magnets to shape the confining magnetic field of a stellarator has the potential to reduce or eliminate the need for non-planar coils. As a proof-of-concept for this idea, here we have developed a procedure for designing an array of cubic permanent magnets that works in tandem with a set of toroidal-field (TF) coils to confine a stellarator plasma. All of the magnets in the design are constrained to have identical geometry and one of three polarization types in order to simplify fabrication while still producing sufficient field accuracy. We present some of the key steps leading to the design, including the geometric arrangement of the magnets around the device, the procedure for optimizing the polarizations according to the three allowable magnet types, and the choice of magnet types to be used. We apply these methods to design an array of rare-Earth permanent magnets that can be paired with a set of planar TF coils to confine a quasi-axisymmetric plasma with a toroidal magnetic field strength of about 0.5 T on axis.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

MONKES: a fast neoclassical code for the evaluation of monoenergetic transport coefficients in stellarator plasmas

Abstract MONKES is a new neoclassical code for the evaluation of monoenergetic transport coefficients in stellarators. By means of a convergence study and benchmarks with other codes, it is shown that MONKES is accurate and efficient. The combination of spectral discretization in spatial and velocity coordinates with block sparsity allows MONKES to compute monoenergetic coefficients at low collisionality, in a single core, in approximately one minute. MONKES is sufficiently fast to be integrated into stellarator optimization codes for direct optimization of the bootstrap current and to be included in predictive transport suites. The code and data from this paper are available at https://github.com/JavierEscoto/MONKES/ .

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Stellarators with enhanced tritium confinement and edge radiation control

A stellarator design is described with the purpose of achieving three goals: (1) enhance the confinement time of tritium. (2) Have a sufficient density of high-Z impurities to radiate the thermal power escaping from the core while having an extremely low impurity density in the core. (3) Maintain a large fraction of the plasma in a burning plasma state with an optimal tritium fraction. Some features of this design could be used in tokamaks. Although having three confinement zones is natural for stellarators, it is not for tokamaks.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Density profiles in stellarators: an overview of particle transport, fuelling and profile shaping studies at TJ-II

We provide an overview of activities carried out at the TJ-II stellarator for improving our understanding of- and developing plasma physics models for particle density profiles in stellarators. Namely, we report on recent progress in turbulent particle transport simulation, validation of pellet deposition models, density profile shaping for performance control and new experimental techniques for edge turbulence and plasma-neutral interaction.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Direct optimization of neoclassical ion transport in stellarator reactors

Abstract We directly optimize stellarator neoclassical ion transport while holding neoclassical electron transport at a moderate level, creating a scenario favorable for impurity expulsion and retaining good ion confinement. Traditional neoclassical stellarator optimization has focused on minimizing ϵ eff , the geometric factor that characterizes the amount of radial transport due to particles in the 1 / ν regime. Under expected reactor-relevant conditions, core electrons will be in the 1 / ν regime and core fuel ions will be in the ν regime. Traditional optimizations thus minimize electron transport and rely on the radial electric field ( E r ) that develops to confine the ions. This often results in an inward-pointing E r that drives high- Z impurities into the core, which may be troublesome in future reactors. In this work, we increase the ratio of the thermal transport coefficients L 11 e / L 11 i , which previous research has shown can create an outward-pointing E r . This effect is very beneficial for impurity expulsion. We obtain self-consistent density, temperature, and E r profiles at reactor-relevant conditions for an optimized equilibrium. This equilibrium is expected to enjoy significantly improved impurity transport properties.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

KEPLER'S OPTICAL SECONDARY ECLIPSE OF HAT-P-7b AND PROBABLE DETECTION OF PLANET-INDUCED STELLAR GRAVITY DARKENING

We present observations spanning 355 orbital phases of HAT-P-7 observed by Kepler from 2009 May to 2011 March (Q1-9). We find a shallower secondary eclipse depth than initially announced, consistent with a low optical albedo and detection of nearly exclusively thermal emission, without a reflected light component. We find an approximately 10 ppm perturbation to the average transit light curve near phase -0.02 that we attribute to a temperature decrease on the surface of the star, phased to the orbit of the planet. This cooler spot is consistent with planet-induced gravity darkening, slightly lagging the sub-planet position due to the finite response time of the stellar atmosphere. The brightness temperature of HAT-P-7b in the Kepler bandpass is T{sub B} = 2733 {+-} 21 K and the amplitude of the deviation in stellar surface temperature due to gravity darkening is approximately -0.18 K. The detection of the spot is not statistically unequivocal due its small amplitude, though additional Kepler observations should be able to verify the astrophysical nature of the anomaly.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

CONFIRMATION OF SMALL DYNAMICAL AND STELLAR MASSES FOR EXTREME EMISSION LINE GALAXIES AT z ∼ 2

Spectroscopic observations from the Large Binocular Telescope and the Very Large Telescope reveal kinematically narrow lines (∼50 km s{sup –1}) for a sample of 14 extreme emission line galaxies at redshifts 1.4 < z < 2.3. These measurements imply that the total dynamical masses of these systems are low (≲ 3 × 10{sup 9} M {sub ☉}). Their large [O III] λ5007 equivalent widths (500-1100 Å) and faint blue continuum emission imply young ages of 10-100 Myr and stellar masses of 10{sup 8}-10{sup 9} M {sub ☉}, confirming the presence of a violent starburst. The dynamical masses represent the first such determinations for low-mass galaxies at z > 1. The stellar mass formed in this vigorous starburst phase represents a large fraction of the total (dynamical) mass, without a significantly massive underlying population of older stars. The occurrence of such intense events in shallow potentials strongly suggests that supernova-driven winds must be of critical importance in the subsequent evolution of these systems.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

EVIDENCE OF MULTIPLE SLOW ACOUSTIC OSCILLATIONS IN THE STELLAR FLARING LOOPS OF PROXIMA CENTAURI

We present the first observational evidence of multiple slow acoustic oscillations in the post-flaring loops of the corona of Proxima Centauri using XMM-Newton observations. We find the signature of periodic oscillations localized in the decay phase of the flare in its soft (0.3-10.0 keV) X-ray emissions. Using the standard wavelet tool, we find multiple periodicities of 1261 s and 687 s. These bursty oscillations persist for durations of 90 minutes and 50 minutes, respectively, for more than three cycles. The intensity oscillations with a period of 1261 s may be the signature of the fundamental mode of slow magnetoacoustic waves with a phase speed of 119 km s{sup –1} in a loop of length 7.5 × 10{sup 9} cm, which is initially heated, producing the flare peak temperature of 33 MK and later cooled down in the decay phase and maintained at an average temperature of 7.2 MK. The other period of 687 s may be associated with the first overtone of slow magnetoacoustic oscillations in the flaring loop. The fundamental mode oscillations show dissipation with a damping time of 47 minutes. The period ratio P {sub 1}/P {sub 2} is found to be 1.83, indicating that such oscillations are most likely excited in longitudinal density stratified stellar loops. We estimate the density scale height of the stellar loop system as ∼23 Mm, which is smaller than the hydrostatic scale height of the hot loop system, and implies the existence of non-equilibrium conditions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

COMPARISON OF PHOTOMETRIC VARIABILITY BEFORE AND AFTER STELLAR FLARES

The energy in the solar acoustic spectrum is known to be correlated with flares, but it is not known if the same is true for stellar flares. In order to answer this question, we have analyzed 73 flares in 39 solar-like stars. These flares were identified in the 854 solar-like stars observed by the Kepler spacecraft that have stellar parameters measured with asteroseismology. Though we were not able to identify a statistically significant enhancement of the energy in the high-frequency part of the post-flare acoustic spectra compared to the pre-flare spectra of these stars, we did identify a larger variability between the energy in the high-frequency part of the post- and pre-flare acoustic spectra compared to spectra taken at random times.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

A STELLAR WIND ORIGIN FOR THE G2 CLOUD: THREE-DIMENSIONAL NUMERICAL SIMULATIONS

We present three-dimensional, adaptive mesh refinement simulations of G2, a cloud of gas moving in a highly eccentric orbit toward the galactic center. We assume that G2 originates from a stellar wind interacting with the environment of the Sgr A* black hole. The stellar wind forms a cometary bubble which becomes increasingly elongated as the star approaches periastron. A few months after periastron passage, streams of material begin to accrete on the central black hole with accretion rates M-dot ∼10{sup −8} M {sub ☉} yr{sup –1}. Predicted Brγ emission maps and position-velocity diagrams show an elongated emission resembling recent observations of G2. A large increase in luminosity is predicted by the emission coming from the shocked wind region during periastron passage. The observations, showing a constant Brγ luminosity, remain puzzling, and are explained here assuming that the emission is dominated by the free-wind region. The observed Brγ luminosity (∼8 × 10{sup 30} erg s{sup –1}) is reproduced by a model with a v{sub w} = 50 km s{sup –1} wind velocity and a 10{sup –7} M {sub ☉} yr{sup –1} mass-loss rate if the emission comes from the shocked wind. A faster and less dense wind reproduces the Brγ luminosity if the emission comes from the inner, free-wind region. The extended cometary wind bubble, largely destroyed by the tidal interaction with the black hole, reforms a few years after periastron passage. As a result, the Brγ emission is more compact after periastron passage.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

A study of outer disc stellar populations of face-on star-forming galaxies in SDSS-IV MaNGA: causes of H α deficiency

ABSTRACT Integral field unit spectra of face-on star-forming galaxies from the MaNGA survey are stacked in radial bins so as to reach an S/N high enough to measure emission lines and Lick indices out to 2.5–3Re. Two-thirds of galaxies have stellar populations in the outer discs that are older, more metal poor, and less dusty than in the inner discs. Recent bursts of star formation have occurred more frequently in the outer disc, but extinction-corrected H α equivalent widths are significantly lower at fixed Dn(4000) in these regions. I examine the properties of a subset of galaxies with the most H α-deficient outer discs. These regions contain young stellar populations that must have formed within the last 0.5 Gyr, but have extinction-corrected H α values well below the values predicted for a standard Kroupa initial mass function. The H α-deficient galaxies have flat Dn(4000) and H δA profiles with little radial fluctuation, indicating that star formation has occurred extremely uniformly across the entire disc. The H α line profiles indicate that the ionized gas kinematics is also very regular across the disc. The main clue to the origin of the H α deficiency is that it sets in at the same radius where the dust extinction abruptly decreases, suggesting a mode of star formation deficient in massive stars in quiescent, H i-dominated gas. Finally, I have carried out a search for galaxies with signatures of unusual H α kinematics and find that 15 per cent of the sample exhibits evidence for significant ionized gas that is displaced from the systemic velocity of the disc.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Exploring the stellar populations of backsplash galaxies

Backsplash galaxies are those that traverse and overshoot cluster cores as they fall into these structures. They are affected by environment, and should stand out in contrast to the infalling population. We target galaxies in the vicinity of clusters (R≳R 200 ) and select a sample in projected phase space (PPS), from the compilation of Sampaio et al. based on SDSS data. We present a statistical analysis, comparing two regions in PPS, with the same projected distance to the cluster but different velocity. The analysis relies on the presence of variations in the stellar population content of backsplash galaxies. We find a lower limit in the fractional contribution of ∼5 per cent with respect to the general sample of infalling galaxies at similar group-centric distance when using single line strength analysis, or ∼15-30 per cent when adopting bivariate distributions. The stellar populations show a subtle but significant difference towards older ages, and a higher fraction of quiescent galaxies. We also compare this set with a general field sample, where a substantially larger difference in galaxy properties is found, with the field sample being consistently younger, metal poorer and with a lower fraction of quiescent galaxies. Noting that our ‘cluster’ sample is located outside of the virial radius, we expect this difference to be caused by pre-processing of the infalling galaxies in the overall higher density regions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Direct microstability optimization of stellarator devices

Turbulent transport is regarded as one of the key issues in magnetic confinement nuclear fusion, both for tokamaks and stellarators. Here, in this work, we show that a significant decrease in a microstability-based proxy, as opposed to a geometric one, for the turbulent heat flux, namely the quasilinear heat flux, can be obtained in an efficient manner by coupling stellarator optimization with linear gyrokinetic simulations. This is accomplished by computing the quasilinear heat flux at each step of the optimization process, as well as the deviation from quasisymmetry, and minimizing their sum, leading to a balance between neoclassical and the turbulent transport proxy.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Optimization of passive superconductors for shaping stellarator magnetic fields

Here, we consider the problem of optimizing a set of passive superconducting coils (PSCs) with currents induced by a background magnetic field rather than power supplies. In the nuclear fusion literature, such coils have been proposed to partially produce the 3D magnetic fields for stellarators and provide passive stabilization. We perform the first optimizations of PSC arrays with respect to the orientation, shape, and location of each coil, jointly minimized with the background fields. We conclude by generating passive coil array solutions for four stellarators.

coil optimization↗
DOE OSTI2024

On a High-Mirror Stellarator Reactor Exploratory Concept With Neutrons Concentrated on Centrifuge Liquids

In the framework of fusion energy research based on magnetic confinement, stellarators allow numerous degrees of freedom for the design of the magnetic trap and plasma shape. Taking advantage of these features, some plasma shapes might benefit several of the many integrated elements involved in commercial fusion reactors, like, e.g., decreasing the number, mass and complexity of replaced activated in-vessel components (IVC) (i.e., by using liquids), extraction of large power, tritium generation, and remote maintenance. Certainly, free-surface liquid materials were proposed for tokamaks and field-reversed configuration (FRC) to try to improve some of such elements, i.e., in advanced power extraction (APEX) studies. Some reactor-relevant quasi-isodynamic (QI) magnetic configurations exhibit a relatively straight sector of plasma and high magnetic mirror. The combination of those elements and possibilities in a single stellarator reactor concept might have some advantages, in spite of the uncertainties due to the current low technological readiness level (TRL). The proposed and studied reactor concept is based on a vacuum vessel having short curved sectors and longer wide cylindrical sectors, which encloses a high-mirror low-vertical-excursion magnetic configuration, and swirling liquids or rotating cylinders, which centrifuge molten Li salts located at the low field region. Thus, the molten salts (if possible covered by a thin layer of liquid lithium) would be located on the internal perimeter of the cylinder, to act as particle exhaust (except for helium), neutron power extractor, and tritium breeder. The high-mirror feature tries to concentrate the neutron power at the cylindrical sectors, which might avoid using breeding materials at the curved sectors. The different elements of the concept are exploratorily studied and defined, and the difficulties assessed.

centrifuge↗
DOE OSTI2013

Self-consistent modeling of radio-frequency plasma generation in stellarators

A self-consistent model of radio-frequency (RF) plasma generation in stellarators in the ion cyclotron frequency range is described. The model includes equations for the particle and energy balance and boundary conditions for Maxwell’s equations. The equation of charged particle balance takes into account the influx of particles due to ionization and their loss via diffusion and convection. The equation of electron energy balance takes into account the RF heating power source, as well as energy losses due to the excitation and electron-impact ionization of gas atoms, energy exchange via Coulomb collisions, and plasma heat conduction. The deposited RF power is calculated by solving the boundary problem for Maxwell’s equations. When describing the dissipation of the energy of the RF field, collisional absorption and Landau damping are taken into account. At each time step, Maxwell’s equations are solved for the current profiles of the plasma density and plasma temperature. The calculations are performed for a cylindrical plasma. The plasma is assumed to be axisymmetric and homogeneous along the plasma column. The system of balance equations is solved using the Crank-Nicholson scheme. Maxwell’s equations are solved in a one-dimensional approximation by using the Fourier transformation along the azimuthal and longitudinal coordinates. Results of simulations of RF plasma generation in the Uragan-2M stellarator by using a frame antenna operating at frequencies lower than the ion cyclotron frequency are presented. The calculations show that the slow wave generated by the antenna is efficiently absorbed at the periphery of the plasma column, due to which only a small fraction of the input power reaches the confinement region. As a result, the temperature on the axis of the plasma column remains low, whereas at the periphery it is substantially higher. This leads to strong absorption of the RF field at the periphery via the Landau mechanism.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Two-slope soft X-ray spectra observed in experiments on electron cyclotron resonance plasma heating in the L-2M stellarator

In experiments on the generation and electron cyclotron resonance heating (ECRH) of plasma in the L-2M stellarator, non-Maxwellian two-slope soft X-ray (SXR) spectra were observed. The temperatures of the thermal and epithermal components of the spectra were measured as functions of the heating power and plasma density. A hypothesis based on the experimental results is suggested to explain the formation mechanism of two-slope SXR spectra in the ECRH experiments at the L-2M stellarator. The measured SXR spectra are compared with the results of numerical simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Physical Parameters of a Reactor-Stellarator with Small Ripples of the Helical Magnetic Field

The paper describes the calculation data on the physical parameters of a reactor-stellarator, where the nonuniformities of the helical field are smaller than the toroidal magnetic field nonuniformities: ε{sub h} < ε{sub t}. Unlike the previous studies, where the ion-component transport coefficients had the collision frequency dependence proportional to ν{sup 1/2}, this being equivalent to the ε{sub h} > ε{sub t} case, in the present calculations, these coefficients were assumed to be in proportion to the first power of the collision frequency, D{sub i} ∝ ν for ν{sub eff} < 2ω{sub E}, and to D{sub i} ∝ ν{sup −1} for the inverse inequality. Here, ω{sub E} is the rotation frequency of plasma in the radial electric field. As before, the plasma electrons corresponded to the mode of D{sub e} ∝ ν{sup −1}. As initial parameters for numerical calculations, a reactor with R = 8 m, r{sub p} = 2 m, and B{sub 0} = 5 Т was taken. A numerical code was used to solve the set of equations that describes the plasma space−time behavior in the reactor-stellarator under the conditions of equal diffusion fluxes. The start of reactor operation in the mode of thermonuclear burning was provided by heating sources with a power of several tens of megawatts. Steady-state operating conditions of a self-sustained thermonuclear reaction were attained by maintaining the plasma density through DT fuel pellet injection into the plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗