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This paper describes temperature measurements made on the high-energy medium-caliber launcher at the Institute for Advanced Technology. Simulations performed in Maxwell 3-D and E-Physics showed that Joule heating from current diffusing into the rails accounts for most of the temperature rise in the conductors. Temporal skin effects increase thermal dissipation significantly over what would be expected by the ohmic losses under fully diffused conditions. Based on this analysis, Joule heating is the overwhelmingly dominant source of heating in low-speed tests. As the velocity of the armature increases, Joule heating remains the dominant source of heat; however, additional mechanisms—which may include frictional heating, arcing energy, aluminum deposition, and temperature-dependent properties—are required to more satisfactorily explain the temperature profile obtained. [ABSTRACT FROM PUBLISHER]

Material ejected from aluminum armatures at the rail–armature interface has been identified as a mechanism that degrades both rails and insulators in a railgun, significantly reducing the bore lifetime. With the goal of controlling the onset of armature ejecta, a series of single-shot tests was conducted in a small railgun with a bore cross section of 22 \times 44 mm. The tests utilized channels of various sizes and geometries machined into the rail contact surface of the armatures to see if ejecta could be controlled. These tests identified several channel patterns as having the potential to delay the onset of armature ejecta. A series of multiple-shot tests was subsequently conducted in a larger railgun having a bore cross section of 38 \times 76 mm. The goal of these experiments was to see if the channel patterns that delayed armature ejecta had a significant impact on rail erosion at start-up. Three test series were conducted. In the first series, three armatures with a nested circular channel pattern were tested. In the second series, three armatures with a large centrally located channel were tested. Both armature designs used equivalent contact areas. These results were then compared to a standard armature contact face with no modifications in a third test series. [ABSTRACT FROM AUTHOR]

The Institute for Advanced Technology has been conducting experiments on plasma-armature railguns for the past several years. To control the damage mechanisms associated with this class of railgun, an electrothermal (ET) launcher preinjects the projectile into the plasma railgun bore milliseconds before the arc is energized. During the initial testing of the ET launcher, it was found that injection velocities were significantly lower than expected for a given input energy. Subsequent experiments using photonic Doppler velocimetry to resolve velocity as a function of time confirmed this finding. Using an ablative capillary discharge code written by Powell and Zielinski in 1991, the velocity discrepancy is analyzed, and the properties of the propellant gas—such as ionization fractions, the mass of the propellant gas, and the magnitude and decay of the pressure pulse seen by the projectile—are determined. [ABSTRACT FROM AUTHOR]

Key Message: This is the first report on association analysis of salt tolerance and identification of SNP markers associated with salt tolerance in cowpea. Cowpea (Vigna unguiculata (L.) Walp) is one of the most important cultivated legumes in Africa. The worldwide annual production in cowpea dry seed is 5.4 million metric tons. However, cowpea is unfavorably affected by salinity stress at germination and seedling stages, which is exacerbated by the effects of climate change. The lack of knowledge on the genetic underlying salt tolerance in cowpea limits the establishment of a breeding strategy for developing salt-tolerant cowpea cultivars. The objectives of this study were to conduct association mapping for salt tolerance at germination and seedling stages and to identify SNP markers associated with salt tolerance in cowpea. We analyzed the salt tolerance index of 116 and 155 cowpea accessions at germination and seedling stages, respectively. A total of 1049 SNPs postulated from genotyping-by-sequencing were used for association analysis. Population structure was inferred using Structure 2.3.4; K optimal was determined using Structure Harvester. TASSEL 5, GAPIT, and FarmCPU involving three models such as single marker regression, general linear model, and mixed linear model were used for the association study. Substantial variation in salt tolerance index for germination rate, plant height reduction, fresh and dry shoot biomass reduction, foliar leaf injury, and inhibition of the first trifoliate leaf was observed. The cowpea accessions were structured into two subpopulations. Three SNPs, Scaffold87490_622, Scaffold87490_630, and C35017374_128 were highly associated with salt tolerance at germination stage. Seven SNPs, Scaffold93827_270, Scaffold68489_600, Scaffold87490_633, Scaffold87490_640, Scaffold82042_3387, C35069468_1916, and Scaffold93942_1089 were found to be associated with salt tolerance at seedling stage. The SNP markers were consistent across the three models and could be used as a tool to select salt-tolerant lines for breeding improved cowpea tolerance to salinity.

Spinach (Spinacia oleracea L., 2n = 2x = 12) is an economically important vegetable crop worldwide and one of the healthiest vegetables due to its high concentrations of nutrients and minerals. The objective of this research was to conduct genetic diversity and population structure analysis of a collection of world-wide spinach genotypes using single nucleotide polymorphisms (SNPs) markers. Genotyping by sequencing (GBS) was used to discover SNPs in spinach genotypes. Three sets of spinach genotypes were used: 1) 268 USDA GRIN spinach germplasm accessions originally collected from 30 countries; 2) 45 commercial spinach F1 hybrids from three countries; and 3) 30 US Arkansas spinach cultivars/breeding lines. The results from this study indicated that there was genetic diversity among the 343 spinach genotypes tested. Furthermore, the genetic background in improved commercial F1 hybrids and in Arkansas cultivars/lines had a different structured populations from the USDA germplasm. In addition, the genetic diversity and population structures were associated with geographic origin and germplasm from the US Arkansas breeding program had a unique genetic background. These data could provide genetic diversity information and the molecular markers for selecting parents in spinach breeding programs.

The genetic diversity of cowpea was analyzed, and the population structure was estimated in a diverse set of 768 cultivated cowpea genotypes from the USDA GRIN cowpea collection, originally collected from 56 countries. Genotyping by sequencing was used to discover single nucleotide polymorphism (SNP) in cowpea and the identified SNP alleles were used to estimate the level of genetic diversity, population structure, and phylogenetic relationships. The aim of this study was to detect the gene pool structure of cowpea and to determine its relationship between different regions and countries. Based on the model-based ancestry analysis, the phylogenetic tree, and the principal component analysis, three well-differentiated genetic populations were postulated from 768 worldwide cowpea genotypes. According to the phylogenetic analyses between each individual, region, and country, we may trace the accession from off-original, back to the two candidate original areas (West and East of Africa) to predict the migration and domestication history during the cowpea dispersal and development. To our knowledge, this is the first report of the analysis of the genetic variation and relationship between globally cultivated cowpea genotypes. The results will help curators, researchers, and breeders to understand, utilize, conserve, and manage the collection for more efficient contribution to international cowpea research.