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Nelson, R.

Publications and source records attributed to Nelson, R..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

The Continuous Electron Beam Accelerator Facility at 12 GeV

This review paper describes the energy-upgraded Continuous Electron Beam Accelerator Facility (CEBAF) accelerator. This superconducting linac has achieved 12 GeV beam energy by adding 11 new high-performance cryomodules containing 88 superconducting cavities that have operated cw at an average accelerating gradient of 20 MV / m . After reviewing the attributes and performance of the previous 6 GeV CEBAF accelerator, we discuss the upgraded CEBAF accelerator system in detail with particular attention paid to the new beam acceleration systems. In addition to doubling the acceleration in each linac, the upgrade included improving the beam recirculation magnets, adding more helium cooling capacity to allow the newly installed modules to run cold, adding a new experimental hall, and improving numerous other accelerator components. We review several of the techniques deployed to operate and analyze the accelerator performance and document system operating experience and performance. In the final portion of the document, we present much of the current planning regarding projects to improve accelerator performance and enhance operating margins, and our plans for ensuring CEBAF operates reliably into the future. For the benefit of potential users of CEBAF, the performance and quality measures for the beam delivered to each of the experimental halls are summarized in the Appendix. Published by the American Physical Society 2024

43 PARTICLE ACCELERATORS↗
DOE OSTI2019

Magnetron R&Ds for High Efficiency CW RF Sources of Particle Accelerators

A proof of principle experiment carried out at JLab's magnetron R&D test stand has demonstrated a -25dB injection signal to phase lock a 2.45GHz 1.2kW CW magnetron within 2.1 Hz of root mean square deviation (RMSD) value with the optimization of magnetic field trimming of ±25% in order to overcome the frequency pushing of more than 4MHz in the RF power output range of 45-100%. In addition, a test stand for a newly built 1497 MHz, 13kW, CW magnetron prototype is ready for the high power injection test with outcome aimed for the CEBAF klystron replacement. Based on these R&D results, a 915MHz, 2?75kW CW commercial heating type magnetron system is being developed as high efficiency (>80%) RF source to drive an electron linac for industrial applications.

Wang, H.↗
DOE OSTI2017

Searches for double beta decay of Xe 134 with EXO-200

Searches for double beta decay of 134 Xe were performed with EXO-200, a single-phase liquid xenon detector designed to search for neutrinoless double beta decay of 136 Xe. Using an exposure of 29.6 kg ∙ yr , the lower limits of T$2νββ\atop{1/2}$ > 8.7 × 10 20 yr and T$0νββ\atop{1/2}$ > 1.1 × 10 23 yr at 90% confidence level were derived, with corresponding half-life sensitivities of 1.2 × 10 21 yr and 1.9 × 10 23 yr . In conclusion, these limits exceed those in the literature for 134 Xe , improving by factors of nearly 10 5 and 2 for the two antineutrino and neutrinoless modes, respectively.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Trace radioactive impurities in final construction materials for EXO-200

Here, we report results from a systematic measurement campaign conducted to identify low radioactivity materials for the construction of the EXO-200 double beta decay experiment. Partial results from this campaign have already been reported in a 2008 paper by the EXO collaboration. Here we release the remaining data, collected since 2007, to the public. The data reported were obtained using a variety of analytic techniques. The measurement sensitivities are among the best in the field. We concluded the construction of the EXO-200 detector and Phase-I data was taken from 2011 to 2014. The detector’s extremely low background implicitly verifies the measurements and the analysis assumptions made during construction and reported in this paper.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2017

Measurement of the drift velocity and transverse diffusion of electrons in liquid xenon with the EXO-200 detector

The EXO-200 Collaboration is searching for neutrinoless double β decay using a liquid xenon (LXe) time projection chamber. This measurement relies on modeling the transport of charge deposits produced by interactions in the LXe to allow discrimination between signal and background events. In this paper, we present measurements of the transverse diffusion constant and drift velocity of electrons at drift fields between 20 V/cm and 615 V/cm using EXO-200 data. Finally, at the operating field of 380 V/cm EXO-200 measures a drift velocity of 1.705 +0.014 –0.010 mm/μs and a transverse diffusion coefficient of 55 ± 4 cm 2 /s.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Generalized multiband typical medium dynamical cluster approximation: Application to (Ga,Mn)N

We generalize the multiband typical medium dynamical cluster approximation and the formalism introduced by Blackman, Esterling, and Berk so that it can deal with localization in multiband disordered systems with both diagonal and off-diagonal disorder with complicated potentials. We also introduce an ansatz for the momentum-resolved typical density of states that greatly improves the numerical stability of the method while preserving the independence of scattering events at different frequencies. Starting from the first-principles effective Hamiltonian, we apply this method to the diluted magnetic semiconductor Ga 1 - x Mn x N , and find the impurity band is completely localized for Mn concentrations x < 0.03 , while for 0.03 < x < 0.10 the impurity band has delocalized states but the chemical potential resides at or above the mobility edge. So, the system is always insulating within the experimental compositional limit ( x ≈ 0.10 ) due to Anderson localization. But, for 0.03 < x < 0.10 hole doping could make the system metallic, allowing double-exchange mediated, or enhanced, ferromagnetism. Finally, this developed method is expected to have a large impact on first-principles studies of Anderson localization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Towards a quantitative description of tunneling conductance of superconductors: Application to LiFeAs

Since the discovery of iron-based superconductors, a number of theories have been put forward to explain the qualitative origin of pairing, but there have been few attempts to make quantitative, material-specific comparisons to experimental results. The spin-fluctuation theory of electronic pairing, based on first-principles electronic structure calculations, makes predictions for the superconducting gap. Within the same framework, the surface wave functions may also be calculated, allowing, e.g., for detailed comparisons between theoretical results and measured scanning tunneling topographs and spectra. We present such a comparison between theory and experiment on the Fe-based superconductor LiFeAs. Our results for the homogeneous surface as well as impurity states are presented as a benchmark test of the theory. For the homogeneous system, we argue that the maxima of topographic image intensity may be located at positions above either the As or Li atoms, depending on tip height and the setpoint current of the measurement. We further report the experimental observation of transitions between As- and Li-registered lattices as functions of both tip height and setpoint bias, in agreement with this prediction. Next, we give a detailed comparison between the simulated scanning tunneling microscopy images of transition-metal defects with experiment. Finally, we discuss possible extensions of the current framework to obtain a theory with true predictive power for scanning tunneling microscopy in Fe-based systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2011

Object-Based Mapping of the Circumpolar Taiga-Tundra Ecotone with MODIS Tree Cover

The circumpolar taiga tundra ecotone was delineated using an image-segmentation-based mapping approach with multi-annual MODIS Vegetation Continuous Fields (VCF) tree cover data. Circumpolar tree canopy cover (TCC) throughout the ecotone was derived by averaging MODIS VCF data from 2000 to 2005 and adjusting the averaged values using linear equations relating MODIS TCC to Quickbird-derived tree cover estimates. The adjustment helped mitigate VCF's overestimation of tree cover in lightly forested regions. An image segmentation procedure was used to group pixels representing similar tree cover into polygonal features (segmentation objects) that form the map of the transition zone. Each polygon represents an area much larger than the 500 m MODIS pixel and characterizes the patterns of sparse forest patches on a regional scale. Those polygons near the boreal/tundra interface with either (1) mean adjusted TCC values from5 to 20%, or (2) mean adjusted TCC values greater than 5% but with a standard deviation less than 5% were used to identify the ecotone. Comparisons of the adjusted average tree cover data were made with (1) two existing tree line definitions aggregated for each 1 degree longitudinal interval in North America and Eurasia, (2) Landsat-derived Canadian proportion of forest cover for Canada, and (3) with canopy cover estimates extracted from airborne profiling lidar data that transected 1238 of the TCC polygons. The adjusted TCC from MODIS VCF shows, on average, less than 12% TCC for all but one regional zone at the intersection with independently delineated tree lines. Adjusted values track closely with Canadian proportion of forest cover data in areas of low tree cover. A comparison of the 1238 TCC polygons with profiling lidar measurements yielded an overall accuracy of 67.7%.

Ranson, K. J.↗
NASA NTRS2011

Titan's Ammonia Feature

NH3 has long been considered an important component in the formation and evolution of the outer planet satellites. NH3 is particularly important for Titan, since it may serve as the reservoir for atmospheric nitrogen. A brightening seen on Titan starting in 2004 may arise from a transient low-lying fog or surface coating of ammonia. The spectral shape suggests the ammonia is anhydrous, a molecule that hydrates quickly in the presence of water.

ammonia↗
NASA NTRS2009

MODIS Tree Cover Validation for the Circumpolar Taiga-Tundra Transition Zone

A validation of the 2005 500m MODIS vegetation continuous fields (VCF) tree cover product in the circumpolar taiga-tundra ecotone was performed using high resolution Quickbird imagery. Assessing the VCF's performance near the northern limits of the boreal forest can help quantify the accuracy of the product within this vegetation transition area. The circumpolar region was divided into longitudinal zones and validation sites were selected in areas of varying tree cover where Quickbird imagery is available in Google Earth. Each site was linked to the corresponding VCF pixel and overlaid with a regular dot grid within the VCF pixel's boundary to estimate percent tree crown cover in the area. Percent tree crown cover was estimated using Quickbird imagery for 396 sites throughout the circumpolar region and related to the VCF's estimates of canopy cover for 2000-2005. Regression results of VCF inter-annual comparisons (2000-2005) and VCF-Quickbird image-interpreted estimates indicate that: (1) Pixel-level, inter-annual comparisons of VCF estimates of percent canopy cover were linearly related (mean R(sup 2) = 0.77) and exhibited an average root mean square error (RMSE) of 10.1 % and an average root mean square difference (RMSD) of 7.3%. (2) A comparison of image-interpreted percent tree crown cover estimates based on dot counts on Quickbird color images by two different interpreters were more variable (R(sup 2) = 0.73, RMSE = 14.8%, RMSD = 18.7%) than VCF inter-annual comparisons. (3) Across the circumpolar boreal region, 2005 VCF-Quickbird comparisons were linearly related, with an R(sup 2) = 0.57, a RMSE = 13.4% and a RMSD = 21.3%, with a tendency to over-estimate areas of low percent tree cover and anomalous VCF results in Scandinavia. The relationship of the VCF estimates and ground reference indicate to potential users that the VCF's tree cover values for individual pixels, particularly those below 20% tree cover, may not be precise enough to monitor 500m pixel-level tree cover in the taiga-tundra transition zone.

Montesano, P. M.↗
NASA NTRS2003

Development of a novel power system for space applications

This paper will describe the power ASICs, module and slices, the key performance capabilities, and the power system architecture of a sample system. In addition the current development status of the X2000 Power System components will be presented.

power systems↗
NASA NTRS2002

A New Diagnostic Technique for the Solar Corona

Over the last 30-40 years spectroscopic observation of the EUV (extreme ultraviolet) line emission has proved invaluable as a diagnostic of the solar coronal plasma state. Line ratios have been used to determine electron density, electron temperature and ion flow velocity. In this paper, we present results obtained with a new measurement technique that uses spectroscopic observations of the white light corona to obtain the electron density, temperature, and flow velocity. A prototype instrument has been designed and built to obtain visible light spectra (3800-4300 A) with modest resolution. This instrument was used to obtain coronal observations during the June 2001 eclipse in Zambia. The data were corrected for sky and instrument transmission to derive the electron temperature and flow speed. Results from these measurements will be discussed.

Nelson, R.↗