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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

Simulation and Experimental Measurements of Inductively Coupled CF4 and CF4/Ar Plasmas

The recently developed code SEMS (semiconductor equipment modeling software)is applied to the simulation of CF4 and CF4/Ar inductively coupled plasmas (ICP). This work builds upon the earlier nitrogen, transformer coupled plasma (TCP) SEMS research by demonstrating its accuracy for more complex reactive mixtures, moving closer to the realization of a virtual plasma reactor. Attention is given to the etching of and/or formation of carbonaceous films on the quartz dielectric window and diagnostic aperatures. The simulations are validated through comparisons with experimental measurements using FTIR (Fourier Transform Infrared) and UV absorption spectroscopy for CFx and SiFx neutral radicals, QMS (quadrupole mass spectrometry) for the ions, and Langmuir probe measurements of electron number density and temperature in an ICP GEC reference cell.

Hash, D. B.↗
NASA NTRS2001

Impact of Gas Heating in Inductively Coupled Plasmas

Recently it has been recognized that the neutral gas in inductively coupled plasma reactors heats up significantly during processing. The resulting gas density variations across the reactor affect reaction rates, radical densities, plasma characteristics, and uniformity within the reactor. A self-consistent model that couples the plasma generation and transport to the gas flow and heating has been developed and used to study CF4 discharges. A Langmuir probe has been used to measure radial profiles of electron density and temperature. The model predictions agree well with the experimental results. As a result of these comparisons along with the poorer performance of the model without the gas-plasma coupling, the importance of gas heating in plasma processing has been verified.

Hash, D. B.↗
NASA NTRS2001

Neutral Gas Temperature Estimates in an Inductively Coupled CF4 Plasma by Fitting Diatomic Emission Spectra

This work examines the accuracy of plasma neutral temperature estimates by fitting the rotational band envelope of different diatomic species in emission. Experiments are performed in an inductively coupled CF4 plasma generated in a Gaseous Electronics Conference reference cell. Visible and ultraviolet emission spectra are collected at a power of 300 W (approximately 0.7 W/cc) and pressure of 30 mtorr. The emission bands of several molecules (CF, CN, C2, CO, and SiF) are fit simultaneously for rotational and vibrational temperatures and compared. Four different rotational temperatures are obtained: 1250 K for CF and CN, 1600 K for CO, 1800 K for C2, and 2300 K for SiF. The vibrational temperatures obtained vary from 1750-5950 K, with the higher vibrational temperatures generally corresponding to the lower rotational temperatures. These results suggest that the different species have achieved different degrees of equilibration between the rotational and vibrational modes and may not be equilibrated with the translational temperatures. The different temperatures are also related to the likelihood that the species are produced by ion bombardment of the surface, with etch products like SiF, CO, and C2 having higher temperatures than species expected to have formed in the gas phase.

Cruden, Brett A.↗
NASA NTRS1999

Measurements of Ion Energy and Ion Flux Distributions in Inductively Coupled Plasmas in CF4/O2/Ar Mixtures

We report mass spectrometric studies of energy distributions and absolute concentrations of ions generated in CF4/O2/Ar inductively coupled rf plasmas. The ions were collected through a 100 mm orifice in the grounded and water cooled lower electrode in a GEC cell configuration. The measurements were made at gas pressures in the 10-50 mTorr range and rf coil power in the 100-300 W range. The observed ions are CF3(+), CF2(+), CF(+), C(+), F(+), COF(+), CO(+), O2(+), and O(+). The relative abundance of these ions varies with pressure and rf power. The energy distribution and absolute concentrations are correlated with electron number density and floating plasma potential measured by a compensated Langmuir probe.

Rao, M. V. V. S.↗
NASA NTRS2001

The Importance of Optical Pathlength Control for Plasma Absorption Measurements

An inductively coupled GEC Cell with modified viewing ports has been used to measure in-situ absorption in CF4 plasmas via Fourier Transform Infrared Spectroscopy, and the results compared to those obtained in a standard viewport configuration. The viewing ports were modified so that the window boundary is inside, rather than outside, of the GEC cell. Because the absorption obtained is a spatially integrated absorption, measurements made represent an averaging of absorbing species inside and outside of the plasma. This modification is made to reduce this spatial averaging and thus allow a more accurate estimation of neutral species concentrations and temperatures within the plasmas. By reducing this pathlength, we find that the apparent CF4 consumption increases from 65% to 95% and the apparent vibrational temperature of CF4 rises by 50-75 K. The apparent fraction of etch product SiF4 decreases from 4% to 2%. The data suggests that these density changes may be due to significant temperature gradients between the plasma and chamber viewports.

Cruden, Brett A.↗
NASA NTRS2001

Detection of Chamber Conditioning by CF4 in the GEC Cell

During oxide etch processes, buildup of fluorocarbon residues on reactor sidewalks can cause to drift and will necessitate time for conditioning and cleaning of the reactor. Various measurements in CF4 and Ar plasmas are made in an attempt to identify a metric able to indicate the chamber condition. Mass spectrometry and a Langmuir probe shows that the buildup of fluorocarbon films on the reactor surface causes a decrease in plasma floating potential, plasma potential, and ion energy in argon plasmas. This change in floating potential is also observed in CF4 plasma operation, and occurs primarily during the first hour and a half of plasma operation. A slight rise in electron density is also observed in the argon plasmas. Because the change is seen in an argon plasma, it is indicative of altered physical, not chemical, plasma-surface interactions. Specifically, the insulating films deposited on metal surfaces alter the electromagnetic fields seen by the plasma, affecting various parameters including the floating potential and electron density. An impedance probe placed on the inductive coil shows a slight reduction in plasma impedance due to this rising electron density. The optical emission of several species, including CF, C2, atomic Si and atomic C, is also monitored for changes in density resulting from the buildup of film on the chamber wall. Changes in the optical emission spectrum are comparable to the noise levels in their measurement.

Cruden, Brett A.↗
NASA NTRS2001

Inductively-Coupled RF Powered O2 Plasma as a Sterilization Source

Low-temperature or cold plasmas have been shown to be effective for the sterilization of sensitive medical devices and electronic equipment. Low-temperature plasma sterilization procedures possess certain advantages over other protocols such as ethylene oxide, gamma radiation, and heat due to the use of inexpensive reagents, the insignificant environmental impacts and the low energy requirements. In addition, plasmas may also be more efficacious in the removal of robust microorganisms due to their higher chemical reactivity. Together, these attributes render cold plasma sterilization as ideal for the surface decontamination requirements for NASA Planetary Protection. Hence, the work described in this study involves the construction, characterization, and application of an inductively-coupled, RF powered oxygen (O2) plasma.

Sharma, S. P.↗
NASA NTRS2011

Kinetic-Scale Magnetic Turbulence and Finite Larmor Radius Effects at Mercury

We use a nonstationary generalization of the higher-order structure function technique to investigate statistical properties of the magnetic field fluctuations recorded by MESSENGER spacecraft during its first flyby (01/14/2008) through the near-Mercury space environment, with the emphasis on key boundary regions participating in the solar wind - magnetosphere interaction. Our analysis shows, for the first time, that kinetic-scale fluctuations play a significant role in the Mercury's magnetosphere up to the largest resolvable timescale (approx.20 s) imposed by the signal nonstationariry, suggesting that turbulence at this plane I is largely controlled by finite Larmor radius effects. In particular, we report the presence of a highly turbulent and extended foreshock system filled with packets of ULF oscillations, broad-band intermittent fluctuations in the magnetosheath, ion-kinetic turbulence in the central plasma sheet of Mercury's magnetotail, and kinetic-scale fluctuations in the inner current sheet encountered at the outbound (dawn-side) magnetopause. Overall, our measurements indicate that the Hermean magnetosphere, as well as the surrounding region, are strongly affected by non-MHD effects introduced by finite sizes of cyclotron orbits of the constituting ion species. Physical mechanisms of these effects and their potentially critical impact on the structure and dynamics of Mercury's magnetic field remain to be understood.

Uritsky, V. M.↗
NASA NTRS2018

The NANOGrav 11 Year Data Set: Pulsar-Timing Constraints on the Stochastic Gravitational-Wave Background

We search for an isotropic stochastic gravitational-wave background (GWB) in the newly released 11 year data set from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). While we find no evidence for a GWB, we place constraints on a population of inspiraling supermassive black hole (SMBH) binaries, a network of decaying cosmic strings, and a primordial GWB. For the first time, we find that the GWB constraints are sensitive to the solar system ephemeris (SSE) model used and that SSE errors can mimic a GWB signal. We developed an approach that bridges systematic SSE differences, producing the first pulsar-timing array (PTA) constraints that are robust against SSE errors. We thus place a 95% upper limit on the GW-strain amplitude of A (sub GWB) < 1.45 × 10 (exp -15) at a frequency of f=1 yr(exp -1) for a fiducial f (exp -2/3) power-law spectrum and with interpulsar correlations modeled. This is a factor of approximately 2 improvement over the NANOGrav nine-year limit calculated using the same procedure. Previous PTA upper limits on the GWB (as well as their astrophysical and cosmological interpretations) will need revision in light of SSE systematic errors. We use our constraints to characterize the combined influence on the GWB of the stellar mass density in galactic cores, the eccentricity of SMBH binaries, and SMBH-galactic-bulge scaling relationships. We constrain the cosmic-string tension using recent simulations, yielding an SSE-marginalized 95% upper limit of G (sub mu) < 5.3 × 10(exp -11) - a factor of approximately 2 better than the published NANOGrav nine-year constraints. Our SSE-marginalized 95% upper limit on the energy density of a primordial GWB (for a radiation-dominated post-inflation universe) is omega (sub GWB)(f) h (exp 2) < 3.4 × 10 (exp -10).

Supermassive Black Holes↗
NASA NTRS2019

Partially Chloritized Smectites: Analogues of Smectites at Gale Crater, Mars

Characterizing the structure and composition of phyllosilicates is important for interpreting the aqueous history of Mars and identifying potential habitable environments. Smectites and chlorites are the most dominant clay types on Mars, and there is evidence of the presence of smectite/chlorite intergrades. Smectite has been detected at Gale Crater, Mars, via orbital observations and in-situ measurements, in abundances up to approximately 25 weight percentage of bulk rock. John Klein (JK) and Cumberland (CB) were analyzed by the Chemistry and Mineralogy (CheMin) and Samples Analysis at Mars (SAM) evolved gas analysis experiment (EGA) instruments, onboard Mars Science Laboratory (MSL), Curiosity, to distinguish clay mineralogy. John Klein has a collapsed 2:1 smectite with a d-spacing of 10 Angstroms, whereas Cumberland smectite did not fully collapse and has a d-spacing of approximately 13.2 Angstroms. It has been suggested that partial chloritization or ‘pillaring’ could be responsible for the expanded Cumberland smectite because pillaring inhibits the collapse of smectites down to 10 Angstrom, even under the desiccating conditions on the martian surface. Clay minerals have been detected in ancient fluvio-lacustrine rocks throughout Curiosity’s traverse and catalog the changes of the lake water chemistry and diagenetic conditions at Gale Crater, Mars. Investigating clay minerals is important for identifying them on the Martian surface, in particular as Curiosity proceeds into the upcoming Clay-bearing Unit.

Tu, V. M.↗
NASA NTRSDate not supplied

Researching the Planetary Environment with an Interstellar Probe

In 2018, a study originated with the idea of a mission that would be feasible to launch in the 2030s, targeting 1000 AU within 50 years using current technology. While the primary objective of such an Interstellar Probe would be to understand the heliosphere and interstellar medium, this probe offers an excellent opportunity for rock, dust, and ice sciences. In the initial stages of its journey through the solar system, this Interstellar Probe would carry out a wide range of potential observations to study the planetary environment, particularly focusing on dust/ice analysis and planetary science through fly-bys of critical science targets, especially in the trans-Neptunian region. A flyby of a trans-Neptunian dwarf planet, such as Quaoar, would provide further geological, compositional, and geophysical context for Earth-based observations. Aside from in-situ remote sensing techniques, VISIR and dust analyses would also benefit in this region of the solar system to determine (i) dominant ice and dust compositions and potential variations depending on heliocentric distance (e.g., chemical or irradiated products); (ii) collections and identification of PAH-type components; and (iii) solar nebula chemical and mechanical processing, such as collisions. The purpose of our poster is to provide a background of the Interstellar Probe’s objectives and possible instrumentation to offer insight on current questions about the planetary environment.

C J Ahrens↗
NASA NTRS1996

In-flight radiation measurements on STS-60

A joint investigation between the United States and Russia to study the radiation environment inside the Space Shuttle flight STS-60 was carried out as part of the Shuttle-Mir Science Program (Phase 1). This is the first direct comparison of a number of different dosimetric measurement techniques between the two countries. STS-60 was launched on 3 February 1994 in a nearly circular 57 degrees x 353 km orbit with five U.S. astronauts and one Russian cosmonaut for 8.3 days. A variety of instruments provided crew radiation exposure, absorbed doses at fixed locations, neutron fluence and dose equivalent, linear energy transfer (LET) spectra of trapped and galactic cosmic radiation, and energy spectra and angular distribution of trapped protons. In general, there is good agreement between the U.S. and Russian measurements. The AP8 Min trapped proton model predicts an average of 1.8 times the measured absorbed dose. The average quality factor determined from measured lineal energy, y, spectra using a tissue equivalent proportional counter (TEPC), is in good agreement with that derived from the high temperature peak in the 6LiF thermoluminescent detectors (TLDs). The radiation exposure in the mid-deck locker from neutrons below 1 MeV was 2.53 +/- 1.33 microSv/day. The absorbed dose rates measured using a tissue equivalent proportional counter, were 171.1 +/- 0.4 and 127.4 +/- 0.4 microGy/day for trapped particles and galactic cosmic rays, respectively. The combined dose rate of 298.5 +/- 0.82 microGy/day is about a factor of 1.4 higher than that measured using TLDs. The westward longitude drift of the South Atlantic Anomaly (SAA) is estimated to be 0.22 +/- 0.02 degrees/y. We evaluated the effects of spacecraft attitudes on TEPC dose rates due to the highly anisotropic low-earth orbit proton environment. Changes in spacecraft attitude resulted in dose-rate variations by factors of up to 2 at the location of the TEPC.

NASA Discipline Radiation Health↗
NASA NTRS1991

Measurements of self-broadening of infrared absorption lines of ozone

Lorentz self-broadening coefficients have been determined for 355 spectral lines belonging to five different infrared vibration-rotation bands of O3 in the spectral region from 4.8 to 17 microns. Six ozone absorption spectra, recorded at room temperature using a Fourier transform spectrometer, were analyzed. The half-width values were obtained through a nonlinear least-squares spectral fitting procedure. The results are compared with previous measurements, and the vibration of the half-widths with vibrational and rotational quantum numbers is examined.

Smith, M. A. H.↗
NASA NTRS1992

The HITRAN molecular data base - Editions of 1991 and 1992

We describe in this paper the modifications, improvements, and enhancements to the HITRAN molecular absorption database that have occurred in the two editions of 1991 and 1992. The current database includes line parameters for 31 species and their isotopomers that are significant for terrestrial atmospheric studies. This line-by-line portion of HITRAN presently contains about 709,000 transitions between 0 and 23,000/cm and contains three molecules not present in earlier versions: COF2, SF6, and H2S. The HITRAN compilation has substantially more information on chlorofluorocarbons and other molecular species that exhibit dense spectra which are not amenable to line-by-line representation. The user access of the database has been advanced, and new media forms are now available for use on personal computers.

Rothman, Laurence S.↗
NASA NTRS1992

Position and morphology of the compact non-thermal radio source at the Galactic Center

We have determined with VLBI the position of the compact nonthermal radio source at the Galactic Center, commonly referred to as SgrA*, in the J2000.0 reference frame of extragalactic radio sources. We have also determined the size of SgrA* at 1.3, 3.6, and 13 cm wavelengths and found that the apparent size of the source increases proportionally to the observing wavelength squared, as expected from source size broadening by interstellar scattering and as reported previously by other authors. We have also established an upper limit of about 8 mJy at 3.6 cm wavelength for any ultracompact component. The actual size of the source is less than 15 AU. Fourier analysis of our very sensitive 3.6 cm observations of this source shows no significant variations of correlated flux density on time scales from 12 to 700 s.

Marcaide, J. M.↗
NASA NTRS2000

Hemodynamic and symptomatic effects of acute interventions on tilt in patients with postural tachycardia syndrome

A variety of approaches have been used to alleviate symptoms in postural tachycardia syndrome (POTS). Drugs reported to be of benefit include midodrine, propranolol, clonidine, and phenobarbital. Other measures used include volume expansion and physical countermaneuvers. These treatments may influence pathophysiologic mechanisms of POTS such as alpha-receptor dysfunction, beta-receptor supersensitivity, venous pooling, and brainstem center dysfunction. The authors prospectively studied hemodynamic indices and symptom scores in patients with POTS who were acutely treated with a variety of interventions. Twenty-one subjects who met the criteria for POTS were studied (20 women, 1 man; mean age, 28.7 +/- 6.8 y; age range, 14-39 y). Patients were studied with a 5-minute head-up tilt protocol, ECG monitoring, and noninvasive beat-to-beat blood pressure monitoring, all before and after the administration of an intervention (intravenous saline, midodrine, propranolol, clonidine, or phenobarbital). The hemodynamic indices studied were heart rate (ECG) and systolic, mean, and diastolic blood pressure. Patients used a balanced verbal scale to record any change in their symptoms between the tilts. Symptom scores improved significantly after the patients received midodrine and saline. Midodrine and propranolol reduced the resting heart rate response to tilt (p <0.005) and the immediate and 5-minute heart rate responses to tilt (p <0.002). Clonidine accentuated the immediate decrease in blood pressure on tilt up (p <0.05). It was concluded that midodrine and intravenous saline are effective in decreasing symptoms on tilt in patients with POTS when given acutely. Effects of treatments on heart rate and blood pressure responses generally reflected the known pharmacologic mechanisms of the agents.

Non-NASA Center↗
NASA NTRS1999

Formation of compact myelin is required for maturation of the axonal cytoskeleton

Although traditional roles ascribed to myelinating glial cells are structural and supportive, the importance of compact myelin for proper functioning of the nervous system can be inferred from mutations in myelin proteins and neuropathologies associated with loss of myelin. Myelinating Schwann cells are known to affect local properties of peripheral axons (de Waegh et al., 1992), but little is known about effects of oligodendrocytes on CNS axons. The shiverer mutant mouse has a deletion in the myelin basic protein gene that eliminates compact myelin in the CNS. In shiverer mice, both local axonal features like phosphorylation of cytoskeletal proteins and neuronal perikaryon functions like cytoskeletal gene expression are altered. This leads to changes in the organization and composition of the axonal cytoskeleton in shiverer unmyelinated axons relative to age-matched wild-type myelinated fibers, although connectivity and patterns of neuronal activity are comparable. Remarkably, transgenic shiverer mice with thin myelin sheaths display an intermediate phenotype indicating that CNS neurons are sensitive to myelin sheath thickness. These results indicate that formation of a normal compact myelin sheath is required for normal maturation of the neuronal cytoskeleton in large CNS neurons.

NASA Discipline Neuroscience↗