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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Resonance laser-plasma excitation of coherent terahertz phonons in the bulk of fluorine-bearing crystals under high-intensity femtosecond laser irradiation

The dynamics of coherent phonons in fluorine-containing crystals was investigated by pump-probe technique in the plasma production regime. Several phonon modes, whose frequencies are overtones of the 0.38-THz fundamental frequency, were simultaneously observed in a lithium fluoride crystal. Phonons with frequencies of 1 and 0.1 THz were discovered in a calcium fluoride crystal and coherent phonons with frequencies of 1 THz and 67 GHz were observed in a barium fluoride crystal. Furthermore, in the latter case the amplitudes of phonon mode oscillations were found to significantly increase 15 ps after laser irradiation. (interaction of laser radiation with matter)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

OBSERVATIONAL STUDY OF THE QUASI-PERIODIC FAST-PROPAGATING MAGNETOSONIC WAVES AND THE ASSOCIATED FLARE ON 2011 MAY 30

On 2011 May 30, quasi-periodic fast-propagating (QFP) magnetosonic waves accompanied by a C2.8 flare were directly imaged by the Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory. The QFP waves successively emanated from the flare kernel, they propagated along a cluster of open coronal loops with a phase speed of {approx}834 km s{sup -1} during the flare's rising phase, and the multiple arc-shaped wave trains can be fitted with a series of concentric circles. We generate the k - {omega} diagram of the Fourier power and find a straight ridge that represents the dispersion relation of the waves. Along the ridge, we find a lot of prominent nodes which represent the available frequencies of the QFP waves. On the other hand, the frequencies of the flare are also obtained by analyzing the flare light curves using the wavelet technique. The results indicate that almost all the main frequencies of the flare are consistent with those of the QFP waves. This suggests that the flare and the QFP waves were possibly excited by a common physical origin. On the other hand, a few low frequencies (e.g., 2.5 mHz (400 s) and 0.7 mHz (1428 s)) revealed by the k - {omega} diagram cannot be found in the accompanying flare. We propose that these low frequencies were possibly due to the leakage of the pressure-driven p-mode oscillations from the photosphere into the low corona, which should be a noticeable mechanism for driving the QFP waves observed in the corona.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

IMAGING OBSERVATIONS OF QUASI-PERIODIC PULSATIONS IN SOLAR FLARE LOOPS WITH SDO/AIA

Quasi-periodic pulsations (QPPs) of flaring emission with periods from a few seconds to tens of minutes have been widely detected from radio bands to {gamma}-ray emissions. However, in the past the spatial information of pulsations could not be utilized well due to the instrument limits. We report here imaging observations of the QPPs in three loop sections during a C1.7 flare with periods of P = 24 s-3 minutes by means of the extreme-ultraviolet 171 A channel of the Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory. We confirm that the QPPs with the shortest period of 24 s were not of an artifact produced by the Nyquist frequency of the AIA 12 s cadence. The QPPs in the three loop sections were interconnected and closely associated with the flare. The detected perturbations propagated along the loops at speeds of 65-200 km s{sup -1}, close to those of acoustic waves in them. The loops were made up of many bright blobs arranged in alternating bright and dark changes in intensity (spatial periodical distribution) with the wavelengths 2.4-5 Mm (as if they were magnetohydrodynamic waves). Furthermore, in the time-distance diagrams, the detected perturbation wavelengths of the QPPs are estimated to be {approx}10 Mm, which evidently do not fit the above ones of the spatial periodic distributions and produce a difference of a factor of 2-4 with them. It is suggested that the short QPPs with periods P < 60 s were possibly sausage-mode oscillations and the long QPPs with periods P > 60 s were the higher (e.g., >2nd) harmonics of slow magnetoacoustic waves.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

POLOIDAL-FIELD INSTABILITY IN MAGNETIZED RELATIVISTIC STARS

We investigate the instability of purely poloidal magnetic fields in nonrotating neutron stars (NSs) by means of three-dimensional general-relativistic magnetohydrodynamics simulations, extending the work presented by Ciolfi et al. in 2011. Our aim is to draw a clear picture of the dynamics associated with the instability and to study the final configuration reached by the system, thus obtaining indications on possible equilibria in a magnetized NS. Furthermore, since the internal rearrangement of magnetic fields is a highly dynamical process and has been suggested to be behind magnetar giant flares, our simulations can provide a realistic estimate of the electromagnetic and gravitational-wave (GW) emission that should accompany the flare event. Our main findings are the following: (1) the initial development of the instability meets all the expectations of perturbative studies in terms of the location of the seed of the instability, the timescale for its growth, and the generation of a toroidal component; (2) in the subsequent nonlinear reorganization of the system, {approx}90% of magnetic energy is lost in few Alfven timescales mainly through electromagnetic emission, and further decreases on a much longer timescale; (3) all stellar models tend to achieve a significant amount of magnetic helicity and the equipartition of energy between poloidal and toroidal magnetic fields and evolve to a new configuration that does not show a subsequent instability on dynamical or Alfven timescales; (4) the electromagnetic emission matches the duration of the initial burst in luminosity observed in giant flares, giving support to the internal rearrangement scenario; and (5) only a small fraction of the energy released during the process is converted into f-mode oscillations and in the consequent GW emission, thus resulting in very low chances of detecting this signal with present and near-future ground-based detectors.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Testing universal relations of neutron stars with a nonlinear matter-gravity coupling theory

Due to our ignorance of the equation of state (EOS) beyond nuclear density, there is still no unique theoretical model for neutron stars (NSs). It is therefore surprising that universal EOS-independent relations connecting different physical quantities of NSs can exist. Lau et al. found that the frequency of the f-mode oscillation, the mass, and the moment of inertia are connected by universal relations. More recently, Yagi and Yunes discovered the I-Love-Q universal relations among the mass, the moment of inertia, the Love number, and the quadrupole moment. In this paper, we study these universal relations in the Eddington-inspired Born-Infeld (EiBI) gravity. This theory differs from general relativity (GR) significantly only at high densities due to the nonlinear coupling between matter and gravity. It thus provides us an ideal case to test how robust the universal relations of NSs are with respect to the change of the gravity theory. Due to the apparent EOS formulation of EiBI gravity developed recently by Delsate and Steinhoff, we are able to study the universal relations in EiBI gravity using the same techniques as those in GR. We find that the universal relations in EiBI gravity are essentially the same as those in GR. Our work shows that, within the currently viable coupling constant, there exists at least one modified gravity theory that is indistinguishable from GR in view of the unexpected universal relations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Final Scientific Report: Experimental Investigation of Reconnection in a Line-tied Plasma

This grant used funding from the NSF/DoE Partnership on Plasma Science to investigate magnetic reconnection phenomena in a line-tied pinch experiment. The experiment was upgraded from a previous device intended to study fusion plasma-related instabilities to a new configuration capable of studying a number of new, previously unstudied configurations. A high spatial and time resolution array of magnetic probes was constructed to measure time evolving structures present as instability and turbulence developed. The most important new equilibrium made possible by this grant was a Zero-Net-Current equilibrium that models the footpoint twisting of solar flux tubes that occurs prior to solar eruptions (flares and coronal mass ejections). This new equilibrium was successfully created in the lab, and it exhibited a host of instabilities. In particular, at low current when the equilibrium was not overly stressed, a saturated internal kink mode oscillation was observed. At high current, 2 D magnetic turbulence developed which we attribute to the lack of a equilibrium brought about by a subcritical transition to turbulence. A second set of experiments involved the turbulent interactions of a collection of flux tubes all being twisted independently, a problem known as the Parker Problem. Current profiles consisting of 2, 3 and 4 guns were used to impose a fine scale drive, and resulted in a new experimental platform in which the injection scale of the magnetic turbulence could be controlled. First experiments in this configuration support the conclusion that an inverse cascade of magnetic energy occurred which self-organized the plasma into a nearly axisymmetric current distribution.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Estimation of Electromechanical Modes of Oscillation in the Eastern Interconnection from Ambient PMU Data

A power system's inter-area electromechanical modes of oscillation determine if the system is small-signal stable. Mode meters are tools that estimate the modes based on ambient synchrophasor measurements, allowing continuous monitoring of the system's stability margin. This paper reports on the application of mode meter algorithms to 21 months of synchrophasor measurements from the United States Eastern Interconnection (EI). Estimates for three modes were updated every minute, providing a much more comprehensive set of results than previous studies based on disturbance recordings. The detailed signal designs and mode meter configurations provided in this paper can be used to accelerate mode meter deployments in the EI. Analysis of the mode estimates reveal for the first time how seasonal load changes and system reconfiguration impact the EI system's dynamics.

Follum, James D.↗
DOE OSTI2013

OBSERVATIONS OF INTENSITY FLUCTUATIONS ATTRIBUTED TO GRANULATION AND FACULAE ON SUN-LIKE STARS FROM THE KEPLER MISSION

Sun-like stars show intensity fluctuations on a number of timescales due to various physical phenomena on their surfaces. These phenomena can convincingly be studied in the frequency spectra of these stars-while the strongest signatures usually originate from spots, granulation, and p-mode oscillations, it has also been suggested that the frequency spectrum of the Sun contains a signature of faculae. We have analyzed three stars observed for 13 months in short cadence (58.84 s sampling) by the Kepler mission. The frequency spectra of all three stars, as for the Sun, contain signatures that we can attribute to granulation, faculae, and p-mode oscillations. The temporal variability of the signatures attributed to granulation, faculae, and p-mode oscillations was analyzed and the analysis indicates a periodic variability in the granulation and faculae signatures-comparable to what is seen in the Sun.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Mode transition of a Hall thruster discharge plasma

A Hall thruster is a cross-field plasma device used for spacecraft propulsion. An important unresolved issue in the development of Hall thrusters concerns the effect of discharge oscillations in the range of 10–30 kHz on their performance. The use of a high speed Langmuir probe system and ultra-fast imaging of the discharge plasma of a Hall thruster suggests that the discharge oscillation mode, often called the breathing mode, is strongly correlated to an axial global ionization mode. Stabilization of the global oscillation mode is achieved as the magnetic field is increased and azimuthally rotating spokes are observed. A hybrid-direct kinetic simulation that takes into account the transport of electronically excited atoms is used to model the discharge plasma of a Hall thruster. The predicted mode transition agrees with experiments in terms of the mean discharge current, the amplitude of discharge current oscillation, and the breathing mode frequency. It is observed that the stabilization of the global oscillation mode is associated with reduced electron transport that suppresses the ionization process inside the channel. As the Joule heating balances the other loss terms including the effects of wall loss and inelastic collisions, the ionization oscillation is damped, and the discharge oscillation stabilizes. A wide range of the stable operation is supported by the formation of a space charge saturated sheath that stabilizes the electron axial drift and balances the Joule heating as the magnetic field increases. Finally, it is indicated from the numerical results that there is a strong correlation between the emitted light intensity and the discharge current.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

ARMAX-Based Transfer Function Model Identification Using Wide-Area Measurement for Adaptive and Coordinated Damping Control

One of the main drawbacks of the existing oscillation damping controllers that are designed based on offline dynamic models is adaptivity to the power system operating condition. With the increasing availability of wide-area measurements and the rapid development of system identification techniques, it is possible to identify a measurement-based transfer function model online that can be used to tune the oscillation damping controller. Such a model could capture all dominant oscillation modes for adaptive and coordinated oscillation damping control. our paper describes a comprehensive approach to identify a low-order transfer function model of a power system using a multi-input multi-output (MIMO) autoregressive moving average exogenous (ARMAX) model. This methodology consists of five steps: 1) input selection; 2) output selection; 3) identification trigger; 4) model estimation; and 5) model validation. The proposed method is validated by using ambient data and ring-down data in the 16-machine 68-bus Northeast Power Coordinating Council system. Our results demonstrate that the measurement-based model using MIMO ARMAX can capture all the dominant oscillation modes. Compared with the MIMO subspace state space model, the MIMO ARMAX model has equivalent accuracy but lower order and improved computational efficiency. The proposed model can be applied for adaptive and coordinated oscillation damping control.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2014

Quasi-periodic pulsations with varying period in multi-wavelength observations of an X-class flare

This work presents an interesting phenomenon of the period variation in quasi-periodic pulsations (QPPs) observed during the impulsive phase of a coronal mass ejection-related X1.1 class flare on 2012 July 6. The period of QPPs was changed from 21 s at soft X-rays (SXR) to 22-23 s at microwaves, to ∼24 s at extreme ultraviolet emissions (EUV), and to 27-32 s at metric-decimetric waves. The microwave, EUV, and SXR QPPs, emitted from flare loops of different heights, were oscillating in phase. Fast kink mode oscillations were proposed to be the modulation mechanism, which may exist in a wide region in the solar atmosphere from the chromosphere to the upper corona or even to the interplanetary space. Changed parameters of flare loops through the solar atmosphere could result in the varying period of QPPs at different wavelengths. The first appearing microwave QPPs and quasi-periodic metric-decimetric type III bursts were generated by energetic electrons. This may imply that particle acceleration or magnetic reconnection were located between these two non-thermal emission sources. Thermal QPPs (in SXR and EUV emissions) occurred later than the nonthermal ones, which would suggest a some time for plasma heating or energy dissipation in flare loops during burst processes. At the beginning of flare, a sudden collapse and expansion of two separated flare loop structures occurred simultaneously with the multi-wavelength QPPs. An implosion in the corona, including both collapse and expansion of flare loops, could be a trigger of loop oscillations in a very large region in the solar atmosphere.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ARE PULSING SOLITARY WAVES RUNNING INSIDE THE SUN?

A precise sequence of frequencies-detected four independent ways-is interpreted as a system of solitary waves below the Sun's convective envelope. Six future observational or theoretical tests of this idea are suggested. Wave properties (rotation rates, radial energy distribution, nuclear excitation strength) follow from conventional dynamics of global oscillation modes after assuming a localized nuclear term strong enough to perturb and hold mode longitudes into alignments that form 'families'. To facilitate future tests, more details are derived for a system of two dozen solitary waves 2 {<=} l {<=} 25. Wave excitation by {sup 3}He and {sup 14}C burning is complex. It spikes by factors M{sub 1} {<=} 10{sup 3} when many waves overlap in longitude but its long-time average is M{sub 2} {<=} 10. Including mixing can raise overall excitation to {approx}50 times that in a standard solar model. These spikes cause tiny phase shifts that tend to pull wave rotation rates toward their ideal values {proportional_to}[l(l + 1)]{sup -1}. A system like this would generate some extra nuclear energy in two spots at low latitude on opposite sides of the Sun. Each covers about 20 Degree-Sign of longitude. Above a certain wave amplitude, the system starts giving distinctly more nuclear excitation to some waves (e.g., l = 9, 14, and 20) than to neighboring l values. The prominence of l = 20 has already been reported. This transition begins at temperature amplitudes {Delta}T/T = 0.03 in the solar core for a typical family of modes, which corresponds to {delta}T/T {approx} 0.001 for one of its many component oscillation modes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Differential rotation in main-sequence solar-like stars: Qualitative inference from asteroseismic data

Understanding differential rotation of Sun-like stars is of great importance for insight into the angular momentum transport in these stars. One means of gaining such information is that of asteroseismology. By a forward modeling approach we analyze in a qualitative manner the impact of different differential rotation profiles on the splittings of p-mode oscillation frequencies. The optimum modes for inference on differential rotation are identified along with the best value of the stellar inclination angle. We find that in general it is not likely that asteroseismology can be used to make an unambiguous distinction between a rotation profile such as a conical Sun-like profile and a cylindrical profile. In addition, it seems unlikely that asteroseismology of Sun-like stars will result in inferences on the radial profile of the differential rotation, such as can be done for red giants. At best, one could possibly obtain the sign of the radial differential rotation gradient. Measurements of the extent of the latitudinal differential from frequency splitting are, however, more promising. One very interesting aspect that could likely be tested from frequency splittings is whether the differential rotation is solar-like or anti-solar-like in nature, in the sense that a solar-like profile has an equator rotating faster than the poles.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Stabilization of tearing modes by oscillating the resonant surface

The effects of the plasma current modulation on the linear instability of the tearing mode are numerically investigated. It is found that the tearing mode can be stabilized if the frequency of the modulation is suitable and the oscillation amplitude of the resonant surface position is large enough. The power needed for the lower-hybrid-current-drive to stabilize the tearing mode by oscillating the position of the resonant surface is comparable to the power consumption of the conventional method of tearing mode stabilization by using the electron-cyclotron-current-drive.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Coherent acoustic phonons in YBa{sub 2}Cu{sub 3}O{sub 7}/La{sub 1/3}Ca{sub 2/3}MnO{sub 3} superlattices

We investigate photo-induced coherent acoustic phonons in complex oxide superlattices consisting of high-Tc superconductor YBa{sub 2}Cu{sub 3}O{sub 7−x} and ferromagnetic manganite La{sub 1/3}Ca{sub 2/3}MnO{sub 3} epitaxial layers with broadband pump-probe spectroscopy. Two oscillatory components have been observed in time-resolved differential reflectivity spectra. Based on the analysis, the slow oscillation mode with a frequency sensitive to the probe wavelength is ascribed to the stimulated Brillouin scattering due to the photon reflection by propagating train of coherent phonons. The fast oscillation mode with a probe-wavelength-insensitive frequency is attributed to the Bragg oscillations caused by specular phonon reflections at oxide interfaces or the electron-coupling induced modulation due to free carrier absorption in the metallic superlattices. Our findings suggest that oxide superlattice is an ideal system to tailor the coherent behaviors of acoustic phonons and to manipulate the thermal and acoustic properties.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Rotational Splitting and Asteroseismic Modeling of the δ Scuti Star EE Camelopardalis

According to the rotational splitting law of g modes, the frequency spectra of EE Cam can be disentangled only with oscillation modes of ℓ = 0, 1, and 2. Fifteen sets of rotational splits were found, and they contain five sets of ℓ = 1 multiplets and 10 sets of ℓ = 2 multiplets. The rotational period of EE Cam is deduced to be P{sub rot}=1.84{sub −0.05}{sup +0.07} days. When we do model fittings, we use two nonradial oscillation modes (f {sub 11} and f {sub 32}), and the fundamental radial mode f {sub 1}. The fitting results show that χ{sup 2} of the best-fitting model is much smaller than those of other theoretical models. The physical parameters of the best-fitting model are M = 2.04 M {sub ⊙}, Z = 0.028, T {sub eff} = 6433 K, logL/L{sub ⊙}=1.416, R = 4.12 R {sub ⊙}, logg=3.518, and χ{sup 2}=0.00035. Furthermore, we find that f {sub 11} and f {sub 32} are mixed modes, which mainly characterize the features of the helium core. The fundamental radial mode f {sub 1} mainly restricts the features of the stellar envelope. Finally, the acoustic radius τ{sub 0} and the period separation Π{sub 0} are determined to be 5.80 hr and 463.7 s, respectively, and the size of the helium core of EE Cam is estimated to be M {sub He} = 0.181 M {sub ⊙} and R {sub He} = 0.0796 R {sub ⊙}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Detection and Interpretation of Long-lived X-Ray Quasi-periodic Pulsations in the X-class Solar Flare on 2013 May 14

Quasi-periodic pulsations (QPP) seen in the time derivative of the GOES soft X-ray light curves are analyzed for the X3.2 event on 2013 May 14. The pulsations are apparent for a total of at least two hours from the impulsive phase to well into the decay phase, with a total of 163 distinct pulses evident to the naked eye. A wavelet analysis shows that the characteristic timescale of these pulsations increases systematically from ∼25 s at 01:10 UT, the time of the GOES peak, to ∼100 s at 02:00 UT. A second “ridge” in the wavelet power spectrum, most likely associated with flaring emission from a different active region, shows an increase from ∼40 s at 01:40 UT to ∼100 s at 03:10 UT. We assume that the QPP that produced the first ridge result from vertical kink-mode oscillations of the newly formed loops following magnetic reconnection in the coronal current sheet. This allows us to estimate the magnetic field strength as a function of altitude given the density, loop length, and QPP timescale as functions of time determined from the GOES light curves and Ramaty High Energy Solar Spectroscopic Imager ( RHESSI ) images. The calculated magnetic field strength of the newly formed loops ranges from ∼500 G at an altitude of 24 Mm to a low value of ∼10 G at 60 Mm, in general agreement with the expected values at these altitudes. Fast sausage-mode oscillations are also discussed and cannot be ruled out as an alternate mechanism for producing the QPP.

79 ASTRONOMY AND ASTROPHYSICS↗