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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

SU-E-T-552: Minimum Monitor Unit Effects On Plan Quality for Multi-Field Optimized Spot Scanning Proton Therapy

Purpose: To investigate the influence of the minimum monitor unit (MU) on the quality of clinical treatment plans for scanned proton therapy. Methods: Delivery system characteristics limit the minimum number of protons that can be delivered per spot, resulting in a min-MU limit. Plan quality can be impacted by the min-MU limit. Two sites were used to investigate the impact of min-MU on treatment plans: pediatric brain tumor at a depth of 5-10 cm; a head and neck tumor at a depth of 1-20 cm. Three field intensity modulated spot scanning proton plans were created for each site with the following parameter variations: min-MU limit range of 0.0000-0.0060; and spot spacing range of 0.5-2.0σ of the nominal spot size at isocenter in water (σ=4mm in this work). Comparisons were based on target homogeneity and normal tissue sparing. Results: The increase of the min-MU with a fixed spot spacing decreases plan quality both in homogeneous target coverage and in the avoidance of critical structures. Both head and neck and pediatric brain plans show a 20% increase in relative dose for the hot spot in the CTV and 10% increase in key critical structures when comparing min-MU limits of 0.0000 and 0.0060 with a fixed spot spacing of 1σ. The DVHs of CTVs show min-MU limits of 0.0000 and 0.0010 produce similar plan quality and quality decreases as the min-MU limit increases beyond 0.0020. As spot spacing approaches 2σ, degradation in plan quality is observed when no min-MU limit is imposed. Conclusion: Given a fixed spot spacing of ≤ 1σ of the spot size in water, plan quality decreases as min- MU increases greater than 0.0020. The effect of min-MU should be taken into consideration while planning spot scanning proton therapy treatments to realize its full potential.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Effects of minimum monitor unit threshold on spot scanning proton plan quality

Purpose: To investigate the influence of the minimum monitor unit (MU) on the quality of clinical treatment plans for scanned proton therapy. Methods: Delivery system characteristics limit the minimum number of protons that can be delivered per spot, resulting in a min-MU limit. Plan quality can be impacted by the min-MU limit. Two sites were used to investigate the impact of min-MU on treatment plans: pediatric brain tumor at a depth of 5–10 cm; a head and neck tumor at a depth of 1–20 cm. Three-field, intensity modulated spot scanning proton plans were created for each site with the following parameter variations: min-MU limit range of 0.0000–0.0060; and spot spacing range of 2–8 mm. Comparisons were based on target homogeneity and normal tissue sparing. For the pediatric brain, two versions of the treatment planning system were also compared to judge the effects of the min-MU limit based on when it is accounted for in the optimization process (Eclipse v.10 and v.13, Varian Medical Systems, Palo Alto, CA). Results: The increase of the min-MU limit with a fixed spot spacing decreases plan quality both in homogeneous target coverage and in the avoidance of critical structures. Both head and neck and pediatric brain plans show a 20% increase in relative dose for the hot spot in the CTV and 10% increase in key critical structures when comparing min-MU limits of 0.0000 and 0.0060 with a fixed spot spacing of 4 mm. The DVHs of CTVs show min-MU limits of 0.0000 and 0.0010 produce similar plan quality and quality decreases as the min-MU limit increases beyond 0.0020. As spot spacing approaches 8 mm, degradation in plan quality is observed when no min-MU limit is imposed. Conclusions: Given a fixed spot spacing of ≤4 mm, plan quality decreases as min-MU increased beyond 0.0020. The effect of min-MU needs to be taken into consideration while planning proton therapy treatments.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

TH-CD-209-01: A Greedy Reassignment Algorithm for the PBS Minimum Monitor Unit Constraint

Purpose: To investigate a Greedy Reassignment algorithm in order to mitigate the effects of low weight spots in proton pencil beam scanning (PBS) treatment plans. Methods: To convert a plan from the treatment planning system’s (TPS) to a deliverable plan, post processing methods can be used to adjust the spot maps to meets the minimum MU constraint. Existing methods include: deleting low weight spots (Cut method), or rounding spots with weight above/below half the limit up/down to the limit/zero (Round method). An alternative method called Greedy Reassignment was developed in this work in which the lowest weight spot in the field was removed and its weight reassigned equally among its nearest neighbors. The process was repeated with the next lowest weight spot until all spots in the field were above the MU constraint. The algorithm performance was evaluated using plans collected from 190 patients (496 fields) treated at our facility. The evaluation criteria were the γ-index pass rate comparing the pre-processed and post-processed dose distributions. A planning metric was further developed to predict the impact of post-processing on treatment plans for various treatment planning, machine, and dose tolerance parameters. Results: For fields with a gamma pass rate of 90±1%, the metric has a standard deviation equal to 18% of the centroid value. This showed that the metric and γ-index pass rate are correlated for the Greedy Reassignment algorithm. Using a 3rd order polynomial fit to the data, the Greedy Reassignment method had 1.8 times better metric at 90% pass rate compared to other post-processing methods. Conclusion: We showed that the Greedy Reassignment method yields deliverable plans that are closest to the optimized-without-MU-constraint plan from the TPS. The metric developed in this work could help design the minimum MU threshold with the goal of keeping the γ-index pass rate above an acceptable value.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Nearest Neighbor Averaging and its Effect on the Critical Level and Minimum Detectable Concentration for Scanning Radiological Survey Instruments that Perform Facility Release Surveys.

Through the SNL New Mexico Small Business Assistance (NMSBA) program, several Sandia engineers worked with the Environmental Restoration Group (ERG) Inc. to verify and validate a novel algorithm used to determine the scanning Critical Level (L c ) and Minimum Detectable Concentration (MDC) (or Minimum Detectable Areal Activity) for the 102F scanning system. Through the use of Monte Carlo statistical simulations the algorithm mathematically demonstrates accuracy in determining the L c and MDC when a nearest-neighbor averaging (NNA) technique was used. To empirically validate this approach, SNL prepared several spiked sources and ran a test with the ERG 102F instrument on a bare concrete floor known to have no radiological contamination other than background naturally occurring radioactive material (NORM). The tests conclude that the NNA technique increases the sensitivity (decreases the L c and MDC) for high-density data maps that are obtained by scanning radiological survey instruments.

Fournier, Sean Donovan↗
DOE OSTI2018

A Minimum Variance Algorithm for Overdetermined TOA Equations with an Altitude Constraint.

We present a direct (non-iterative) method for solving for the location of a radio frequency (RF) emitter, or an RF navigation receiver, using four or more time of arrival (TOA) measurements and an assumed altitude above an ellipsoidal earth. Both the emitter tracking problem and the navigation application are governed by the same equations, but with slightly different interpreta- tions of several variables. We treat the assumed altitude as a soft constraint, with a specified noise level, just as the TOA measurements are handled, with their respective noise levels. With 4 or more TOA measurements and the assumed altitude, the problem is overdetermined and is solved in the weighted least squares sense for the 4 unknowns, the 3-dimensional position and time. We call the new technique the TAQMV (TOA Altitude Quartic Minimum Variance) algorithm, and it achieves the minimum possible error variance for given levels of TOA and altitude estimate noise. The method algebraically produces four solutions, the least-squares solution, and potentially three other low residual solutions, if they exist. In the lightly overdermined cases where multiple local minima in the residual error surface are more likely to occur, this algebraic approach can produce all of the minima even when an iterative approach fails to converge. Algorithm performance in terms of solution error variance and divergence rate for bas eline (iterative) and proposed approach are given in tables.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2024

Probing room temperature indirect and minimum direct band gaps of h-BN

Abstract Hexagonal boron nitride (h-BN) has attracted considerable interest as an ultrawide bandgap (UWBG) semiconductor. Experimental studies focused on the detailed near band-edge structure of h-BN at room temperature are still lacking. We report a direct experimental measurement of the near band-edge structure performed on h-BN quasi-bulk wafers via photocurrent excitation spectroscopy (PES). PES resolved the band-to-band transitions near M- and K-points in the Brillion zone (BZ), from which the room temperature indirect band gap of E g M K ∼6.02 eV, minimum direct bandgap at M-point of E g M = 6.36 eV and next lowest direct energy bandgap at K-point of E g K = 6.56 eV , have been simultaneously determined for the first time experimentally. The measured energy differences between K- and M-points in the conduction band minimum (CBM) and valence band maximum (VBM) are Δ E C M K = 0.54 eV and Δ E V M K = 0.34 eV, respectively, in good agreement with the calculation results. Significantly differing from its III-nitride wurtzite counterparts, in which only electrons and holes in the conduction and valence band extremes at the Γ-point are predominantly involved in the optical and transport processes, the results highlighted that charge carriers associated with both M- and K-valleys control to the optical excitation, recombination and charge transport processes in h-BN.

Physics↗
DOE OSTI2017

Inferring the Heliospheric Magnetic Field Back through Maunder Minimum

Recent solar conditions include a prolonged solar minimum (2005–2009) and a solar maximum that has not fully recovered in terms of the Heliospheric Magnetic Field (HMF) strength when compared to the previous maximum values. These anomalies may indicate that we are entering an era of lower solar activity than observed at other times during the space age. We study past solar grand minima, especially the Maunder period (1645–1715) to gain further insight into grand minima. We find the timescale parameters associated with three processes attributed to the magnetic flux balance in the heliosphere using chi-square analysis. We use HMF time series reconstructed based on geomagnetic data and near-Earth spacecraft measurements (OMNI) data to find the fundamental timescales that influence heliospheric field evolution through conversion or opening of magnetic flux from coronal mass ejections (CMEs) into the ambient heliospheric field, removal or loss of the ambient heliospheric field through magnetic reconnection, and interchange reconnection between CME magnetic flux and ambient heliospheric magnetic flux. We also investigate the existence of a floor in the heliospheric magnetic flux, in the absence of CMEs, and show that a floor ⩽1.49nT is sufficient to successfully describe the HMF evolution. The minimum value for the HMF at 1 au in the model-predicted historic record is 3.13 ± 0.35 nT. Our model results favorably reproduce paleocosmic data and near-Earth spacecraft measurements data and show how the HMF may evolve through periods of extremely low activity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

An Hp-Adaptive Minimum Action Method Based on a Posteriori Error Estimate

In this work, we develop an hp-adaptive minimum action method (MAM). MAM plays an important role to minimize the Freidlin-Wentzell action functional, which is the central object of the Freidlin-Wentzell theory of large deviations for transitions induced by small random perturbations. Because of the demanding computation cost, especially in spatially extended systems, numerical efficiency is a critical issue for MAM. Difficulties come from discretizations of both time and space. One hurdle for the application of MAM to large scale systems is the global reparametrization in time direction, which is required by most versions of MAM. We recently introduced a new version of MAM in [25], called tMAM, where we used some simple heuristic criteria to demonstrate that tMAM can be effectively coupled with h-adaptivity. The target of this paper is to integrate hp-adaptivity into tMAM using a posterior error estimation techniques. More specifically, we use the arc length constraint given by geometric minimum action method (gMAM) to define an indicator of the effect of linear time scaling in tMAM, and the derivative recovery technique to construct an error indicator and a regularity indicator for hp-refinement. Numerical results are presented.

Wan, Xiaoliang↗
DOE OSTI2024

Refinement of Pitting Factor Basis to Evaluate Minimum Nitrite Requirement in Dilute Waste Chemistries

Considering that chloride concentrations are an order of magnitude lower at SRS than the maximum chloride previously tested to evaluate pitting susceptibility of carbon steel, it has been proposed that the Pitting Factor methodology be refined to determine if a reduction in the minimum nitrite requirement is warranted in more dilute solutions and those with chloride concentrations more indicative of waste tank chemistries at SRS. Through the use of cyclic potentiodynamic polarization, the susceptibility of A537 carbon steel to pitting corrosion was evaluated. The Pitting Factor was shown to accurately predict pitting susceptibility throughout the chemistry envelope evaluated. In addition, it was shown that nitrite is not required for inhibition of pitting corrosion in tank conditions with chloride concentrations observed at SRS and up to 4 M nitrate. Based upon these findings, new control limits were proposed that removed the minimum nitrite requirement within the chemistry envelope evaluated up to 50 °C.

36 MATERIALS SCIENCE↗
NASA NTRS1969

Minimum impulse guidance.

Linearized theory for minimum fuel guidance in neighborhood of minimum fuel space trajectory, unrestricted thrust magnitude and allowances for midcourse impulses

Edelbaum, T. N.↗
NASA NTRS1951

Determination of Shapes of Boattail Bodies of Revolution for Minimum Wave Drag

By use of an approximate equation for the wave drag of slender bodies of revolution in a supersonic flow field, the optimum shapes of certain boattail bodies are determined for minimum wave drag. The properties of three specific families of bodies are determined, the first family consisting of bodies having a given length and base area and a contour passing through a prescribed point between the nose and base, the second family having fixed length, base area, and maximum area, and the third family having given length, volume, and base area. The method presented is easily generalized to determine minimum-wave-drag profile shapes which have contours that must pass through any prescribed number of points. According to linearized theory, the optimum profiles are found to have infinite slope at the nose but zero radius of curvature so that the bodies appear to have pointed noses, a zero slope at the body base, and no variation of wave drag with Mach number. For those bodies having a specified intermediate.diameter (that is, location and magnitude given), the maximum body diameter is shown to be larger, in general, than the specified diameter. It is also shown that, for bodies having a specified maximum diameter, the location of the maximum diameter is not arbitrary but is determined from the ratio of base diameter to maximum diameter.

Adams, Mac C.↗
NASA NTRS1970

Minimum impulse guidance

Linearized theory for minimum fuel guidance in neighborhood of minimum fuel space trajectory, unrestricted thrust magnitude and allowances for midcourse impulses

Edelbaum, T. N.↗
NASA NTRS1971

Performance index sensitivity of minimum-fuel systems.

The purpose of this paper is to investigate the effects of small parameter variations on the performance index of minimum-fuel systems with initial and final manifolds. The pulse-shaped variations produced by the parameter change on the piecewise-constant control are taken into account in order to derive the trajectory sensitivity equation. It is proved that the variation of the performance index is related to those of the trajectory and the parameter by a simple expression. This expression extends, to the class of minimum-fuel problems, a re sult obtained by Kokotovic et al. for systems with continuous, unconstrained controls.

Courtin, P.↗
NASA NTRS1972

A discrete variable approximation to minimum weight panel designs subject to a supersonic flutter-speed constraint.

A numerical technique is presented that determines the minimum-weight thickness distribution for a one-dimensional, simply supported panel subjected to a parallel supersonic flow on one side. An aeroelastic eigenvalue, which characterizes flutter speed, is held constant. The governing differential equations are approximated by sets of difference equations adjoined to the weight function via a penalty function. A conjugate gradient method then solves the resulting sequence of unconstrained minimization problems. Numerical results are obtained without imposing a minimum thickness constraint; the effect of constant inplane tensile and compressive stresses is also investigated.

Pierson, B. L.↗
NASA NTRS1972

Minimum-energy control of a class of electrically driven vehicles.

A minimum-energy controller is designed and built for a class of electrically driven vehicles according to the theoretical concepts determined by the application of modern control theory. Theoretical results are obtained by making several justifiable assumptions in the dynamical equations of the system and solving the resulting stochastic optimal control problem by Bellman's dynamic programming technique. Several practical and economical considerations are taken into account for the mechanization of the minimum-energy control law.

Sahinkaya, Y. E.↗
NASA NTRS1972

A comparison of open and closed loop applications of the minimum distance guidance technique

A comparison is made of open and closed loop applications of a second order guidance algorithm, using the minimum distance strategy. A nonlinear reoptimization procedure is used as the ideal guidance history. The system model used for the comparison is a low-thrust vehicle performing a minimum time, three-dimensional, heliocentric Earth-Mars transfer. For the example problem considered, closed loop guidance proves to be much more accurate on satisfaction of the final state than the open loop procedure. On the other hand, closed loop guidance proves to be much more vulnerable to perturbation by highly nonlinear regions in the trajectory. Finally, the results indicate that for this problem the best loop closure interval is at each integration step, about one day, or more often, if possible.

Lattimore, J. P., Jr.↗
NASA NTRS1971

The minimum distance approach to classification

The work to advance the state-of-the-art of miminum distance classification is reportd. This is accomplished through a combination of theoretical and comprehensive experimental investigations based on multispectral scanner data. A survey of the literature for suitable distance measures was conducted and the results of this survey are presented. It is shown that minimum distance classification, using density estimators and Kullback-Leibler numbers as the distance measure, is equivalent to a form of maximum likelihood sample classification. It is also shown that for the parametric case, minimum distance classification is equivalent to nearest neighbor classification in the parameter space.

Wacker, A. G.↗
NASA NTRS1972

Minimum mass of a neutron star.

The equation of state at subnuclear densities is considered together with the average binding energy per nucleon and the possibilities for a conversion of a neutron star into a white dwarf. It is found that the stable minimum mass with a core of pure neutron gas is equal to 0.067 units in terms of the solar mass, and with a core clustered with heavy nuclei equals 0.093 units. The real minimum mass is probably somewhere between these two numbers.

Leung, Y. C.↗