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At least 775 records · Page 43

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DOE OSTI2016

SU-F-T-530: Characterization of a 60-Leaf Motorized MLC Designed for Cobalt-60 Units

Purpose: In a continuing effort to improve conformal radiation therapy with Cobalt-60 units, a 60-leaf MLC was designed, manufactured, and released to market. This work describes the physics measurements taken to characterize the clinical performance of this MLC. Methods: A 60 leaf MLC was custom designed with tungsten leaves of 4.5 cm height, single focused, achieving field size of 30×30 cm^2 when mounted on a 100cm SAD Cobalt-60 unit. Leakage and output factor measurements were performed using a single ion chamber in a solid water phantom. Penumbra and surface dose were measured using scanning chambers and diodes in a water phantom. Radiation-light coincidence measurements were performed using radiographic films. Results: With MLC mounted, measured penumbras at all depths are smaller than with jaws only. Surface doses were not significantly affected by the presence of MLC, and remained below values recommended by regulatory bodies. Light-radiation coincidences were found to be better than 3 mm for all field sizes. Leakage through the MLC was found to be strongly dependent on field size, increasing from 1.0 % for a 10×10 cm field to 2.0% for a 30×30 cm field. Such results meet the requirements of IEC 60601-2-11. The MLC was found to have significant influence on the output factor, when field size defined by MLC is significantly smaller than field size defined by jaws. Such effect is also observed on linear accelerators, but it is more pronounced on Cobalt-60 units. A 10×10 “diamond” MLC shape inside a 14×14 cm jaw showed output factor that is 5.7% higher than 10×10 cm field defined by matching MLC and jaws. Conclusion: The MLC offers clinically acceptable performance in penumbra, surface dose, and light-radiation coincidence. Several units of this MLC have recently been installed and used clinically. Validation of Cobalt-60 based IMRT with this MLC is ongoing. The authors are employees of Best Theratrnics Ltd.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

FPGA-based Klystron linearization implementations in scope of ILC

We report the development and implementation of four FPGA-based predistortion-type klystron linearization algorithms. Klystron linearization is essential for the realization of ILC, since it is required to operate the klystrons 7% in power below their saturation. The work presented was performed in international collaborations at the Fermi National Accelerator Laboratory (FNAL), USA and the Deutsches Elektronen Synchrotron (DESY), Germany. With the newly developed algorithms, the generation of correction factors on the FPGA was improved compared to past algorithms, avoiding quantization and decreasing memory requirements. At FNAL, three algorithms were tested at the Advanced Superconducting Test Accelerator (ASTA), demonstrating a successful implementation for one algorithm and a proof of principle for two algorithms. Furthermore, the functionality of the algorithm implemented at DESY was demonstrated successfully in a simulation.

43 PARTICLE ACCELERATORS↗

NASA NTRS1984

Accelerated convergence for incompressible flow calculations

Two improved algorithms which solve the steady-state Navier-Stokes equations, PISO and SIMPLER, are studied. Computations were carried out on progressively finer grids for the driven cavity and flow over a backward-facing step. The effects of relaxation factor, number of grid nodes and number of sweeps through the pressure equations are studied to evaluate the performance of the PISO and SIMPLER schemes. Results show that these improved schemes accelerate the convergence rate of the solution generally by a factor of two as compared to the SIMPLE method.

Neely, G. M.↗

NASA NTRS1985

Accelerated convergence for incompressible flow calculations

Neely, G. M.↗

NASA NTRS1989

Calculation of structural dynamic forces and stresses using mode acceleration

While the standard mode acceleration formulation in structural dynamics has often been interpreted to suggest that the reason for improved convergence obtainable is that the dynamic correction factor is divided by the modal frequencies-squared, an alternative formulation is presented which clearly indicates that the only difference between mode acceleration and mode displacement data recovery is the addition of a static correction term. Attention is given to the advantages in numerical implementation associated with this alternative, as well as to an illustrative example.

Blelloch, Paul↗

NASA NTRS2003

Test of the Equivalence Principle in an Einstein Elevator

The laboratory activity consisted in the construction of a laboratory prototype of a differential accelerometer. The laboratory prototype has been used to conduct key tests on the differential instrument. We demonstrated the ability to damp quickly transient oscillations by utilizing a resistive load in the feedback loops and then removing that load to reestablish a high quality factor of the detector. A rotating divide with tilt control was also built. This device was utilized to impart (through the Earth's gravity) common-mode perturbations to the differential accelerometer. These calibration disturbances have been used to trim the acceleration outputs of the individual proof masses in order to obtain a common-mode rejection factor better than 10(exp -4) in a sufficiently large frequency band centered at the spin frequency.

Shapiro, Irwin I.↗

DOE OSTI2020

Recent and planned hydrodynamic instability experiments on indirect-drive implosions on the National Ignition Facility

At National Ignition Facility (NIF), yield amplification due to alpha particle heating approached ~3 in the highest performing inertial confinement fusion (ICF) implosions, while yield amplification of ~15-30 is needed for ignition. Hydrodynamic instabilities are a major factor in degradation of implosions while understanding and mitigation of the instabilities are critical to achieving ignition. This article describes recent and planned hydrodynamic instability experiments with several focused platforms that have been developed to directly measure these instabilities in all phases of ICF implosions. Measurements of ripple-shock generation at OMEGA laser have indicated initial seeds for the instabilities in three ablators - plastic (CH), beryllium, and high-density carbon (HDC). Hydrodynamic Growth Radiography (HGR) platform was used to measure instability growth at the ablation front in the acceleration phase of implosions. This platform used pre-imposed 2-D perturbations for growth factor measurements at different perturbation wavelengths and was also used to measure growth of “native roughness” modulations, fill tubes, and capsule support membranes or “tents”. Also, in the acceleration phase several new experimental platforms have been or are being developed to measure instability growth at the ablator-ice interface. In the deceleration phase of implosions, “self-emission” and “self-backlighting” platforms were developed to measure perturbations near peak compression. This article reviews recent progress and results.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

NASA NTRS1991

Stereoscopic camera and viewing systems with undistorted depth presentation and reduced or eliminated erroneous acceleration and deceleration perceptions, or with perceptions produced or enhanced for special effects

Methods for providing stereoscopic image presentation and stereoscopic configurations using stereoscopic viewing systems having converged or parallel cameras may be set up to reduce or eliminate erroneously perceived accelerations and decelerations by proper selection of parameters, such as an image magnification factor, q, and intercamera distance, 2w. For converged cameras, q is selected to be equal to Ve - qwl = 0, where V is the camera distance, e is half the interocular distance of an observer, w is half the intercamera distance, and l is the actual distance from the first nodal point of each camera to the convergence point, and for parallel cameras, q is selected to be equal to e/w. While converged cameras cannot be set up to provide fully undistorted three-dimensional views, they can be set up to provide a linear relationship between real and apparent depth and thus minimize erroneously perceived accelerations and decelerations for three sagittal planes, x = -w, x = 0, and x = +w which are indicated to the observer. Parallel cameras can be set up to provide fully undistorted three-dimensional views by controlling the location of the observer and by magnification and shifting of left and right images. In addition, the teachings of this disclosure can be used to provide methods of stereoscopic image presentation and stereoscopic camera configurations to produce a nonlinear relation between perceived and real depth, and erroneously produce or enhance perceived accelerations and decelerations in order to provide special effects for entertainment, training, or educational purposes.

Diner, Daniel B.↗

DOE OSTI2015

TU-F-CAMPUS-T-04: Variations in Nominally Identical Small Fields From Photon Jaw Reproducibility and Associated Effects On Small Field Dosimetric Parameters

Purpose: To investigate uncertainties in small field output factors and detector specific correction factors from variations in field size for nominally identical fields using measurements and Monte Carlo simulations. Methods: Repeated measurements of small field output factors are made with the Exradin W1 (plastic scintillation detector) and the PTW microDiamond (synthetic diamond detector) in beams from the Elekta Precise linear accelerator. We investigate corrections for a 0.6x0.6 cm{sup 2} nominal field size shaped with secondary photon jaws at 100 cm source to surface distance (SSD). Measurements of small field profiles are made in a water phantom at 10 cm depth using both detectors and are subsequently used for accurate detector positioning. Supplementary Monte Carlo simulations with EGSnrc are used to calculate the absorbed dose to the detector and absorbed dose to water under the same conditions when varying field size. The jaws in the BEAMnrc model of the accelerator are varied by a reasonable amount to investigate the same situation without the influence of measurements uncertainties (such as detector positioning or variation in beam output). Results: For both detectors, small field output factor measurements differ by up to 11 % when repeated measurements are made in nominally identical 0.6x0.6 cm{sup 2} fields. Variations in the FWHM of measured profiles are consistent with field size variations reported by the accelerator. Monte Carlo simulations of the dose to detector vary by up to 16 % under worst case variations in field size. These variations are also present in calculations of absorbed dose to water. However, calculated detector specific correction factors are within 1 % when varying field size because of cancellation of effects. Conclusion: Clinical physicists should be aware of potentially significant uncertainties in measured output factors required for dosimetry of small fields due to field size variations for nominally identical fields.

07 ISOTOPE AND RADIATION SOURCES↗

DOE OSTI2016

Intermediate accelerated solutions as generic late-time attractors in a modified Jordan-Brans-Dicke theory

In this paper we investigate the evolution of a Jordan-Brans-Dicke scalar field, Φ, with a power-law potential in the presence of a second scalar field, φ, with an exponential potential, in both the Jordan and the Einstein frames. We present the relation of our model with the induced gravity model with power-law potential and the integrability of this kind of models is discussed when the quintessence field φ is massless, and has a small velocity. The fact that for some fine-tuned values of the parameters we may get some integrable cosmological models, makes our choice of potentials very interesting. We prove that in Jordan-Brans-Dicke theory, the de Sitter solution is not a natural attractor. Instead, we show that the attractor in the Jordan frame corresponds to an ''intermediate accelerated'' solution of the form a(t) ≅ e{sup α{sub 1} t{sup p{sup {sub 1}}}}, as t → ∞ where α{sub 1} > 0 and 0 < p{sub 1} < 1, for a wide range of parameters. Furthermore, when we work in the Einstein frame we get that the attractor is also an ''intermediate accelerated'' solution of the form a(t) ≅ e{sup α{sub 2} tp{sub 2}} as t → ∞ where α{sub 2} > 0 and 0<p{sub 2}<1, for the same conditions on the parameter space as in the Jordan frame. In the special case of a quadratic potential in the Jordan frame, or for a constant potential in the Einstein's frame, the above intermediate solutions are of saddle type. These results were proved using the center manifold theorem, which is not based on linear approximation. Finally, we present a specific elaboration of our extension of the induced gravity model in the Jordan frame, which corresponds to a particular choice of a linear potential of Φ. The dynamical system is then reduced to a two dimensional one, and the late-time attractor is linked with the exact solution found for the induced gravity model. In this example the ''intermediate accelerated'' solution does not exist, and the attractor solution has an asymptotic de Sitter-like evolution law for the scale factor. Apart from some fine-tuned examples such as the linear, and quadratic potential U(Φ) in the Jordan frame, it is true that ''intermediate accelerated'' solutions are generic late-time attractors in a modified Jordan-Brans-Dicke theory.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

Some cosmological consequences of Weyl invariance

We examine some Weyl invariant cosmological models in the framework of generalized dilaton gravity, in which the action is made of a set of N conformally coupled scalar fields. It will be shown that when the FRW ansatz for the spacetime metric is assumed, the Ward identity for conformal invariance guarantees that the gravitational equations hold whenever the scalar fields EM do so. It follows that any scale factor can solve the theory provided a non-trivial profile for a dilaton field. In particular, accelerated expansion is a natural solution to the full set of equations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2022

Adaptive time scaling for accelerating explicit finite element analysis

A method for accelerating an explicit finite element analysis (FEA) simulation of a modeled system or process includes performing an initial iteration of the FEA simulation according to a baseline time interval via an FEA computing network, and calculating a criteria ratio of a predetermined set of scaling criteria for the modeled system or process. The method includes determining a time-scaling factor using the criteria ratio via the FEA computing network as a function of the criteria ratio, and then applying the time-scaling factor to the baseline time interval to generate a scaled time interval. The scaled time interval accelerates simulation time of the FEA simulation. The method includes performing a subsequent iteration of the explicit FEA simulation at the scaled time interval using the FEA computing network. The process continues for subsequent iterations, with the time-scaling factor adapting with each iteration.

Chen, Jian↗

DOE OSTI2019

Hierarchical sensitivity analysis for simulating barrier island geomorphologic responses to future storms and sea-level rise

This paper presents a new application of an advanced hierarchical sensitivity analysis of a new climate model of barrier island geomorphological evolution. The implemented sensitivity analysis in this study integrates a hierarchical uncertainty framework with a variance-based global sensitivity analysis to decompose the different model input uncertainties. The analysis can provide quantitative and accurate measurements for the relative importance of uncertain model input factors while considering their dependence relationships. The climate model used in this research was the barrier island profile (BIP) model, which is a new computer code developed to simulate barrier island morphological evolution over periods ranging from years to decades under the impacts of accelerated future sea-level rise and long-term changes in the storm climate. In the application of the model, the BIP model was used to evaluate the responses of a series of barrier island cross-sections derived for Santa Rosa Island, Florida, to random storm events and five potential accelerated rates of sea-level rise projected over the next century. The uncertain model input factors thus include the scenario uncertainty caused by alternative future sea-level rise scenarios and the parametric uncertainties of random storm parameters and dune characteristics. The study results reveal that the occurrence of storms is the most important factor for the evolution of sand dunes on the barrier island and the impact of sea-level rise is essential to the morphological change of the island backshore environment. The analysis can provide helpful insights for coastal management and planning. This hierarchical sensitivity analysis is mathematically general and rigorous and can be applied to a wide range of climate models.

Dai, Heng↗

DOE OSTI2013

DIFFUSIVE SHOCK ACCELERATION AT COSMOLOGICAL SHOCK WAVES

We reexamine nonlinear diffusive shock acceleration (DSA) at cosmological shocks in the large-scale structure of the universe, incorporating wave-particle interactions that are expected to operate in collisionless shocks. Adopting simple phenomenological models for magnetic field amplification (MFA) by cosmic-ray (CR) streaming instabilities and Alfvenic drift, we perform kinetic DSA simulations for a wide range of sonic and Alfvenic Mach numbers and evaluate the CR injection fraction and acceleration efficiency. In our DSA model, the CR acceleration efficiency is determined mainly by the sonic Mach number M{sub s} , while the MFA factor depends on the Alfvenic Mach number and the degree of shock modification by CRs. We show that at strong CR modified shocks, if scattering centers drift with an effective Alfven speed in the amplified magnetic field, the CR energy spectrum is steepened and the acceleration efficiency is reduced significantly, compared to the cases without such effects. As a result, the postshock CR pressure saturates roughly at {approx}20% of the shock ram pressure for strong shocks with M{sub s} {approx}> 10. In the test-particle regime (M{sub s} {approx}< 3), it is expected that the magnetic field is not amplified and the Alfvenic drift effects are insignificant, although relevant plasma physical processes at low Mach number shocks remain largely uncertain.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1985

A simple microgravity table for the Orbiter or Space Station

Methods of limiting perturbations in microgravity experiments are proposed. An acceleration level below 10 to the -4th m/s-squared is necessary to maintain an undisturbed microgravity environment. Machinery vibrations, crew motion, and the firing of vernier thrusters produce acceleration levels greate than 10 to the -4th m/s-squared. The use of a weak spring system or simple electromagnets to isolate an experimental table from these factors is described. The manners in which crew motion and vernier firing are countered by the springs are examined. The steady acceleration caused by atmospheric drag, gravity gradient force, and steady rotation can be maintained below 10 to the -th m/s-squared; however, the springs can protect the table from these accelerations if required.

Garriott, O. K.↗

NASA NTRS1996

Orbiter Aerodynamic Acceleration Flight Measurements in the Rarefied-Flow Transition Regime

Acceleration data taken from the Orbital Acceleration Research Experiment (OARE) during reentry on STS-62 have been analyzed using calibration factors taken on orbit. This is the first Orbiter mission which collected OARE data during the Orbiter reentry phase. The data examined include the flight regime from orbital altitudes down to about 90 km which covers the free-molecule-flow regime and the upper altitude fringes of the rarefied-flow transition into the hypersonic continuum. Ancillary flight data on Orbiter position, orientation, velocity, and rotation rates have been used in models to transform the measured accelerations to the Orbiter center-of-gravity, from which aerodynamic accelerations along the Orbiter body axes have been calculated. Residual offsets introduced in the measurements by unmodeled Orbiter forces are identified and discussed. Direct comparisons are made between the OARE flight data and an independent micro-gravity accelerometer experiment, the High Resolution Accelerometer Package (HiRAP), which also obtained flight data on reentry during the mission down to about 95 km. The resulting OARE aerodynamic acceleration measurements along the Orbiter's body axis, aid the normal to axial acceleration ratio in the free-molecule-flow and transition-flow regimes are presented and compared with numerical simulations from three direct simulation Monte Carlo codes.

Blanchard, Robert C.↗

DOE OSTI2020

Accelerating Radiation Computations for Dynamical Models With Targeted Machine Learning and Code Optimization

Abstract Atmospheric radiation is the main driver of weather and climate, yet due to a complicated absorption spectrum, the precise treatment of radiative transfer in numerical weather and climate models is computationally unfeasible. Radiation parameterizations need to maximize computational efficiency as well as accuracy, and for predicting the future climate many greenhouse gases need to be included. In this work, neural networks (NNs) were developed to replace the gas optics computations in a modern radiation scheme (RTE+RRTMGP) by using carefully constructed models and training data. The NNs, implemented in Fortran and utilizing BLAS for batched inference, are faster by a factor of 1–6, depending on the software and hardware platforms. We combined the accelerated gas optics with a refactored radiative transfer solver, resulting in clear‐sky longwave (shortwave) fluxes being 3.5 (1.8) faster to compute on an Intel platform. The accuracy, evaluated with benchmark line‐by‐line computations across a large range of atmospheric conditions, is very similar to the original scheme with errors in heating rates and top‐of‐atmosphere radiative forcings typically below 0.1 K day −1 and 0.5 W m −2 , respectively. These results show that targeted machine learning, code restructuring techniques, and the use of numerical libraries can yield material gains in efficiency while retaining accuracy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Nonideal Fields Solve the Injection Problem in Relativistic Reconnection

Magnetic reconnection in relativistic plasmas is well established as a fast and efficient particle accelerator, capable of explaining the most dramatic astrophysical flares. With particle-in-cell simulations, we demonstrate the importance of nonideal fields for the early stages (“injection”) of particle acceleration. Most of the particles ending up with high energies (near or above the mean magnetic energy per particle) must have passed through nonideal regions where the assumptions of ideal magnetohydrodynamics are broken (i.e., regions with E > B or nonzero E ∥ = E ∙ B/B), whereas most of the particles that do not experience nonideal fields end up with Lorentz factors of order unity. So, injection by nonideal fields is a necessary prerequisite for further acceleration. Our results have important implications for the origin of nonthermal particles in high-energy astrophysical sources.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗