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At least 1,765 records · Page 98
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Hydrogen Balmer Line Broadening in Solar and Stellar Flares

The broadening of the hydrogen lines during flares is thought to result from increased charge (electron, proton) density in the flare chromosphere. However, disagreements between theory and modeling prescriptions have precluded an accurate diagnostic of the degree of ionization and compression resulting from flare heating in the chromosphere. To resolve this issue, we have incorporated the unified theory of electric pressure broadening of the hydrogen lines into the non-LTE radiative-transfer code RH. This broadening prescription produces a much more realistic spectrum of the quiescent, A0 star Vega compared to the analytic approximations used as a damping parameter in the Voigt profiles. We test recent radiative-hydrodynamic (RHD) simulations of the atmospheric response to high nonthermal electron beam fluxes with the new broadening prescription and find that the Balmer lines are overbroadened at the densest times in the simulations. Adding many simultaneously heated and cooling model loops as a “multithread” model improves the agreement with the observations. We revisit the three-component phenomenological flare model of the YZ CMi Megaflare using recent and new RHD models. The evolution of the broadening, line flux ratios, and continuum flux ratios are well-reproduced by a multithread model with high-flux nonthermal electron beam heating, an extended decay phase model, and a “hot spot” atmosphere heated by an ultrarelativistic electron beam with reasonable filling factors: ∼0.1%, 1%, and 0.1% of the visible stellar hemisphere, respectively. The new modeling motivates future work to understand the origin of the extended gradual phase emission.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

Dynamics and control of escape and rescue from a tumbling spacecraft

The results of 18 months of investigations are reported. A movable mass control system to convert the tumbling motion of a spacecraft into simple spin was studied along with the optimization techniques for generating displacement profiles for a tumbling asymmetrical body. Equations of motion are discussed for two asymmetrical vehicles with flexible beams and one spacecraft with flexible solar arrays. The characteristics which allow reasonable safety and reliability in bailout are also discussed.

Kaplan, M. H.↗
NASA NTRS1979

Crystallography of decahedral and icosahedral particles. II - High symmetry orientations

Based on the exact crystal structure of decahedral and icosahedral particles, high energy electron diffraction patterns and image profiles have been derived for various high symmetry orientations of the particles with respect to the incident beam. These results form a basis for the identification of small metal particle structures with advanced methods of transmission electron microscopy.

Yang, C. Y.↗
NASA NTRS1982

Resonant Doppler velocimetry in supersonic nitrogen flow

The development of the Resonant Doppler Velocimeter (RDV) is discussed. It is a new nonintrusive laser technique for flow diagnosis. The RDV technique is applied to supersonic nitrogen flow with sodium atoms as tracer particles. The measurements are achieved by shining a tunable single frequency laser beam into the flow. The resonant absorption spectrum of the seeded species is determined by observing the fluorescence signal intensity as a function of excitation wavelength. By comparing the peak absorption wavelength with a reference frequency marker, the flow velocity along the excitation beam can be obtained through the Doppler shift relation. By fitting the spectrum with a theoretical line profile, the static temperature and pressure of the flow an be determined.

Cheng, S. W. S.↗
NASA NTRS1987

Potential for on-orbit manufacture of large space structures using the pultrusion process

On-orbit manufacture of lightweight, high-strength, advanced-composite structures using the pultrusion process is proposed. This process is adaptable to a zero-gravity environment by using preimpregnated graphite-fiber reinforcement systems. The reinforcement material is preimpregnated with a high-performance thermoplastic resin at a ground station, is coiled on spools for compact storage, and is transported into Earth orbit. A pultrusion machine is installed in the Shuttle cargo bay from which very long lengths of the desired structure is fabricated on-orbit. Potential structural profiles include rods, angles, channels, hat sections, tubes, honeycomb-cored panels, and T, H, and I beams. A potential pultrudable thermoplastic/graphite composite material is presented as a model for determining the effect on Earth-to-orbit package density of an on-orbit manufacture, the package density is increased by 132 percent, and payload volume requirement is decreased by 56.3 percent. The fabrication method has the potential for on-orbit manufacture of structural members for space platforms, large space antennas, and long tethers.

Wilson, Maywood L.↗
NASA NTRS2009

Performance of a Cylindrical Hall-Effect Thruster Using Permanent Magnets

While annular Hall thrusters can operate at high efficiency at kW power levels, it is difficult to construct one that operates over a broad envelope from 1 kW down to 100 W while maintaining an efficiency of 45-55%. Scaling to low power while holding the main dimensionless parameters constant requires a decrease in the thruster channel size and an increase in the magnetic field strength. Increasing the magnetic field becomes technically challenging since the field can saturate the miniaturized inner components of the magnetic circuit and scaling down the magnetic circuit leaves very little room for magnetic pole pieces and heat shields. In addition, the central magnetic pole piece defining the interior wall of the annular channel can experience excessive heat loads in a miniaturized Hall thruster, with the temperature eventually exceeding the Curie temperature of the material and in extreme circumstances leading to accelerated erosion of the channel wall. An alternative approach is to employ a cylindrical Hall thruster (CHT) geometry. Laboratory model CHTs have operated at power levels ranging from 50 W up to 1 kW. These thrusters exhibit performance characteristics that are comparable to conventional, annular Hall thrusters of similar size. Compared to the annular Hall thruster, the CHTs insulator surface area to discharge chamber volume ratio is lower. Consequently, there is the potential for reduced wall losses in the channel of a CHT, and any reduction in wall losses should translate into lower channel heating rates and reduced erosion, making the CHT geometry promising for low-power applications. This potential for high performance in the low-power regime has served as the impetus for research and development efforts aimed at understanding and improving CHT performance. Recently, a 2.6 cm channel diameter permanent magnet CHT (shown in Fig. 1) was tested. This thruster has the promise of reduced power consumption over previous CHT iterations that employed electromagnets. Data are presented to expose the effect different controllable parameters have on the discharge and to summarize performance measurements (thrust, Isp, efficiency) obtained using a thrust stand. In addition, beam current data are presented to show the effect of the magnetic field topology on the plume profile and current utilization and to gain insight into the thruster s operation. These data extend and improve upon the results previously presented by the authors in Ref. [1].

Polzin, Kurt A.↗
NASA NTRS2017

Hydrogen Balmer Line Broadening in Solar and Stellar Flares

The broadening of the hydrogen lines during flares is thought to result from increased charge (electron, proton) density in the flare chromosphere. However, disagreements between theory and modeling prescriptions have precluded an accurate diagnostic of the degree of ionization and compression resulting from flare heating in the chromosphere. To resolve this issue, we have incorporated the unified theory of electric pressure broadening of the hydrogen lines into the non-LTE radiative-transfer code RH. This broadening prescription produces a much more realistic spectrum of the quiescent, A0 star Vega compared to the analytic approximations used as a damping parameter in the Voigt profiles. We test recent radiative-hydrodynamic (RHD) simulations of the atmospheric response to high nonthermal electron beam fluxes with the new broadening prescription and find that the Balmer lines are overbroadened at the densest times in the simulations. Adding many simultaneously heated and cooling model loops as a 'multithread' model improves the agreement with the observations. We revisit the three component phenomenological flare model of the YZ CMi Megaflare using recent and new RHD models. The evolution of the broadening, line flux ratios, and continuum flux ratios are well-reproduced by a multithread model with high-flux nonthermal electron beam heating, an extended decay phase model, and a 'hot spot' atmosphere heated by an ultra relativistic electron beam with reasonable filling factors: approximately 0.1%, 1%, and 0.1% of the visible stellar hemisphere, respectively. The new modeling motivates future work to understand the origin of the extended gradual phase emission.

Kowalski, Adam F.↗
DOE OSTI2013

Dosimetric properties of an amorphous silicon EPID for verification of modulated electron radiotherapy

Purpose: To investigate the dosimetric properties of an electronic portal imaging device (EPID) for electron beam detection and to evaluate its potential for quality assurance (QA) of modulated electron radiotherapy (MERT). Methods: A commercially available EPID was used to detect electron beams shaped by a photon multileaf collimator (MLC) at a source-surface distance of 70 cm. The fundamental dosimetric properties such as reproducibility, dose linearity, field size response, energy response, and saturation were investigated for electron beams. A new method to acquire the flood-field for the EPID calibration was tested. For validation purpose, profiles of open fields and various MLC fields (square and irregular) were measured with a diode in water and compared to the EPID measurements. Finally, in order to use the EPID for QA of MERT delivery, a method was developed to reconstruct EPID two-dimensional (2D) dose distributions in a water-equivalent depth of 1.5 cm. Comparisons were performed with film measurement for static and dynamic monoenergy fields as well as for multienergy fields composed by several segments of different electron energies. Results: The advantageous EPID dosimetric properties already known for photons as reproducibility, linearity with dose, and dose rate were found to be identical for electron detection. The flood-field calibration method was proven to be effective and the EPID was capable to accurately reproduce the dose measured in water at 1.0 cm depth for 6 MeV, 1.3 cm for 9 MeV, and 1.5 cm for 12, 15, and 18 MeV. The deviations between the output factors measured with EPID and in water at these depths were within {+-}1.2% for all the energies with a mean deviation of 0.1%. The average gamma pass rate (criteria: 1.5%, 1.5 mm) for profile comparison between EPID and measurements in water was better than 99% for all the energies considered in this study. When comparing the reconstructed EPID 2D dose distributions at 1.5 cm depth to film measurements, the gamma pass rate (criteria: 2%, 2 mm) was better than 97% for all the tested cases. Conclusions: This study demonstrates the high potential of the EPID for electron dosimetry, and in particular, confirms the possibility to use it as an efficient verification tool for MERT delivery.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2014

Si etching with reactive neutral beams of very low energy

A Si etching process has been investigated with reactive neutral beams (NBs) extracted using a low acceleration voltage of less than 100 V from CF{sub 4} and Ar mixed plasmas. The etched Si profile shows that the etching process is predominantly anisotropic. The reactive NB has a constant Si etching rate in the acceleration voltage range from 20 V to 80 V. It is considered that low-energy NBs can trigger Si etching because F radicals adsorb onto the Si surface and weaken Si–Si bonds. The etching rate per unit beam flux is 33 times higher than that with Ar NB. These results show that the low-energy reactive NB is useful for damage-free high speed Si etching.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Uniaxial, Pure Bending, and Column Buckling Experiments on Superelastic NiTi Rods and Tubes

Many existing shape memory alloy (SMA) devices consist of slender beams and frames. To better understand SMA beam behavior, we experimentally examined the isothermal, room temperature response of superelastic NiTi rods and tubes, of similar outer diameters, subjected to four different modes of loading. Pure tension, pure compression, and pure bending experiments were first performed to establish and compare the baseline uniaxial and bending behaviors of rods and tubes. Column buckling experiments were then performed on rod and tube columns of several slenderness ratios to investigate their mechanical responses, phase transformation kinetics under combined uniaxial and bending deformation, and the interaction between material and structural instabilities. In all experiments, stereo digital image correlation measured local displacement fields in order to capture phenomena such as strain localization and propagating phase boundaries. Superelastic mechanical behavior and the nature of stress-induced phase transformation were found to be strongly affected by specimen geometry and the deformation mode. Under uniaxial tension, both the rod and tube had well-defined loading and unloading plateaus in their superelastic responses, during which stress-induced phase transformation propagated along the length of the specimen in the form of a high/low strain front. Due to the dependence of strain localization on kinematic compatibility, the high/low strain front morphologies differed between the rod and tube: for the rod, the high/low strain front consisted of a diffuse “neck”, while the high/low strain front in the tube consisted of distinct, criss-crossing “fingers.” During uniaxial compression, both cross-sectional forms exhibited higher transformation stresses and smaller transformation strains than uniaxial tension, highlighting the now well-known tension-compression asymmetry of SMAs. Additionally, phase transformation localization and propagation were absent under compressive loading. During pure bending, the moment-curvature response of both forms exhibited plateaus and strain localization during forward and reverse transformations. Rod specimens developed localized, high-curvature regions that propagated along the specimen axis and caused shear strain near the high/low curvature interface; whereas, the tube specimens exhibited finger/wedge-like high strain regions over the tensile side of the tube which caused nonlinear strain profiles through the thickness of the specimen that did not propagate. Here, it was therefore found that classical beam theory assumptions did not hold in the presence of phase transformation localization (although, the assumptions did hold on average for the tube). During column buckling, the structures were loaded into the post-buckling regime yet recovered nearly-straight forms upon unloading. Strain localization was observed only for high aspect ratio (slender) tubes, but the mechanical responses were similar to that of rods of the same slenderness ratio. Also, an interesting “unbuckling” phenomenon was discovered in certain low aspect ratio (stout) columns, where late post-buckling straightening was observed despite continuous monotonic loading. Thus, these behaviors are some of the challenging phenomena which must be captured when developing SMA constitutive models and executing structural simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Time constant of defect relaxation in ion-irradiated 3 C -SiC

Above room temperature, the buildup of radiation damage in SiC is a dynamic process governed by the mobility and interaction of ballistically generated point defects. Here in this work, we study the dynamics of radiation defects in 3C-SiC bombarded at 100 °C with 500 keV Ar ions, with the total ion dose split into a train of equal pulses. Damage–depth profiles are measured by ion channeling for a series of samples irradiated under identical conditions except for different durations of the passive part of the beam cycle. Results reveal an effective defect relaxation time constant of ~3 ms (for second order kinetics) and a dynamic annealing efficiency of ~40% for defects in both Si and C sublattices. Finally, this demonstrates a crucial role of dynamic annealing at elevated temperatures and provides evidence of the strong coupling of defect accumulation processes in the two sublattices of 3C-SiC.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Control of internal transport barriers in magnetically confined tokamak burning plasmas

The initiation, termination, and control of internal transport barriers associated with E × B flow shear near local minima of magnetic shear are examined for burning plasmas to determine if the positive feedback loops between profiles, instability, transport, and flow shear operate in regimes with fusion self-heating. A five-field transport model for the evolution of profiles of density, ion and electron temperature, ion and electron fluctuations, and radial electric field is utilized to examine the efficacy of controls associated with external inputs of heat and particles, including neutral beam injection, RF, pellets, and gas puffing. The response of the plasma to these inputs is studied in the presence of self-heating. The latter is affected by the external inputs and their modification of profiles and is, therefore, not an external control. Provided sufficient external power is applied, internal transport barriers can be created and controlled, both in ion and electron channels. Barrier control is sensitive to the locations of power deposition and pellet ablation, as well as temporal sequencing of external inputs.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

SU-F-T-431: Dosimetric Validation of Acuros XB Algorithm for Photon Dose Calculation in Water

Purpose: To validate the Acuros XB algorithm implemented in Eclipse Treatment planning system version 11 (Varian Medical System, Inc., Palo Alto, CA, USA) for photon dose calculation. Methods: Acuros XB is a Linear Boltzmann transport equation (LBTE) solver that solves LBTE equation explicitly and gives result equivalent to Monte Carlo. 6MV photon beam from Varian Clinac-iX (2300CD) was used for dosimetric validation of Acuros XB. Percentage depth dose (PDD) and profiles (at dmax, 5, 10, 20 and 30 cm) measurements were performed in water for field size ranging from 2×2,4×4, 6×6, 10×10, 20×20, 30×30 and 40×40 cm{sup 2}. Acuros XB results were compared against measurements and anisotropic analytical algorithm (AAA) algorithm. Results: Acuros XB result shows good agreement with measurements, and were comparable to AAA algorithm. Result for PDD and profiles shows less than one percent difference from measurements, and from calculated PDD and profiles by AAA algorithm for all field size. TPS calculated Gamma error histogram values, average gamma errors in PDD curves before dmax and after dmax were 0.28, 0.15 for Acuros XB and 0.24, 0.17 for AAA respectively, average gamma error in profile curves in central region, penumbra region and outside field region were 0.17, 0.21, 0.42 for Acuros XB and 0.10, 0.22, 0.35 for AAA respectively. Conclusion: The dosimetric validation of Acuros XB algorithms in water medium was satisfactory. Acuros XB algorithm has potential to perform photon dose calculation with high accuracy, which is more desirable for modern radiotherapy environment.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-F-T-577: Comparison of Small Field Dosimetry Measurements in Fields Shaped with Conical Applicators On Two Different Accelerating Systems

Purpose: To investigate small field dosimetry measurements and associated uncertainties when conical applicators are used to shape treatment fields from two different accelerating systems. Methods: Output factor measurements are made in water in beams from the CyberKnife radiosurgery system, which uses conical applicators to shape fields from a (flattening filter-free) 6 MV beam, and in a 6 MV beam from the Elekta Precise linear accelerator (with flattening filter) with BrainLab external conical applicators fitted to shape the field. The measurements use various detectors: (i) an Exradin A16 ion chamber, (ii) two Exradin W1 plastic scintillation detectors, (iii) a Sun Nuclear Edge diode, and (iv) two PTW microDiamond synthetic diamond detectors. Profiles are used for accurate detector positioning and to specify field size (FWHM). Output factor measurements are corrected with detector specific correction factors taken from the literature where available and/or from Monte Carlo simulations using the EGSnrc code system. Results: Differences in measurements of up to 1.7% are observed with a given detector type in the same beam (i.e., intra-detector variability). Corrected results from different detectors in the same beam (inter-detector differences) show deviations up to 3 %. Combining data for all detectors and comparing results from the two accelerators results in a 5.9% maximum difference for the smallest field sizes (FWHM=5.2–5.6 mm), well outside the combined uncertainties (∼1% for the smallest beams) and/or differences among detectors. This suggests that the FWHM of a measured profile is not a good specifier to compare results from different small fields with the same nominal energy. Conclusion: Large differences in results for both intra-detector variability and inter-detector differences suggest potentially high uncertainties in detector-specific correction factors. Differences between the results measured in circular fields from different accelerating systems provide insight into sources of variability in small field dosimetric measurements reported in the literature.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2013

Superficial dosimetry imaging based on Čerenkov emission for external beam radiotherapy with megavoltage x-ray beam

Purpose: Čerenkov radiation emission occurs in all tissue, when charged particles (either primary or secondary) travel at velocity above the threshold for the Čerenkov effect (about 220 KeV in tissue for electrons). This study presents the first examination of optical Čerenkov emission as a surrogate for the absorbed superficial dose for MV x-ray beams.Methods: In this study, Monte Carlo simulations of flat and curved surfaces were studied to analyze the energy spectra of charged particles produced in different regions near the surfaces when irradiated by MV x-ray beams. Čerenkov emission intensity and radiation dose were directly simulated in voxelized flat and cylindrical phantoms. The sampling region of superficial dosimetry based on Čerenkov radiation was simulated in layered skin models. Angular distributions of optical emission from the surfaces were investigated. Tissue mimicking phantoms with flat and curved surfaces were imaged with a time domain gating system. The beam field sizes (50 × 50–200 × 200 mm{sup 2}), incident angles (0°–70°) and imaging regions were all varied.Results: The entrance or exit region of the tissue has nearly homogeneous energy spectra across the beam, such that their Čerenkov emission is proportional to dose. Directly simulated local intensity of Čerenkov and radiation dose in voxelized flat and cylindrical phantoms further validate that this signal is proportional to radiation dose with absolute average discrepancy within 2%, and the largest within 5% typically at the beam edges. The effective sampling depth could be tuned from near 0 up to 6 mm by spectral filtering. The angular profiles near the theoretical Lambertian emission distribution for a perfect diffusive medium, suggesting that angular correction of Čerenkov images may not be required even for curved surface. The acquisition speed and signal to noise ratio of the time domain gating system were investigated for different acquisition procedures, and the results show there is good potential for real-time superficial dose monitoring. Dose imaging under normal ambient room lighting was validated, using gated detection and a breast phantom.Conclusions: This study indicates that Čerenkov emission imaging might provide a valuable way to superficial dosimetry imaging in real time for external beam radiotherapy with megavoltage x-ray beams.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2016

A Complete Reporting of MCNP6 Validation Results for Electron Energy Deposition in Single-Layer Extended Media for Source Energies <= 1-MeV

In this paper, we expand on previous validation work by Dixon and Hughes. That is, we present a more complete suite of validation results with respect to to the well-known Lockwood energy deposition experiment. Lockwood et al. measured energy deposition in materials including beryllium, carbon, aluminum, iron, copper, molybdenum, tantalum, and uranium, for both single- and multi-layer 1-D geometries. Source configurations included mono-energetic, mono-directional electron beams with energies of 0.05-MeV, 0.1-MeV, 0.3- MeV, 0.5-MeV, and 1-MeV, in both normal and off-normal angles of incidence. These experiments are particularly valuable for validating electron transport codes, because they are closely represented by simulating pencil beams incident on 1-D semi-infinite slabs with and without material interfaces. Herein, we include total energy deposition and energy deposition profiles for the single-layer experiments reported by Lockwood et al. (a more complete multi-layer validation will follow in another report).

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Rotation profile flattening and toroidal flow shear reversal due to the coupling of magnetic islands in tokamaks

The electromagnetic coupling of helical modes, even those having different toroidal mode numbers, modifies the distribution of toroidal angular momentum in tokamak discharges. This can have deleterious effects on other transport channels as well as on magnetohydrodynamic (MHD) stability and disruptivity. At low levels of externally injected momentum, the coupling of core-localized modes initiates a chain of events, whereby flattening of the core rotation profile inside successive rational surfaces leads to the onset of a large m/n = 2/1 tearing mode and locked-mode disruption. With increased torque from neutral beam injection, neoclassical tearing modes in the core may phase-lock to each other without locking to external fields or structures that are stationary in the laboratory frame. The dynamic processes observed in these cases are in general agreement with theory, and detailed diagnosis allows for momentum transport analysis to be performed, revealing a significant torque density that peaks near the 2/1 rational surface. However, as the coupled rational surfaces are brought closer together by reducing q{sub 95}, additional momentum transport in excess of that required to attain a phase-locked state is sometimes observed. Rather than maintaining zero differential rotation (as is predicted to be dynamically stable by single-fluid, resistive MHD theory), these discharges develop hollow toroidal plasma fluid rotation profiles with reversed plasma flow shear in the region between the m/n = 3/2 and 2/1 islands. The additional forces expressed in this state are not readily accounted for, and therefore, analysis of these data highlights the impact of mode coupling on torque balance and the challenges associated with predicting the rotation dynamics of a fusion reactor—a key issue for ITER.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Radar - ANL Wind Profiler with RASS, Yakima - Raw Data

**Winds** A radar wind profiler measures the Doppler shift of electromagnetic energy scattered back from atmospheric turbulence and hydrometeors along 3-5 vertical and off-vertical point beam directions. Back-scattered signal strength and radial-component velocities are remotely sensed along all beam directions and combined to derive the horizontal wind field over the radar. These data typically are sampled and averaged hourly and usually have 6-m and/or 100-m vertical resolutions up to 4 km for the 915 MHz and 8 km for the 449 MHz systems. **Temperature** To measure atmospheric temperature, a radio acoustic sound system (RASS) is used in conjunction with the wind profile. These data typically are sampled and averaged for five minutes each hour and have a 60-m vertical resolution up to 1.5 km for the 915 MHz and 60-m up to 3.5k m for the 449 MHz.

17 WIND ENERGY↗