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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

Track Structure and the Biological Effectiveness of Accelerated Particles for the Induction of Chromosome Damage

Track structure models predict that at a fixed value of LET, particles with lower charge number, Z will have a higher biological effectiveness compared to particles with a higher Z. In this report we investigated how track structure effects induction of chromosomal aberration in human cells. Human lymphocytes were irradiated in vitro with various energies of accelerated iron, silicon, neon, or titanium ions and chromosome damage was assessed in using three color FISH chromosome painting in chemically induced PCC samples collected a first cell division post irradiation. The LET values for these ions ranged from 30 to195 keV/micron. Of the particles studied, Neon ions have the highest biological effectiveness for induction of total chromosome damage, which is consistent with track structure model predictions. For complex-type exchanges 64 MeV/ u Neon and 450 MeV/u Iron were equally effective and induced the most complex damage. In addition we present data on chromosomes exchanges induced by six different energies of protons (5 MeV/u to 2.5 GeV/u). The linear dose response term was similar for all energies of protons suggesting that the effect of the higher LET at low proton energies is balanced by the production of nuclear secondaries from the high energy protons.

George, K.↗
NASA NTRS2012

The Biological Effectiveness of Accelerated Particles for the Induction of Chromosome Damage: Track Structure Effects and Cytogenetic Signatures of High-LET Exposure

Track structure models predict that at a fixed value of LET, particles with lower charge number, Z will have a higher biological effectiveness compared to particles with a higher Z. In this report we investigated how track structure effects induction of chromosomal aberration in human cells. Human lymphocytes were irradiated in vitro with various energies of accelerated iron, silicon, neon, or titanium ions and chromosome damage was assessed in using three color FISH chromosome painting in chemically induced PCC samples collected a first cell division post irradiation. The LET values for these ions ranged from 30 to 195 keV/micrometers. Of the particles studied, Neon ions have the highest biological effectiveness for induction of total chromosome damage, which is consistent with track structure model predictions. For complex-type exchanges 64 MeV/ u Neon and 450 MeV/u Iron were equally effective and induced the most complex damage. In addition we present data on chromosomes exchanges induced by six different energies of protons (5 MeV/u to 2.5 GeV/u). The linear dose response term was similar for all energies of protons suggesting that the effect of the higher LET at low proton energies is balanced by the production of nuclear secondaries from the high energy protons. All energies of protons have a much higher percentage of complex-type chromosome exchanges than gamma rays, signifying a cytogenetic signature for proton exposures.

George, K.↗
NASA NTRSDate not supplied

Validation of Remotely Sensed and Modeled Soil Moisture at Forested and Unforested Sites

Soil moisture is an important driver for forest ecosystems, influencing fire occurrence and extent, insect and pathogen impacts, and tree growth, which creates a need for regular, globally extensive soil moisture information that only satellite-based sensors or models can achieve. However, the reliability of soil moisture measurements in forests is not well understood due to a lack of suitable validation sites (especially relative to unforested ecosystems) and interference caused by high vegetation water content on remotely sensed measurements; although recent studies have started to address this gap [1], [2], [3], [4]. Here we validate the performance of multiyear remotely sensed (SMAP/Sentinel), remotely sensed data assimilation modelled (SMAP-L4), and modelled (NLDAS) surface and root zone (0-1 m) soil moisture datasets with data from in-situ sensors at 39 National Ecological Observatory Network (NEON) sites throughout the contiguous US. Due to differences in spatial resolution, NEON soil moisture (~0.2 km measurement zone) correlations were expected to be stronger with the SMAP/Sentinel product (3 km resolution) than with coarser resolution SMAP-L4 (9 km resolution) or NLDAS products (13 km resolution). However, given the sensitivity of satellite measurements to vegetation water content we expected a deterioration in the correlations based on remotely sensed measurements (SMAP/Sentinel and SMAP-L4) as aboveground biomass increased, whereas the model-based data (NLDAS) was expected to be largely insensitive to vegetation type. We recognize that the SMAP/Sentinel product was developed for unforested regions, therefore our application is outside its primary use case. Soil moisture is measured at up to 8 depths in five soil plots spaced up to 40 m apart at each NEON terrestrial site. Correlation parameters were calculated for the three remotely sensed and modelled data products relative to in-situ measurement following Entekhabi et al. [5]. The datasets comprised 94 (SMAP-L4), 28 (SMAP/Sentinel), and 106 (NLDAS) sites-years for surface soils and 13 (SMAP-L4) and 14 (NLDAS) site-years for the root zone. At unforested sites, the performance of the three remotely sensed and modelled data products was similar for surface soils (Table 1). For example, unbiased RMSD (ubRMSD), which SMAP uses as its primary performance metric [6], ranged from 0.05 to 0.06 m3 m-3 (Table 1), indicating the ability of all three products to track changes in soil moisture over time. The performance of the three products deteriorated at forested sites, however, while the difference in performance was modest for SMAP-L4 and NLDAS, the deterioration in SMAP/Sentinel performance was substantial. For instance, SMAP/Sentinel ubRMSD increased from 0.06 to 0.11 m3 m-3 and absolute mean difference (Abs MD; which includes measurement bias and spatial representativeness errors) increased from 0.06 to 0.16 m3 m-3, indicating both a reduction in ability to track temporal changes and absolute amounts of soil moisture in forest ecosystems. SMAP-L4 and NLDAS had lower unbiased RMSD for root zone (0-1 m) than surface soils at both forested and unforested sites (Tables 1 and 2; SMAP/Sentinel does not produce a root zone measurement). However, in most cases the correlation coefficient (r) was lower for the root zone than surface soils, suggesting the lower unbiased RMSD may be attributed to greater temporal stability of soil moisture in the root zone rather than improved data product performance. Mean difference and absolute mean difference, which encompass measurement bias and spatial representativeness errors, were greater for root zone than surface soils at unforested sites for both data products, but the opposite was generally true at forested sites. As with surface soils, there was relatively little change in the performance of SMAP-L4 and NLDAS between the unforested and forested sites. In summary, all three data products were able to adequately represent soil moisture at unforested sites, at least when aggregating across sites. However, while the performance of all three products deteriorated at forested sites, SMAP-L4 and NLDAS maintained sufficient performance to remain suitable for some use cases (ubRMSD <0.06 m3 m-3 and RMSD <0.13 m3 m-3). In contrast, the relatively poorer performance of the SMAP/Sentinel product at forested sites seems insufficient for most use cases (ubRMSD >0.1 m3 m-3 and RMSD >0.2 m3 m-3). We attribute the large reduction in the performance of the SMAP/Sentinel product in forests to its use of C-band wavelengths, which are particularly sensitive to vegetation interference, and apparently outweighed any gains provided by its higher spatial resolution. A combined SMAP/NISAR soil moisture product may provide improved performance relative to SMAP/Sentinel due to NISAR’s use of L-band wavelengths, which are less sensitive to vegetation (NISAR is scheduled for launch in early 2024).

Edward Ayres↗
DOE OSTI2016

Real-time radiative divertor feedback control development for the NSTX-U tokamak using a vacuum ultraviolet spectrometer

A radiative divertor technique is planned for the NSTX-U tokamak to prevent excessive erosion and thermal damage of divertor plasma-facing components in H-mode plasma discharges with auxiliary heating up to 12 MW. In the radiative (partially detached) divertor, extrinsically seeded deuterium or impurity gases are used to increase plasma volumetric power and momentum losses. A real-time feedback control of the gas seeding rate is planned for discharges of up to 5 s duration. The outer divertor leg plasma electron temperature T{sub e} estimated spectroscopically in real time will be used as a control parameter. A vacuum ultraviolet spectrometer McPherson Model 251 with a fast charged-coupled device detector is developed for temperature monitoring between 5 and 30 eV, based on the Δn = 0, 1 line intensity ratios of carbon, nitrogen, or neon ion lines in the spectral range 300–1600 Å. A collisional-radiative model-based line intensity ratio will be used for relative calibration. A real-time T{sub e}-dependent signal within a characteristic divertor detachment equilibration time of ∼10–15 ms is expected.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

Determination of the plasma parameters in the PF-3 facility by the methods of X-ray spectroscopy

NeIX and NeX spectra emitted by the PF-3 high-current (2 MA) plasma focus facility are measured. A numerical model describing the spectral intensities of the emission of helium- and hydrogen-like neon ions from an optically thick plasma is proposed. The electron temperature T{sub e} and electron density n{sub e} in the plasma of the PF-3 facility are determined by comparing the calculated and measured emission spectra of neon.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Lasers, extreme UV and soft X-ray

Three decades ago, large ICF lasers that occupied entire buildings were used as the energy sources to drive the first X-ray lasers. Today X-ray lasers are tabletop, spatially coherent, high-repetition rate lasers that enable many of the standard optical techniques such as interferometry to be extended to the soft X-ray regime between wavelengths of 10 and 50 nm. Over the last decade X-ray laser performance has been improved by the use of the grazing incidence geometry, diode-pumped solid-state lasers, and seeding techniques. The dominant X-ray laser schemes are the monopole collisional excitation lasers either driven by chirped pulse amplification (CPA) laser systems or capillary discharge. The CPA systems drive lasing in neon-like or nickel-like ions, typically in the 10 – 30 nm range, while the capillary system works best for neon-like argon at 46.9 nm. Most researchers use nickel-like ion lasers near 14 nm because they are well matched to the Mo:Si multilayer mirrors that have peak reflectivity near 13 nm and are used in many applications. As a result, the last decade has seen the birth of the X-ray free electron laser (XFEL) that can reach wavelengths down to 0.15 nm and the inner-shell Ne laser at 1.46 nm.

42 ENGINEERING↗
DOE OSTI2017

Plasma Radiation Source on the Basis of the Gas Puff with Outer Plasma Shell in the Circuit of a Mega-Ampere Load Current Doubler

Characteristics of Z-pinch plasma radiation in the form of a double shell neon gas puff with outer plasma shell are investigated in the microsecond implosion mode. Experiments are performed using a GIT-12 mega-joule generator with load current doubler having a ferromagnetic core at implosion currents up to 5 MA. Conditions for matching of the nonlinear load with the mega-ampere current multiplier circuit are determined. The load parameters (plasma shell characteristics and mass and geometry of gas puff shells) are optimized on the energy supplied to the gas puff and n energy characteristics of radiation. It is established that the best modes of K-shell radiation in neon are realized for such radial distribution of the gas-puff material at which the compression velocity of the shell is close to a constant and amounts to 27–30 cm/μs. In these modes, up to 40% of energy supplied to the gas puff is converted into K-shell radiation. The reasons limiting the efficiency of the radiation source with increasing implosion current are analyzed. A modernized version of the energy supply from the current doubler to the Z-pinch is proposed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

A comparison of He and Ne FIB imaging of cracks in microindented silicon nitride

Highlights: • Silicon nitride was successfully imaged with helium and neon ion microscopy. • Indent-induced cracking was resolved on conductively-coated and uncoated samples. • Cracking was found to be primarily intergranular in nature. • The grain microstructure was most visible on the carbon-coated sample. • For the carbon-coated sample, localized charging was observed along crack pathways. - Abstract: Helium ion microscopy (HIM) offers potential as a high spatial resolution technique for imaging insulating samples that are susceptible to charging artifacts. In this study helium and neon ion microscopy are used to image cracking in microindented samples of the non-conductive ceramic silicon nitride. The crack morphology of radial cracks emanating from the microindentations has been characterized for two different compositions of silicon nitride, with and without conductive coatings. Gold coating enhances crack edge contrast, but masks grain contrast for both He and Ne ion-induced secondary electron (ISE) imaging. Carbon coating enables the crystalline and glassy phases to be distinguished, more clearly with Ne-ISE, and the cracking pathway is found to be primarily intergranular. Zones of <100 nm diameter depleted ion-induced secondary electron emission along the crack paths are identified, consistent with charging ‘hotspots’.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Diagnostics of fast formation of distributed plasma discharges using X-band microwaves

We present measurements of high power (25.7 kW), pulsed (800 ns), X-band (9.382 GHz) microwave breakdown plasmas, including reflected power measurements, mixer reflected amplitude and phase measurements, optical emission spectroscopy (OES) measurements, and an analysis that estimates the average electron density and electron temperature. In addition, a six-region, 1-D model was used to determine plasma parameters and compare with the experimental results. The experimental results show that using a 43 Hz repetition rate with an 800 ns pulse, fast (<300 ns) breakdown occurs in neon measured between 50 Torr and 250 Torr, producing plasma that lasts for over 7 μs. It also leads to large microwave reflections (70%) and an on-axis transmission attenuation of −15 dB. Moreover, a comparison between a 1-D model and mixer measurements shows that at 100 Torr, the neon plasma electron density peaked at 2 × 10{sup 12} cm{sup −3}, and the electron temperature peaked at 2.5 eV assuming a Maxwellian distribution. The addition of 2% Ar in Ne reduced the breakdown time and allowed OES measurements to determine the effective electron temperature. OES measurements of mixed (Ne/Ar: 98/2) argon line ratios (420.1 nm/419.8 nm) were used to determine the average effective electron temperature T{sub e(eff)} = 1.2 eV, averaged over the entire 7μs plasma lifetime. They indicate that the electron energy distribution was not Maxwellian but, instead, tended towards a Druyvesteyn character.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

The effect of hydrogen bonding on torsional dynamics: A combined far-infrared jet and matrix isolation study of methanol dimer

The effect of strong intermolecular hydrogen bonding on torsional degrees of freedom is investigated by far-infrared absorption spectroscopy for different methanol dimer isotopologues isolated in supersonic jet expansions or embedded in inert neon matrices at low temperatures. For the vacuum-isolated and Ne-embedded methanol dimer, the hydrogen bond OH librational mode of the donor subunit is finally observed at ∼560 cm{sup −1}, blue-shifted by more than 300 cm{sup −1} relative to the OH torsional fundamental of the free methanol monomer. The OH torsional mode of the acceptor embedded in neon is observed at ∼286 cm{sup −1}. The experimental findings are held against harmonic predictions from local coupled-cluster methods with single and double excitations and a perturbative treatment of triple excitations [LCCSD(T)] and anharmonic. VPT2 corrections at canonical MP2 and density functional theory (DFT) levels in order to quantify the contribution of vibrational anharmonicity for this important class of intermolecular hydrogen bond vibrational motion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Multispecies density peaking in gyrokinetic turbulence simulations of low collisionality Alcator C-Mod plasmas

Peaked density profiles in low-collisionality AUG and JET H-mode plasmas are probably caused by a turbulently driven particle pinch, and Alcator C-Mod experiments confirmed that collisionality is a critical parameter. Density peaking in reactors could produce a number of important effects, some beneficial, such as enhanced fusion power and transport of fuel ions from the edge to the core, while others are undesirable, such as lower beta limits, reduced radiation from the plasma edge, and consequently higher divertor heat loads. Fundamental understanding of the pinch will enable planning to optimize these impacts. We show that density peaking is predicted by nonlinear gyrokinetic turbulence simulations based on measured profile data from low collisionality H-mode plasma in Alcator C-Mod. Multiple ion species are included to determine whether hydrogenic density peaking has an isotope dependence or is influenced by typical levels of low-Z impurities, and whether impurity density peaking depends on the species. We find that the deuterium density profile is slightly more peaked than that of hydrogen, and that experimentally relevant levels of boron have no appreciable effect on hydrogenic density peaking. The ratio of density at r/a = 0.44 to that at r/a = 0.74 is 1.2 for the majority D and minority H ions (and for electrons), and increases with impurity Z: 1.1 for helium, 1.15 for boron, 1.3 for neon, 1.4 for argon, and 1.5 for molybdenum. The ion temperature profile is varied to match better the predicted heat flux with the experimental transport analysis, but the resulting factor of two change in heat transport has only a weak effect on the predicted density peaking.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Response to “Comment on ‘Two-dimensional positive column structure in a discharge tube with radius discontinuity’” [Phys. Plasmas 22, 094701 (2015)]

For a quantitative validation of the nonlocal approach for simulation of positive column of dc gas discharge in neon, we have performed a comparative self-consistent calculations of parameters of longitudinally uniform positive column using two approaches—a full kinetic equation with a radially inhomogeneous electron distribution function (exact calculation) and a nonlocal approach. The calculations were performed for discharge current 1 mA and neon pressures 40 and 90 Pa. It is shown that discrepancies between parameters calculated with the two approaches—radial distributions of electron number densities, ionization rates, space potentials—are minor. Physical reasons of a validity of the nonlocal approach for present conditions are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Mitigation of upward and downward vertical displacement event heat loads with upper or lower massive gas injection in DIII-D

Intentionally triggered upward and downward vertical displacement events (VDEs) leading to disruptions were pre-emptively mitigated with neon massive gas injection (MGI) coming from either above or below the plasma. Global indicators of disruption mitigation effectiveness (conducted heat loads, radiated power, and vessel motion) do not show a clear improvement when mitigating with the gas jet located closer to the VDE impact area. A clear trend of improved mitigation is observed for earlier MGI timing relative to the VDE impact time. The plasma edge magnetic perturbation is seen to lock to a preferential phase during the VDE thermal quench, but this phase is not clearly matched by preliminary attempts to fit to the conducted heat load phase. Clear indications of plasma infra-red (IR) emission are observed both before and during the disruptions. This IR emission can affect calculation of disruption heat loads; here, the time decay of post-disruption IR signals is used to correct for this effect.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Thermal quench mitigation and current quench control by injection of mixed species shattered pellets in DIII-D

Injection of large shattered pellets composed of variable quantities of the main ion species (deuterium) and high-Z impurities (neon) in the DIII-D tokamak demonstrates control of thermal quench (TQ) and current quench (CQ) properties in mitigated disruptions. As the pellet composition is varied, TQ radiation fractions increase continuously with the quantity of radiating impurity in the pellet, with a corresponding decrease in divertor heating. Post-TQ plasma resistivities increase as a result of the higher radiation fraction, allowing control of current decay timescales based on the pellet composition. Magnetic reconstructions during the CQ show that control of the current decay rate allows continuous variation of the minimum safety factor during the vertically unstable disruption, reducing the halo current fraction and resulting vessel displacement. Both TQ and CQ characteristics are observed to saturate at relatively low quantities of neon, indicating that effective mitigation of disruption loads by shattered pellet injection (SPI) can be achieved with modest impurity quantities, within injection quantities anticipated for ITER. This mixed species SPI technique provides a possible approach for tuning disruption properties to remain within the limited ranges allowed in the ITER design.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

ELEMENTAL ABUNDANCES IN THE EJECTA OF OLD CLASSICAL NOVAE FROM LATE-EPOCH SPITZER SPECTRA

We present Spitzer Space Telescope mid-infrared IRS spectra, supplemented by ground-based optical observations, of the classical novae V1974 Cyg, V382 Vel, and V1494 Aql more than 11, 8, and 4 years after outburst, respectively. The spectra are dominated by forbidden emission from neon and oxygen, though in some cases, there are weak signatures of magnesium, sulfur, and argon. We investigate the geometry and distribution of the late time ejecta by examination of the emission line profiles. Using nebular analysis in the low-density regime, we estimate lower limits on the abundances in these novae. In V1974 Cyg and V382 Vel, our observations confirm the abundance estimates presented by other authors and support the claims that these eruptions occurred on ONe white dwarfs (WDs). We report the first detection of neon emission in V1494 Aql and show that the system most likely contains a CO WD.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Setting the stage for circumstellar interaction in core-collapse supernovae. II. Wave-driven mass loss in supernova progenitors

Supernovae (SNe) powered by interaction with circumstellar material provide evidence for intense stellar mass loss during the final years before core collapse. We have argued that during and after core neon burning, internal gravity waves excited by core convection can tap into the core fusion power and transport a super-Eddington energy flux out to the stellar envelope, potentially unbinding ∼1 solar mass of material. In this work, we explore the internal conditions of SN progenitors using the MESA one-dimensional stellar evolution code in search of those most susceptible to wave-driven mass loss. We focus on simple, order of magnitude considerations applicable to a wide range of progenitors. Wave-driven mass loss during core neon and oxygen fusion happens preferentially in either lower mass (∼20 solar mass zero-age main sequence) stars or massive, sub-solar metallicity stars. Roughly 20% of the SN progenitors we survey can excite 10{sup 46-48} erg of energy in waves that can potentially drive mass loss within a few months to a decade of core collapse. This energy can generate circumstellar environments with 10{sup –3}-1 solar masses reaching 100 AU before explosion. We predict a correlation between the energy associated with pre-SN mass ejection and the time to core collapse, with the most intense mass loss preferentially occurring closer to core collapse. During silicon burning, wave energy may inflate 10{sup –3}-1 solar masses of the envelope to 10-100 s of solar radii. This suggests that some nominally compact SN progenitors (Type Ibc progenitors) will have a significantly different SN shock breakout signature than traditionally assumed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

The remarkable deaths of 9–11 solar mass stars

The post-helium-burning evolution of stars from 7 M{sub ⊙} to 11 M{sub ⊙} is complicated by the lingering effects of degeneracy and off-center ignition. Here, stars in this mass range are studied using a standard set of stellar physics. Two important aspects of the study are the direct coupling of a reaction network of roughly 220 nuclei to the structure calculation at all stages and the use of a subgrid model to describe the convective bounded flame that develops during neon and oxygen burning. Below 9.0 M{sub ⊙} degenerate oxygen–neon cores form that may become either white dwarfs or electron-capture supernovae. Above 10.3 M{sub ⊙} the evolution proceeds “normally” to iron-core collapse, without composition inversions or degenerate flashes. Emphasis here is upon the stars in between, which typically ignite oxygen burning off-center. After oxygen burns in a convectively bounded flame, silicon burning ignites in a degenerate flash that commences closer to the stellar center and with increasing violence for stars of larger mass. In some cases the silicon flash is so violent that it could lead to the early ejection of the hydrogen envelope. This might have interesting observable consequences. For example, the death of a 10.0 M{sub ⊙} star could produce two supernova-like displays, a faint low-energy event due to the silicon flash, and an unusually bright supernova many months later as the low-energy ejecta from core collapse collides with the previously ejected envelope. The potential relation to the Crab supernova is discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ESTIMATION OF THE NEON/OXYGEN ABUNDANCE RATIO AT THE HELIOSPHERIC TERMINATION SHOCK AND IN THE LOCAL INTERSTELLAR MEDIUM FROM IBEX OBSERVATIONS

We report the first direct measurement of the Ne/O abundance ratio of the interstellar neutral gas flowing into the inner heliosphere. From the first year of Interstellar Boundary Explorer IBEX data collected in spring 2009, we derive the fluxes of interstellar neutral oxygen and neon. Using the flux ratio at the location of IBEX at 1 AU at the time of the observations, and using the ionization rates of neon and oxygen prevailing in the heliosphere during the period of solar minimum, we estimate the neon/oxygen ratios at the heliospheric termination shock and in the gas phase of the inflowing local interstellar medium. Our estimate is (Ne/O){sub gas,ISM} = 0.27 {+-} 0.10, which is-within the large given uncertainties-consistent with earlier measurements from pickup ions. Our value is larger than the solar abundance ratio, possibly indicating that a significant fraction of oxygen in the local interstellar medium is hidden in grains and/or ices.

79 ASTRONOMY AND ASTROPHYSICS↗