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At least 1,153 records · Page 64
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Retrieving fields from proton radiography without source profiles

Proton radiography is a technique in high-energy density science to diagnose magnetic and/or electric fields in a plasma by firing a proton beam and detecting its modulated intensity profile on a screen. Current approaches to retrieve the integrated field from the modulated intensity profile require the unmodulated beam intensity profile before the interaction, which is rarely available experimentally due to shot-to-shot variability. In this paper, we present a statistical method to retrieve the integrated field without needing to know the exact source profile. We apply our method to experimental data, showing the robustness of our approach. Furthermore, our proposed technique allows for the retrieval not only of the path-integrated fields, but also of the statistical properties of the fields.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Tungsten erosion during L-mode discharges in the DIII-D SAS-VW divertor

A series of L-mode plasma discharges was performed in the DIII-D tokamak to assess the impact of outer strike point (OSP) position and toroidal magnetic field direction on erosion and core contamination potential of the recently-installed, tungsten-coated Small Angle Slot (SAS-VW) divertor. In one discharge, in-slot emission spectroscopy measured an<48 % increase in the W gross erosion rate when the OSP was moved 3 cm outwards, away from the V-shaped vertex of the slot divertor. However, the effective W yield (erosion rate divided by the incident D flux) was, overall, insensitive to changes in OSP location. Consistently low estimates of the effective W yield based on measurements taken a few cm outwards from the vertex suggest potentially significant C surface contamination. No W emission signal was detected when orienting the toroidal magnetic field such that the ion B×∇B drift direction is pointed away from the X-point. However, measurements of W content in the plasma core for both toroidal magnetic field directions suggest the presence of additional, unmeasured sources of erosion. The difference in the measured core W density with OSP position is much greater than the difference in the measured erosion rates, which may suggest that the leakage of eroded impurities out of the divertor is governed primarily through the parallel ion temperature gradient and friction forces.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

MHD-induced beta limits in the Large Helical Device

Using the extended-magnetohydrodynamics code, M3D-C1, we perform a systematic numerical study of the effect of externally applied heating on the achievable plasma beta in a ten field-period heliotron. Heat sources of varying intensity are applied to a vacuum magnetic field that is representative of the standard configuration of the Large Helical Device, with R 0 = 3.66 m, where R 0 is the radial position of the magnetic axis in vacuum. As the system is driven to a state that is unstable to low-n magnetohydrodynamic (MHD) modes, nonlinear mode interactions lead to the formation of chaotic magnetic fields. With sufficiently strong heating, a collapse of the electron temperature profile is observed. This demonstrates the necessity of simulating the self-consistent evolution of plasma profiles, without imposing assumptions on the structure of the magnetic field, to accurately determine transport properties in stellarator plasmas. It also highlights the value of these advanced simulation capabilities for accelerating the development of high-performance stellarator operating scenarios.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

First experimental confirmation of island SOL geometry effects in a high radiation regime on W7-X

Abstract This work characterizes the detachment behavior and radiation characteristics of the low&#xD;iota configuration in the Wendelstein 7-X (W7-X) stellarator. The island scrape-off layer (SOL) of the&#xD;low iota has a poloidal mode number of 6 islands surrounding the last closed flux surface (LCFS).&#xD;The island geometry of the low iota configuration is significantly different to that of the standard&#xD;magnetic field configuration, whose detachment characteristics have already been described in previous&#xD;work[2, 3, 4]. Experimental results show that the radiation pattern in the low iota configuration&#xD;is starkly different to that of the standard magnetic field configuration, with radiation concentrated&#xD;at the island SOL O-points, rather than the X-points. Additionally, this O-point localized radiation&#xD;pattern is associated with unstable detachment, with both radiation oscillations in experiments and the&#xD;lack of a self-consistent plasma solution at high radiated power fraction in EMC3-Eirene simulations.&#xD;EMC3-Eirene simulations are used to understand the radiation distribution. It was found that the O-&#xD;point localized radiation arises first from local impurity accumulation near the parallel flow stagnation,&#xD;which is located close to the geometrical center of the island (”O-point”). The local cooling in this&#xD;region leads to plasma condensation in the islands in closest magnetic connection to the divertor target&#xD;plates. The heat source to this region of the island, which is thermally isolated from the upstream heat&#xD;source in terms of parallel transport, must arise via perpendicular heat transport. This heat source is&#xD;expected to be large for the low iota configuration due to its very small internal island field line pitch.&#xD;This work highlights the importance (complementary to previous work, e. g. [5, 6]) of the internal&#xD;island field line pitch not only on the radiation pattern, but also the detachment performance of the&#xD;island divertor.

Winters, Victoria (ORCID:0000000181087774)↗
DOE OSTI2021

Attend and Decode: 4D fMRI Task State Decoding Using Attention Models

Source code for Brain Attend and Decode paper. Functional magnetic resonance imaging (fMRI) is a neuroimaging modality that captures the blood oxygen level in a subject's brain while the subject either rests or performs a variety of functional tasks under different conditions. Given fMRI data, the problem of inferring the task, known as task state decoding, is challenging due to the high dimensionality (hundreds of million sampling points per datum) and complex spatio-temporal blood flow patterns inherent in the data. In this work, we propose to tackle the fMRI task state decoding problem by casting it as a 4D spatiotemporal classification problem. We present a novel architecture called Brain Attend and Decode (BAnD), that uses residual convolutional neural networks for spatial feature extraction and self-attention mechanisms for temporal modeling. We achieve significant performance gain compared to previous works on a 7-task benchmark from the large-scale Human Connectome Project-Young Adult (HCP-YA) dataset. We also investigate the transferability of BAnD's extracted features on unseen HCP tasks, either by freezing the spatial feature extraction layers and retraining the temporal model, or finetuning the entire model. The pre-trained features from BAnD are useful on similar tasks while finetuning them yields competitive results on unseen tasks/conditions.

Ng, BrendaM.↗
DOE OSTI2023

Development of MHD for NekRS

A spectral-element-based formulation of incompressible MHD is presented in the context of the open-source fluid-thermal code, Nek5000/RS. The formulation supports magnetic fields in a solid domain that surrounds the fluid domain. Several steady-state and time-transient model problems are presented as part of the code verification process. Nek5000/RS is designed for large-scale turbulence simulations, which will be the next step with this new MHD capability.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2024

A Real-Time Testbed for Smart Inverter Cyber Security Studies

Distributed energy resources (DER) have become a popular solution to modern-day issues surrounding the efficiency and reliability of power generation, as well as climate change concerns. Energy centers are shifting towards incorporating smart inverters with embedded functionalities such as high voltage ride through (HVRT), low voltage ride through (LVRT), active and reactive power compensation. However, the integration of smart inverters leave DER systems highly vulnerable to cybersecurity threats. The distributed network protocol 3 (DNP3) is a common method of communication between grid-tied hardware. Despite its popularity, the level of security leaves all hardware connected to the grid at risk of severe cyber-attacks. Thus, it is important to study any potential cybersecurity threats towards grid-tied smart inverters to mitigate cybersecurity vulnerabilities and refine existing cyber-security protections. This report describes the proposed testbed design to study cybersecurity threats to smart inverters. The testbed utilizes a real-time simulation case in RSCAD that includes a grid-tied wind turbine (WT) topology featuring two back-to-back two-level voltage source converters (BTB,2L-VSCs) and a permanent magnet synchronous machine (PMSM). The simulated case runs within the NovaCor real time digital simulator (RTDS). This report focuses on the design and implementation of a single module of the GTNETx2 card as a distributed network protocol and the configuration of an IEEE 1518 DNP database file that includes input and output variables mapped to different connection points in the grid that transmit and receive discrete, analog, and binary signals on command. This allows realistic emulation of the communication between the smart inverter and the grid for cybersecurity studies.

97 MATHEMATICS AND COMPUTING↗
NASA NTRS1972

Analysis of solar X-ray data

Type III solar bursts occurring in the absence of solar flares were observed to be accompanied by weak X-radiation. The energy scale of an OSO-3 soft X-ray ion chamber was assessed using realistic theoretical X-ray spectra. Relationships between soft solar X-rays and solar activity were investigated. These included optical studies, the role of the Type III acceleration mechanism in establishing the soft X-ray source volume, H alpha flare intensity variations, and gross magnetic field structure.

Teske, R. G.↗
NASA NTRS1972

Geology of Utah and Nevada by ERTS imagery

The author has identified the following significant results. Imagery for a portion of the study area was received, catalogued for in-house availability, and examined for relevant data. Topographic, geologic, aeromagnetic, and gravity maps were gathered to form a basis of comparison. Overlay drawings were prepared indicating significant features for further study and eventual field checking. The number of significant observations appears to be very great, and it is apparent that methods of handling the acquired information for easy access, for planning field checks, and for final presentation, must be improved. The tracing of lithologic variations and contacts is readily noted on many of the images. Some of the features vary from what is shown on geological maps but the correlation, or lack of same, is still to be studied in the field and by comparison of ground truth geological maps with the specific images. Numerous lineaments, based on tonal differences, are noted in the districts, some of which extend for tens of kilometers. Several positive and negative magnetic anomalies are noted with the suggestion that the sources of the mineralizing fluids were derived from hidden intrusives.

Jensen, M. L.↗
NASA NTRS1973

Development of a rotating gravity gradiometer for earth orbit applications (AAFE)

Some preliminary mission studies are described along with the design, fabrication, and test of a breadboard model of an earth orbital, rotating gravity gradiometer with a design goal of 10 to the minus 11th power/sec sq (0.01 EU) in a 35-sec integration time. The proposed mission uses a Scout vehicle to launch one (or two orthogonally oriented) spin-stabilized satellites into a 330-km circular polar orbit some 20 days before an equinox. During the short orbital lifetime, the experiment would obtain two complete maps of the gravity gradient field with a resolution approaching 270 km (degree 75). The breadboard model of the gradiometer demonstrated a combined thermal and electronic noise threshold of 0.015 EU per data channel. The design changes needed to reduce the noise to less than 0.01 EU were identified. Variations of the sensor output signal with temperature were experimentally determined and a suitable method of temperature compensation was developed and tested. Other possible error sources, such as sensor interaction with satellite dynamics and magnetic fields, were studied analytically and shown to be small.

Forward, R. L.↗
NASA NTRS1975

Electromagnetic hiss and relativistic electron losses in the inner zone

It is shown that ELF hiss is present in the inner radiation zone (L less than 2) during both storms and substorms. This presence is ascribed to propagation of waves into this region from a distributed source located at high altitude near the equatorial plasmapause. Two major effects allow hiss to reach the inner zone during storms and substorms: (1) the observed tendency for plasmaspheric hiss to be greatly intensified during the recovery phase as the outward-moving plasmapause encounters the electrons that have been freshly injected into the magnetosphere; (2) the displacement of the plasmapause, and the presumed source region, to low altitudes during high levels of magnetic activity.

Tsurutani, B. T.↗
NASA NTRS1980

X-ray spectra of accreting degenerate stars

X-ray emission from accreting nonmagnetic degenerate dwarfs is discussed. Emphasis is placed on the numerical calculations by Kylafis and Lamb (1979) with regard to the entire range of accretion rates and stellar masses. It is concluded that at moderate and high accretion rates the spectral temperature varies dramatically and the star exhibits a pronounced correlation between X-ray spectral temperature and luminosity. Also considered are the results of Lamb and Masters (1979) of calculations of high-harmonic cyclotron emission from a hot plasma, from which they developed a model of X-ray and UV emission from accreting magnetic degenerate dwarfs. It is found that the position and relative strength of the spectral components change with variations in the accretion rate and differ for different magnetic fields; in addition, magnetic degenerate dwarfs are predicted to be strong UV sources with only a few percent of the total accretion luminosity ordinarily appearing as soft and hard X-rays.

Kylafis, N. D.↗
NASA NTRS1981

Precision timekeeping using a small passive hydrogen maser

The timekeeping ability of a prototype passive hydrogen maser which is a factor of 5 smaller in size, weight, and cost than any previously designed, was compared to UTC (NBS) based on 10 cesium frequency standards including a large primary standard, NBS-4. The frequency of the prototype was monitored as a function of source pressure, cavity temperature, microwave power, modulation width, and magnetic field. Based on these measurements, a frequency stability of better than 6 x 10 to the -15 power was expected, implying a timekeeping ability of order 0.5 ns/day. Measurements vs UTC(NBS) indicate a joint timekeeping a stability of order 1.2 ns/day. Simultaneous measurements made between NBS-4, UTC(6600), and the small passive me maser show peak to peak time variations of the small maser vs UTC(6600) was 10 ns for the full 32 days if the average rate and drift are considered. Frequency stability of the small prototype vs UTC (NBS) was to 1.1 x 10 to the -14th power for tau = 1 to 8 days.

Walls, F. L.↗
NASA NTRS1982

Ducted auroral kilometric radiation

Certain discrete, intense wave signals attributed to auroral kilometric radiation (AKR) were observed with ISEE-l while it was within the plasmaspheric shadow zone for direct propagation. It is believed that wave ducting by thin depletions of the plasma density aligned with the magnetic field accounts for such signals, and that their discrete nature is caused by the satellite intercepting individual ducts. These ducts, which were also observed as coincident decreases of the upper hybrid resonance frequency, appeared to be twenty-percent depletions roughly one hundred kilometers across. The AKR, which is emitted approximately perpendicular to the magnetic field, apparently entered these ducts equatorward of the source after the waves had been refracted parallel to the duct axis. A diffuse background was also observed which is consistent with the leakage from similar ducts at lower L-values. These observations establish the existence of ducted AKR, its signature on the satellite wave spectrograms, and new evidence for depletion ducts within the plasmasphere.

Calvert, W.↗
NASA NTRS1982

Neutral composition in the polar thermosphere - Observations made on Dynamics Explorer

A neutral mass spectrometer on the lower altitude satellite of the Dynamics Explorer program is providing observations of compositional variability in the thermosphere. The 90 deg inclination of the satellite orbit provides excellent coverage over the polar caps. Two data sets, one in the Southern Hemisphere and the other in the Northern, are used to further illustrate the strong magnetic control of some of the heat and momentum sources at high latitudes and the complex morphology that results.

Carignan, G. R.↗