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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2005

ASK Talks with Bill Townsend

Recently retiring from his position as Deputy Director of NASA's Goddard Space Flight Center in Greenbelt, Maryland, Bill Townsend is now the Vice President and General Manager of Civil Space Systems of Ball Aerospace and Technologies Corporation. Prior to his assignment to Goddard in 1998, Mr.Townsend had served as the Deputy Associate Administrator (Programs) for the Office of Earth Science since 1993. For a 20 month period beginning June 1996, he was also the acting Associate Administrator for the Enterprise.

Townsend, Bill↗
NASA NTRS1988

Townsend coefficients for electron scattering over dielectric surfaces

A method for describing the probability of initiating flashover discharges across dielectric surfaces is presented in which a transport coefficient for electron multiplication similar to the Townsend coefficient used for gas discharges is defined. The coefficient is a function of the scaling parameter (charge released from the cathode)/(cathode-anode separation) and is also a measure of the growth of the sheath on the dielectric surface resulting from electron scattering. Results are discussed as to when the source of seed electrons does not necessarily depend upon field emission at the cathode-vacuum-dielectric triple point. For these conditions, there is a different functional dependence of flashover probability on voltage and geometry than when field emission provides the seed electrons. As a result, criteria previously used to predict flashover discharges may not apply.

Peck, Timothy L.↗
NASA NTRS1998

Ion-Molecule Reactions and Ion Kinetics in DC Townsend Discharges in Dielectric Gases

In gas discharges the identify, transport, relative intensities, and kinetic energy distributions (KEDs) of ions may be dominated by ion-molecule reactions. In order to understand these discharges, there is a need for cross section data for various ion-molecule reactions, including symmetric and asymmetric charge-transfer, collisional detachment, and ion conversion reactions, especially for low to intermediate ion-impact energies. In view of the importance of dielectric gases, we have recently measured the KEDs of various positive and negative ions produced in DC Townsend discharges in SF6, CF4, CHF3, O2, and CO2. The relative abundance and mean energies of ions are obtained from the measured KEDs. The effective cross sections (Qeff) describing the KEDs of various ions can be derived by using the mean ion energies and the electric field-to-gas density ratios (E/N). In the case of SF6 and O2 discharges, the determined cross sections (Qeff) for the negative-ion molecule reactions are consistent with previously measured cross section data obtained by crossed-beam experiment. The availability of these cross section values enabled us to understand the ion data reasonably well. However, the lack of adequate cross section data for positive ions in all of the above gases is a major obstacle in understanding ion transport in these gas discharges.

Rao, M. V. V. S.↗
NASA NTRS2013

Decoupling Contributions from Canopy Structure and Leaf Optics is Critical for Remote Sensing Leaf Biochemistry (Reply to Townsend, et al.)

Townsend et al. (1) agree that we explained that the apparent relationship (2) between foliar nitrogen (%N) and near-infrared (NIR) canopy reflectance was largely attributable to structure (which is in turn caused by variation in fraction of broadleaf canopy). Our conclusion that the observed correlation with %N was spurious (i.e., lacking a causal basis) is, thus, clearly justified: we demonstrated that structure explained the great majority of observed correlation, where the structural influence was derived precisely via reconciling the observed correlation with radiative-transfer theory. What this also suggests is that such correlations, although observed, do not uniquely provide information on canopy biochemical constituents.

Knyazikhin, Yuri↗
NASA NTRS2017

The Martian Dust Devil Electron Avalanche: Laboratory Measurements of the E-Field Fortifying Effects of Dust-Electron Absorption

Analogous to terrestrial dust devils, charged dust in Mars dust devils should become vertically stratified in the convective features, creating large scale E-fields. This E-field in a Martian-like atmosphere has been shown to stimulate the development of a Townsend discharge (electron avalanche) that acts to dissipate charge in regions where charge build-up occurs. While the stratification of the charged dust is a source of the electrical energy, the uncharged particulates in the dust population may absorb a portion of these avalanching electrons, thereby inhibiting dissipation and leading to the development of anomalously large E-field values. We performed a laboratory study that does indeed show the presence of enhanced E-field strengths between an anode and cathode when dust-absorbing filaments (acting as particulates) are placed in the avalanching electron flow. Further, the E-field threshold condition to create an impulsive spark discharge increases to larger values as more filaments are placed between the anode and cathode. We conclude that the spatially separated charged dust creates the charge centers and E-fields in a dust devil, but the under-charged portion of the population acts to reduce Townsend electron dissipation currents, further fortifying the development of larger-than-expected E-fields.

Farrell, W. M.↗
NASA NTRS1985

A simplified optical model description of heavy ion fragmentation

The abrasion-ablation model of heavy-ion frgmentation by target nuclei of Townsend (1983) and the EVAP Monte Carlo computer program (Guthrie, 1970) for decay probabilities are applied to calculate the production cross sections of isotopes of S, P, Si, and Al for the fragmentation of 213-MeV/nucleon Ar-40 ions by C-12 targets and of Fe, Mn, Cr, V, Ti, Sc, and Ca for 1.88-GeV Fe-56 fragmentation by Pb-208 and by C-12. The results are presented in tables and graphs and shown to be in good agreement with the experimental data of Viyogi et al. (1979) and Westfall et al. (1979). The implications of these studies for the design of radiation shielding for long-term manned space missions are indicated.

Townsend, L. W.↗
NASA NTRS1973

Multiwire proportional chamber development

The development of large area multiwire proportional chambers, to be used as high resolution spatial detectors in cosmic ray experiments is described. A readout system was developed which uses a directly coupled, lumped element delay-line whose characteristics are independent of the MWPC design. A complete analysis of the delay-line and the readout electronic system shows that a spatial resolution of about 0.1 mm can be reached with the MWPC operating in the strictly proportional region. This was confirmed by measurements with a small MWPC and Fe-55 X-rays. A simplified analysis was carried out to estimate the theoretical limit of spatial resolution due to delta-rays, spread of the discharge along the anode wire, and inclined trajectories. To calculate the gas gain of MWPC's of different geometrical configurations a method was developed which is based on the knowledge of the first Townsend coefficient of the chamber gas.

Doolittle, R. F.↗
NASA NTRS1974

Electron avalanche in liquid xenon

Measurements in liquid Xe are reported of the characteristic function that describes the avalanche, the first Townsend coefficient. An electron avalanche occurs when the drifting electrons attain sufficient kinetic energy from the electric field to cause additional ionization. Data on the avalanche were obtained by studying the pulses in a liquid-filled proportional chamber as a function of applied voltage and anode diameter. The results of the investigation show that the avalanche of free electrons in purified liquid Xe is adequately described by the same formalism used for the avalanche of electrons in gases.

Derenzo, S. E.↗
NASA NTRS1981

Organized motion in turbulent flow

A review of organized motion in turbulent flow indicates that the transport properties of most shear flows are dominated by large-scale vortex nonrandom motions. The mean velocity profile of a turbulent boundary layer consists of a viscous sublayer, buffer layer, and a logarithmic outer layer; an empirical formula of Coles (1956) applies to various pressure gradients. The boundary layer coherent structure was isolated by the correlation methods of Townsend (1956) and flow visualization by direct observations of complex unsteady turbulent motions. The near-wall studies of Willmart and Wooldridge (1962) used the space-time correlation for pressure fluctuations at the wall under a thick turbulent boundary layer; finally, organized motion in free shear flows and transition-control of mixing demonstrated that the Reynolds number invariance of turbulence shows wide scatter.

Cantwell, B. J.↗
NASA NTRS1989

Polarization effects in low-energy e-CH4 collisions

Polarization effects on the cross sections for elastic scattering of electrons by the CH4 molecule, for collision energies from 7.5 down to 0.02 eV, are presently studied on the basis of Takatsuka and McKoy's (1981) Schwinger multichannel formulation. While the closed-channel expansion through which polarization effects are encompassed is not fully converged, the cross sections obtained nevertheless exhibit a Ramsauer-Townsend minimum in the 0.1-0.2 eV range; this is about 0.3 lower than the experimentally obtained value. At 7.5 eV, however, the calculated differential cross-sections are in excellent agreement with measured values.

Lima, M. A. P.↗
NASA NTRS1990

An assessment of the conditions for critical velocity ionization at the weakly magnetized planets

It has been proposed that critical velocity ionization (CVI) may occur in the magnetosheaths of weakly magnetized planets where the solar wind flows through the planets' upper atmospheres. One can examine this possibility for Venus and Mars by using magnetosheath flow and exosphere models to determine whether the criteria for CVI are met. These criteria include Brenning's empirical condition on the cross-field flow velocity (it must not exceed the local magnetosonic velocity) and the 'Townsend condition' requiring that the integrated probability of impact ionization by a hot electron be greater than 1 along a streamline past the planet. Application of the Spreiter-Stahara gas dynamic flow model and the Nagy and coworkers' models for the exospheres lead to the conclusion that the conditions can be met in a limited region near Venus. However, evidence for CVI has not been identified in in-situ data. Since key details of the planet-solar wind interaction can be explained in terms of photoionization alone, other signatures of the process must be sought in the observations if the presence of CVI is to be positively inferred.

Luhmann, J. G.↗
NASA NTRS1991

Modeling of surface flashover on spacecraft

A model for predicting the onset of surface flashover discharges (SFDs) in the context of high voltage pulse power modulators was developed and used to investigate mechanisms leading to the onset of SFDs. We demonstrated that it is possible to analyze surface discharges in a manner similar to gas phase discharges using transport coefficients such as the first Townsend coefficient. Our parameterization of various methods to prevent, or at least delay, the onset of SFDs was not particularly successful in that many of the strategies that we investigated do not yield significantly improved performance. The only safe strategy to reduce the occurrence of SFDs is to prevent the dielectric from being charged in the first place. This leads one to consider passive or active schemes which employ the low pressure of attaching gases which flood the surface prior or coincident to pulsing the high voltage apparatus. Our calculations indicate that only small amounts gas (10s Torr effective pressure at substrate) would be sufficient for many of the anticipated applications. If the surface is flooded only when high voltage is applied across the dielectric, the gas consumption would be nominal.

Kushner, Mark J.↗
NASA NTRS1990

Numerical simulation of low Prandtl number turbulent mixing

Numerical simulations of turbulent mixing of strongly diffusive scalar fields were carried out with and without subgrid-scale modeling of the small-scale strain field. For low Reynolds number flows, when the rate of strain field (determined primarily by the small scales) is fully resolved, the scalar microstructure was found to collapse under Batchelor rate-of-strain scaling even for small Prandtl numbers, in agreement with Kerr. For high Reynolds number flows, when small-scale straining is modeled with a subgrid-scale model, the scalar microstructure follows the Batchelor, Howells, and Townsend predictions that the small-scale rate-of-strain is irrelevant.

Gibson, C.↗
NASA NTRS1991

Characteristics of a high pressure gas proportional counter filled with xenon

The characteristics of a conventional cylindrical geometry proportional counter filled with high pressure xenon gas up to 10 atm. were fundamentally investigated for use as a detector in hard X-ray astronomy. With a 2 percent methane gas mixture the energy resolutions at 10 atm. were 9.8 percent and 7.3 percent for 22 keV and 60 keV X-rays, respectively. From calculations of the Townsend ionization coefficient, it is shown that proportional counters at high pressure operate at weaker reduced electric field than low pressure counters. The characteristics of a parallel grid proportional counter at low pressure showed similar pressure dependence. It is suggested that this is the fundamental reason for the degradation of resolution observed with increasing pressure.

Sakurai, H.↗
NASA NTRS1996

A study of the turbulence structures of wall-bounded shear flows

This project extends the study of the structure of wall-bounded flows using the topological properties of eddying motions as developed by Chong et al. (1990), Soria et al. (1992, 1994), and as recently extended by Blackburn et al. (1996) and Chacin et al. (1996). In these works, regions of flow which are focal in nature are identified by being enclosed by an isosurface of a positive small value of the discriminant of the velocity gradient tensor. These regions resemble the attached vortex loops suggested first by Theodorsen (1955). Such loops are incorporated in the attached eddy model versions of Perry & Chong (1982), Perry et al. (1986), and Perry & Marusic (1995), which are extensions of a model first formulated by Townsend (1976). The DNS data of wall bounded flows studied here are from the zero pressure gradient flow of Spalart (1988) and the boundary layer with separation and reattachment of Na & Moin (1996). The flow structures are examined from the viewpoint of the attached eddy hypothesis.

Chong, M. S.↗