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NASA NTRS2013

Stratified Magnetically Driven Accretion-Disk Winds and Their Relations To Jets

We explore the poloidal structure of two-dimensional magnetohydrodynamic (MHD) winds in relation to their potential association with the X-ray warm absorbers (WAs) and the highly ionized ultra-fast outflows (UFOs) in active galactic nuclei (AGNs), in a single unifying approach. We present the density n(r, theta), ionization parameter xi(r, theta), and velocity structure v(r, theta) of such ionized winds for typical values of their fluid-to-magnetic flux ratio, F, and specific angular momentum, H, for which wind solutions become super-Alfvenic. We explore the geometrical shape of winds for different values of these parameters and delineate the values that produce the widest and narrowest opening angles of these winds, quantities necessary in the determination of the statistics of AGN obscuration. We find that winds with smaller H show a poloidal geometry of narrower opening angles with their Alfv´en surface at lower inclination angles and therefore they produce the highest line of sight (LoS) velocities for observers at higher latitudes with the respect to the disk plane. We further note a physical and spatial correlation between the X-ray WAs and UFOs that form along the same LoS to the observer but at different radii, r, and distinct values of n, xi, and v consistent with the latest spectroscopic data of radio-quiet Seyfert galaxies. We also show that, at least in the case of 3C 111, the winds' pressure is sufficient to contain the relativistic plasma responsible for its radio emission. Stratified MHD disk winds could therefore serve as a unique means to understand and unify the diverse AGN outflows.

accretion↗

NASA NTRS2013

Mode Transitions in Hall Effect Thrusters

Mode transitions have been commonly observed in Hall Effect Thruster (HET) operation where a small change in a thruster operating parameter such as discharge voltage, magnetic field or mass flow rate causes the thruster discharge current mean value and oscillation amplitude to increase significantly. Mode transitions in a 6-kW-class HET called the H6 are induced by varying the magnetic field intensity while holding all other operating parameters constant and measurements are acquired with ion saturation probes and ultra-fast imaging. Global and local oscillation modes are identified. In the global mode, the entire discharge channel oscillates in unison and azimuthal perturbations (spokes) are either absent or negligible. Downstream azimuthally spaced probes show no signal delay between each other and are very well correlated to the discharge current signal. In the local mode, signals from the azimuthally spaced probes exhibit a clear delay indicating the passage of "spokes" and are not well correlated to the discharge current. These spokes are localized oscillations propagating in the ExB direction that are typically 10-20% of the mean value. In contrast, the oscillations in the global mode can be 100% of the mean value. The transition between global and local modes occurs at higher relative magnetic field strengths for higher mass flow rates or higher discharge voltages. The thrust is constant through mode transition but the thrust-to-power decreased by 25% due to increasing discharge current. The plume shows significant differences between modes with the global mode significantly brighter in the channel and the near-field plasma plume as well as exhibiting a luminous spike on thruster centerline. Mode transitions provide valuable insight to thruster operation and suggest improved methods for thruster performance characterization.

interplanetary exploration↗

NASA NTRS2013

Azimuthal Spoke Propagation in Hall Effect Thrusters

Spokes are azimuthally propagating perturbations in the plasma discharge of Hall Effect Thrusters (HETs) that travel in the E x B direction and have been observed in many different systems. The propagation of azimuthal spokes are investigated in a 6 kW HET known as the H6 using ultra-fast imaging and azimuthally spaced probes. A spoke surface is a 2-D plot of azimuthal light intensity evolution over time calculated from 87,500 frames/s videos. The spoke velocity has been determined using three methods with similar results: manual fitting of diagonal lines on the spoke surface, linear cross-correlation between azimuthal locations and an approximated dispersion relation. The spoke velocity for three discharge voltages (300, 400 and 450 V) and three anode mass flow rates (14.7, 19.5 and 25.2 mg/s) yielded spoke velocities between 1500 and 2200 m/s across a range of normalized magnetic field settings. The spoke velocity was inversely dependent on magnetic field strength for low B-field settings and asymptoted at B-field higher values. The velocities and frequencies are compared to standard drifts and plasma waves such as E x B drift, electrostatic ion cyclotron, magnetosonic and various drift waves. The empirically approximated dispersion relation yielded a characteristic velocity that matched the ion acoustic speed for 5 eV electrons that exist in the near-anode and near-field plume regions of the discharge channel based on internal measurements. Thruster performance has been linked to operating mode where thrust-to-power is maximized when azimuthal spokes are present so investigating the underlying mechanism of spokes will benefit thruster operation.

Cylindrical Hall Thruster (CHT)↗

NASA NTRS2016

Particle Streak Velocimetry of Supersonic Nozzle Flows

A novel velocimetry technique to probe the exhaust flow of a laboratory scale combustor is being developed. The technique combines the advantages of standard particle velocimetry techniques and the ultra-fast imaging capabilities of a streak camera to probe high speed flows near continuously with improved spatial and velocity resolution. This "Particle Streak Velocimetry" technique tracks laser illuminated seed particles at up to 236 picosecond temporal resolution allowing time-resolved measurement of one-dimensional flows exceeding 2000 m/s as are found in rocket nozzles and many other applications. Developmental tests with cold nitrogen have been performed to validate and troubleshoot the technique with supersonic flows of much lower velocity and without background noise due to combusting flow. Flow velocities on the order of 500 m/s have been probed with titanium dioxide particles and a continuous-wave laser diode. Single frame images containing multiple streaks are analyzed to find the average slope of all incident particles corresponding to the centerline axial flow velocity. Long term objectives for these tests are correlation of specific impulse to theoretical combustion predictions and direct comparisons between candidate green fuels and the industry standard, monomethylhydrazine, each tested under identical conditions.

Willits, J. D.↗

NASA NTRS2016

Fast Ionized X-Ray Absorbers in AGNs

We investigate the physics of the X-ray ionized absorbers often identified as warm absorbers (WAs) and ultra-fast outflows (UFOs) in Seyfert AGNs from spectroscopic studies in the context of magnetically-driven accretion-disk wind scenario. Launched and accelerated by the action of a global magnetic field anchored to an underlying accretion disk around a black hole, outflowing plasma is irradiated and ionized by an AGN radiation field characterized by its spectral energy density (SED). By numerically solving the Grad-Shafranov equation in the magnetohydrodynamic (MHD) framework, the physical property of the magnetized disk-wind is determined by a wind parameter set, which is then incorporated into radiative transfer calculations with xstar photoionization code under heating-cooling equilibrium state to compute the absorber's properties such as column density N(sub H), line-of-sight (LoS) velocity v, ionization parameter xi, among others. Assuming that the wind density scales as n varies as r(exp. -1), we calculate theoretical absorption measure distribution (AMD) for various ions seen in AGNs as well as line spectra especially for the Fe K alpha absorption feature by focusing on a bright quasar PG 1211+143 as a case study and show the model's plausibility. In this note we demonstrate that the proposed MHD-driven disk-wind scenario is not only consistent with the observed X-ray data, but also help better constrain the underlying nature of the AGN environment in a close proximity to a central engine.

accretion↗

NASA NTRS2017

The Broadband Spectral Variability of Holmberg IX X-1

We present results from four new broadband X-ray observations of the extreme ultraluminous X-ray source Holmberg IX X-1 (L (sub X) greater than 10 (sup 40) ergs per second), performed by Suzaku and NuSTAR in coordination. Combined with the archival data, we now have broadband observations of this remarkable source from six separate epochs. Two of these new observations probe lower fluxes than seen previously, allowing us to extend our knowledge of the broadband spectral variability exhibited. The spectra are well fit by two thermal blackbody components that dominate the emission below 10 kiloelectronvolts, as well as a steep (Gamma approximately equal to 3.5) power-law tail thatdominates above approximately 15 kiloelectronvolts. Remarkably, while the 0.3-10.0 kiloelectronvolts flux varies by a factor of approximately 3 between all these epochs, the 15-40 kiloelectronvolts flux varies by only approximately 20 percent. Although the spectral variability is strongest in the approximately 1-10 kiloelectronvolts band, both of the thermal components are required to vary when all epochs are considered. We also revisit the search for iron absorption features by leveraging the high-energy NuSTAR data to improve our sensitivity to extreme velocity outflows in light of the ultra-fast outflow recently detected in NGC 1313 X-1. Iron absorption from a similar outflow along our line of sight can be ruled out in this case. We discuss these results in the context of super-Eddington accretion models that invoke a funnel-like geometry for the inner flow, and propose a scenario in which we have an almost face-on view of a funnel that expands to larger radii with increasing flux, resulting in an increasing degree of geometrical collimation for the emission from intermediate-temperature regions.

X-rays: binaries↗

NASA NTRS2019

Laser Energy Monitor for Triple-Pulse 2-μm IPDA Lidar Application

Integrated path differential absorption (IPDA) lidar is an active remote sensing technique for monitoring different atmospheric species. The technique relies on wavelength differentiation between strong and weak absorbing features normalized to the transmitted energy. An advanced 2-μm triple-pulse IPDA lidar was developed at NASA Langley Research Center for active sensing of carbon dioxide and water vapor simultaneously. The IPDA transmitter produces three successive laser pulses separated by a short interval (200 μs) with a repetition rate of 50Hz. Measurement of laser pulse energy accurately is a prerequisite for the retrieval of gas mixing ratios from IPDA. The design and calibration of a 2-μm triple-pulse laser energy monitor are presented. Due to the short interval between the three transmitted pulses, conventional thermal energy monitors underestimate the total transmitted energy. The design is based on a high speed, extended range InGaAs pin quantum detector suitable for separating the three pulse events. Pulse integration is applied for converting the detected pulse power into energy. The results obtained from the laser energy monitor were compared to an ultra-fast energy-meter reference for energy scaling and verification. High correlations between the pin energy monitor and the total transmitted energy were obtained. The objective of this development is to reduce measurement biases and errors using the triple-pulse IPDA technique.

Refaat, Tamer F.↗

NASA NTRSDate not supplied

Development of a High-Fidelity CLARREO Pathfinder Simulator

CLARREO Pathfinder is a hyperspectral instrument designed to resolve reflectance of solar radiation from Earth in the wavelength range of 350 nm to 2300 nm with an uncertainty of 0.3% (k=2) or less. Its success will demonstrate CLARREO’s ability to provide on-orbit SI-Traceable calibration of measured spectral reflectance with an advanced accuracy. The CLARREO pathfinder simulator has been developed to generate high-fidelity inter-calibration event data products that can be used for pre-launch inter-calibration algorithm study. A high-fidelity simulation for CLARREO inter-calibration events includes computationally intensive radiative transfer calculations for observations within each individual collocation footprint. A principal component based radiative transfer model is incorporated in the simulator to enable an ultra-fast forward simulation, ensuring a low latency data processing for month- to year-long intercalibration events. This paper introduces the design and development of the CLARREO pathfinder simulator and some of its applications for inter-calibration matching error evaluation.

Wan Wu↗

NASA NTRS1999

A Low-Power High-Speed Smart Sensor Design for Space Exploration Missions

In this paper, an ultra-fast low-power smart vision system-on-a-chip design is proposed to provide effective solutions for real time machine vision applications by taking advantages of advances in integrated sensing/processing designs, electronic neural networks, advanced microprocessors, and sub-micron VLSI systemns-on-a-chip (SOAC) technology.

space↗

NASA NTRS2013

Azimuthal spoke propagation in Hall Effect Thrusters

Spokes are azimuthally propagating perturbations in the plasma discharge of Hall Effect Thrusters (HETs) that travel in the E × B direction and have been observed in many different systems. The propagation of azimuthal spokes are investigated in a 6 kW HET known as the H6 using ultra-fast imaging and azimuthally spaced probes. A spoke surface is a 2-D plot of azimuthal light intensity evolution over time calculated from 87,500 frames/s videos. The spoke velocity has been determined using three methods with similar results: manual fitting of diagonal lines on the spoke surface, linear cross-correlation between azimuthal locations and an approximated dispersion relation. The spoke velocity for three discharge voltages (300, 400 and 450 V) and three anode mass flow rates (14.7, 19.5 and 25.2 mg/s) yielded spoke velocities between 1500 and 2200 m/s across a range of normalized magnetic field settings. The spoke velocity was inversely dependent on magnetic field strength for low B-field settings and asymptoted at B-field higher values. The velocities and frequencies are compared to standard drifts and plasma waves such as E × B drift, electrostatic ion cyclotron, magnetosonic and various drift waves. The empirically approximated dispersion relation yielded a characteristic velocity that matched the ion acoustic speed for 5 eV electrons that exist in the near-anode and near-field plume regions of the discharge channel based on internal measurements. Thruster performance has been linked to operating mode where thrust-to-power is maximized when azimuthal spokes are present so investigating the underlying mechanism of spokes will benefit thruster operation.

Polk, James E.↗

NASA NTRS2021

Speed Limits for Radiation-driven SMBH Winds

Context. Ultra-fast outflows (UFOs) have become an established feature in analyses of the X-ray spectra of active galactic nuclei (AGN). According to the standard picture, they are launched at accretion disc scales with relativistic velocities, up to 0.3-0.4 times the speed of light. Their high kinetic power is enough to induce an efficient feedback on a galactic scale, possibly contributing to the co-evolution between the central supermassive black hole (SMBH) and the host galaxy. It is, therefore, of paramount importance to gain a full understanding of UFO physics and, in particular, of the forces driving their acceleration and the relation to the accretion flow from which they originate. Aims. In this paper, we investigate the impact of special relativity effects on the radiative pressure exerted onto the outflow. The radiation received by the wind decreases for increasing outflow velocity, v, implying that the standard Eddington limit argument has to be corrected according to v. Due to the limited ability of the radiation to counteract the black hole gravitational attraction, we expect to find lower typical velocities with respect to the non-relativistic scenario. Methods. We integrated the relativistic-corrected outflow equation of motion for a realistic set of starting conditions. We concentrated on a range of ionisations, column densities, and launching radii consistent with those typically estimated for UFOs. We explore a one-dimensional, spherical geometry and a three-dimensional setting with a rotating, thin accretion disc. Results. We find that the inclusion of special relativity effects leads to sizeable differences in the wind dynamics and that v is reduced up to 50% with respect to the non-relativistic treatment. We compare our results with a sample of UFOs from the literature and we find that the relativistic-corrected velocities are systematically lower than the reported ones, indicating the need for an additional mechanism, such as magnetic driving, to explain the highest velocity components. Finally, we note that these conclusions, derived for AGN winds, are generally applicable.

A Luminari↗

NASA NTRSDate not supplied

Single Field-of-View Sounding Atmospheric Products

The Single Field-of-view Sounder Atmospheric Products (SiFSAP) answer the need for a novel high spatial resolution atmospheric data product for major hyper-spectral infrared (IR) sounder missions. SiFSAP include a complete set of atmospheric vertical profiles, cloud, and surface properties, which are physically retrieved from top-of-atmosphere (TOA) spectral radiances under all-sky conditions via a rigorously defined radiative transfer relationship. By using a state-of-art fast radiative transfer model and a carefully designed optimal estimation based physical retrieval scheme, the SiFSAP algorithm ensures both an ultra-fast data processing speed needed for operational weather applications and the radiometric consistency desired by long-term climate studies. SiFSAP supplement existing operational products by providing data at the native resolution of the sounder instruments, the direct and accurate retrieval of cloud scattering properties, and the establishment of ‘radiance closure.’

Wan Wu↗

NASA NTRSDate not supplied

Single Field-of-View Sounding Atmospheric Products

The Single Field-of-view Sounder Atmospheric Products (SiFSAP) answer the need for a novel high spatial resolution atmospheric data product for major hyper-spectral infrared (IR) sounder missions. SiFSAP include a complete set of atmospheric vertical profiles, cloud, and surface properties, which are physically retrieved from top-of-atmosphere (TOA) spectral radiances under all-sky conditions via a rigorously defined radiative transfer relationship. By using a state-of-art fast radiative transfer model and a carefully designed optimal estimation based physical retrieval scheme, the SiFSAP algorithm ensures both an ultra-fast data processing speed needed for operational weather applications and the radiometric consistency desired by long-term climate studies. SiFSAP supplement existing operational products by providing data at the native spatial resolution of the sounder instruments, the direct and accurate retrieval of cloud scattering properties, and the establishment of ‘radiance closure.’

single-field-of-view, retrieval, hyperspectral sou↗

NASA NTRSDate not supplied

Hyperspectral Sounder Products: SiFSAP and ClimFiSP

Infrared hyperspectral sounders have demonstrated to provide very high value information for a range of weather and climate applications. Traditional retrieval algorithms of hyperspectral sounders use the ‘cloud clearing’ methodology in the retrieval process due to the lack of means to account for cloud scattering effect. The corresponding Level-2 data products are therefore limited by two major factors: the degradation of spatial resolution as compared with the native resolution of the instruments and the lack of ‘radiance closure’ that is critically needed for climate trend studies. The single field-of-view (FOV) sounder atmospheric products (SiFSAP) algorithm has been developed to supplement existing operational products. The SiFSAP algorithm uses the principal component based radiative transfer model (PCRTM) that is ultra-fast and has been extensively validated for the cloud scattering simulation capability. The cloud properties are physically retrieved along with profiles of temperature, moisture, and trace gases of interest, surface properties simultaneously by fitting single FOV spectral measurements. Therefore, SiFSAP ensure the radiometric consistency between the retrieved geophysical properties and the top-of-atmosphere (TOA) spectral measurements. This presentation provides a general introduction of SiFSAP and details about the physical retrieval algorithm. Also introduced here is a derivative data product of SiFSAP, the climate fingerprinting Sounder Products (ClimFiSP). ClimFiSP include the space-time averaged properties of key climate variables that are derived from the space-time averaged radiances via the spectral fingerprinting scheme. The spectral fingerprinting method allows a low latency data update on ClimFiSP following any potential calibration update on satellite measurements. ClimFiSP also include the rigorously defined uncertainty characterization for trends or anomalies of climate variables, being critical for the construction of long-term climate data record. Both SiFSAP and ClimFiSP will be available to users through NASA's Goddard Earth Sciences Data and Information Services Center (GES DISC).

Wan Wu↗

NASA NTRSDate not supplied

Hierarchical screening for Li-based solid electrolytes using fast, interpretable machine-learned potentials

Li-based solid-state electrolyte materials enable safer, all-solid-state batteries but the computational search for candidates with favorable stability and Li-ion conductivity is challenging due to the size of the search space and the cost of evaluating transport properties with ab initio methods. The prohibitive cost of high-throughput screening with DFT has lead to the development of surrogate models using geometric analysis, empirical potentials, and descriptors for ionic transport. Here, I will discuss a hierarchical screening approach for identifying promising materials using a combination of density functional theory, bond-valence methods, and machine learning potentials generated with the Ultra-Fast Force Fields (UF3) framework. We show how the inexpensive bond-valence method can be used to guide the generation of training samples for machine learning, in addition to filtering candidates. Finally, we apply the hierarchical workflow to screen for ionic conductivity across a database of Li-containing compounds.

Materials discovery↗

NASA NTRSDate not supplied

Ultrasonic Clothes Washer/Dryer Combination for Moon, Mars and ISS Applications

Our study aims to investigate the effectiveness of an ultrasonic-based combo garment washing and drying system for space applications. Our system leverages our technological innovations in direct-contact ultrasonic fabric drying in combination with ultrasonic fabric washing. The main objective of this investigation is to gauge the effectiveness of washing & drying textile garments ultrasonically. In this paper, we will report the results of experiments conducted to assess the effectiveness of washing clothes ultrasonically, including ultrasonic intensity testing, stain removal testing, and fabric degradation testing. This study's outcome could lead to the production of an Ultra-Fast Ultrasonic Washer/Dryer Combination unit that could reduce clothing resupply costs for crewed missions to the Moon, Mars, and the ISS.

washer↗

NASA NTRSDate not supplied

Hierarchical Screening for Li-Based Solid Electrolytes Using Fast, Interpretable Machine-Learned Potentials

Materials discovery↗

DOE OSTI2024

On ultrafast x-ray scattering methods for magnetism

With the introduction of x-ray free electron laser sources around the world, new scientific approaches for visualizing matter at fundamental length and time-scales have become possible. As it relates to magnetism and ‘magnetic-type’ systems, advanced methods are being developed for studying ultrafast magnetic responses on the time-scales at which they occur. We describe three capabilities which have the potential to seed new directions in this area and present original results from each: pump-probe x-ray scattering with low energy excitation, x-ray photon fluctuation spectroscopy, and ultrafast diffuse x-ray scattering. By combining these experimental techniques with advanced modeling together with machine learning, we describe how the combination of these domains allows for a new understanding in the field of magnetism. Finally, we give an outlook for future areas of investigation and the newly developed instruments which will take us there.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗