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Results for “equilibrium potential”

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 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Effects of secondary yield parameter variation on predicted equilibrium potential of an object in a charging environment

The sensitivity of predicted equilibrium potential to changes in secondary electron yield parameters was investigated using MATCHG, a simple charging code which incorporates the NASCAP material property formulations. The equilibrium potential was found to be a sensitive function of one of the two parameters specifying secondary electron yield due to proton impact and of essentially all the parameters specifying yield due to electron impact. The information on the electron generated secondary yield parameters was discovered to be obtainable from monoenergetic beam charging data if charging rates as well as equilibrium potentials are accurately recorded.

Purvis, C. K.↗
NASA NTRS1979

Effects of secondary yield parameter variation on predicted equilibrium potential of an object in a charging environment

A study is presented in which the sensitivity of predicted equilibrium potential to changes in secondary electron yield parameters is investigated using MATCHG, a simple charging code which incorporates the NASCAP material property formulations. It is found that equilibrium potential is a sensitive function of one of the two parameters specifying secondary electron yield due to proton impact and of essentially all the parameters specifying yield due to electron impact. In addition, it is found that information on the electron generated secondary yield parameters can be obtained from monoenergetic beam charging data if charging rates as well as equilibrium potentials are accurately recorded.

Purvis, C. K.↗
NASA NTRS1979

Parameter dependence of equilibrium charging potentials

Equilibrium surface potentials for slab configurations (representative e.g. of a large solar power satellite) are determined under extensive parametric variations of materials, solar exposure and substorm characteristics. The dependence of the charging process for dielectric surfaces on the various material response characteristics is described. The results are used as a guide to the material selection and design of large space systems to minimize dielectric breakdowns and reduce parasitic leakage currents on SPS.

Suh, P. K.↗
NASA NTRS1982

Differential charging of high-voltage spacecraft - The equilibrium potential of insulated surfaces

A theory is presented for the steady-state potential of insulated surfaces near exposed high voltages. The term 'insulated surfaces' is used to mean either dielectric surfaces or electrically isolated metallic surfaces. The potential is bounded below by the zero of the material's I-V curve assuming total suppression of secondary electrons, and above by assuming total extraction of secondaries. Within these bounds, the material's surface potential is determined consistently with the solution to Poisson's equation external to the vehicle. The theory is compared with rocket experiments and with SCATHA satellite data. Also, an explanation is suggested for the observed 'snapover' of solar cell coverslips from near plasma ground potential to near the potential of positively biased interconnects with increasing bias voltage.

Katz, I.↗
NASA NTRS1961

Measurements of Sheath Currents and Equilibrium Potential on the Explorer VIII Satellite (1960 xi)

Experimental data were obtained from the Explorer VIII satellite on five parameters pertinent to the problem of the interaction of space vehicles with an ionized atmosphere. The five parameters are: photoemission current due to electrons emitted from the satellite surfaces as a result of solar radiation; electron and positive ion currents due to the diffusion of charged particles from the medium to the spacecraft; the vehicle potential relative to the medium, and the ambient electron temperature. Included in the experimental data is the aspect dependence of the photoemission and diffusion currents. On the basis of the observations, certain characteristics of the satellite's plasma sheath are postulated.

Bourdeau, R. E.↗
NASA NTRS1994

Evidence against a hypothesis of vestibular efferent function

Efferent stimulation and nicotinic agonists can either decrease or increase the frequency of occurrence of EPSPs recorded from VIIIth nerve afferents in the frog. It has been hypothesized that the distribution of hair cell resting membrane potentials overlaps the equilibrium potential dictated by the nicotinic-gated channels on the hair cells. Nicotinic mediated increases in EPSP frequency would then be due to depolarization of hair cells that were more hyperpolarized at rest, while decreases in EPSP frequency would be due to hyperpolarization of hair cells more depolarized at rest. In order to test this hypothesis, while recording from afferents which showed an increase in EPSP frequency due to bath application of the nicotinic agonist DMPP (1,1-dimethyl-4-phenylpiperizinium iodide), hair cells were depolarized with 10 mM K+ in the bath, and then the effects of DMPP on EPSP frequency were assessed. In this situation, DMPP still increased EPSP frequency, suggesting that the equilibrium potential for the nicotinic-gated channel was much more positive than the resting potentials of the hair cells. An alternative hypothesis then seems likely, that the nicotinic receptors on hair cells are able to activate different iontophores that result in either hair cell depolarization or hyperpolarization, dependent upon which iontophore predominates in the hair cells innervating a particular afferent.

NASA Discipline Neuroscience↗
NASA NTRS2006

Secondary Electron Emission and the Exploration of Space

The emission of secondary electrons from surfaces exposed to the space plasma and radiation environment is a process of great importance to space system engineering design and operations. A spacecraft will collect charge until it reaches an equilibrium potential gov,erned by the balance of incoming electron and ion currents from the space environment with outgoing secondary, backscattered, and photoelectron currents. Laboratory measurements of secondary electron yields are an important parameter for use in spacecraft charging analyses because the magnitude and sign of the equilibrium potential depends on both the energy spectrum of electrons and ions in the space environment and the electrical properties of the surface materials (including the energy dependent secondary electron yields). Typical benign equilibrium potentials range &om a few tens of volts positive in interplanetary space to a few volts negative in low Earth orbit. However, spacecraft are known to charge to negative potentials exceeding one to ten kilovolts in some environments and anomalies or system failures due to electrostatic discharges originating from highly charged surfaces becomes a serious concern. This presentation will provide a review of the spacecraft charging process with special emphasis on the role of secondary electrons in controlling the current balance process. Charging examples will include spacecraft in Earth orbit and interplanetary space as well as dust charging on the lunar surface, a phenomenon of importance to future lunar surface operations.

Minow, Joseph I.↗
NASA NTRS2002

An Investigation of Applications for Thermodynamic Work Potential Methods: Working Tables and Charts for Estimation of Thermodynamic Work Potential in Equilibrium Mixtures of Jet-A and Air

The objective of this report is to provide a tool to facilitate the application of thermodynamic work potential methods to aircraft and engine analysis. This starts with a discussion of the theoretical background underlying these methods, which is then used to derive various equations useful for thermodynamic analysis of aircraft engines. The work potential analysis method is implemented in the form of a set of working charts and tables that can be used to graphically evaluate work potential stored in high-enthalpy gas. The range of validity for these tables is 300 to 36,000 R, pressures between between 0.01 atm and 100 atm, and fuel-air ratios from zero to stoichiometric. The derivations and charts assume mixtures of Jet-A and air as the working fluid. The thermodynamic properties presented in these charts were calculated based upon standard thermodynamic curve fits.

Mavris, Dimitri↗
NASA NTRS2008

Measurements of Charging of Apollo 17 Lunar Dust Grains by Electron Impact

It is well known since the Apollo missions that the lunar surface is covered with a thick layer of micron size dust grains with unusually high adhesive characteristics. The dust grains observed to be levitated and transported on the lunar surface are believed to have a hazardous impact on the robotic and human missions to the Moon. The observed dust phenomena are attributed to the lunar dust being charged positively during the day by UV photoelectric emissions, and negatively during the night by the solar wind electrons. The current dust charging and the levitation models, however, do not fully explain the observed phenomena, with the uncertainty of dust charging processes and the equilibrium potentials of the individual dust grains. It is well recognized that the charging properties of individual dust grains are substantially different from those determined from measurements made on bulk materials that are currently available. An experimental facility has been developed in the Dusty Plasma Laboratory at MSFC for investigating the charging and optical properties of individual micron/sub-micron size positively or negatively charged dust grains by levitating them in an electrodynamic balance in simulated space environments. In this paper, we present the laboratory measurements on charging of Apollo 17 individual lunar dust grains by a low energy electron beam. The charging rates and the equilibrium potentials produced by direct electron impact and by secondary electron emission process are discussed.

Abbas, Mian M.↗
NASA NTRS1986

The electrostatics of a dusty plasma

The potential distribution in a plasma containing dust grains were derived where the Debye length can be larger or smaller than the average intergrain spacing. Three models were treated for the grain-plasma system, with the assumption that the system of dust and plasma is charge-neutral: a permeable grain model, an impermeable grain model, and a capacitor model that does not require the nearest neighbor approximation of the other two models. A gauge-invariant form of Poisson's equation was used which is linearized about the average potential in the system. The charging currents to a grain are functions of the difference between the grain potential and this average potential. Expressions were obtained for the equilibrium potential of the grain and for the gauge-invariant capacitance between the grain and the plasma. The charge on a grain is determined by the product of this capacitance and the grain-plasma potential difference.

Whipple, E. C.↗
NASA NTRS1993

Water uptake by growing cells: an assessment of the controlling roles of wall relaxation, solute uptake, and hydraulic conductance

Growing plant cells increase in volume principally by water uptake into the vacuole. There are only three general mechanisms by which a cell can modulate the process of water uptake: (a) by relaxing wall stress to reduce cell turgor pressure (thereby reducing cell water potential), (b) by modifying the solute content of the cell or its surroundings (likewise affecting water potential), and (c) by changing the hydraulic conductance of the water uptake pathway (this works only for cells remote from water potential equilibrium). Recent studies supporting each of these potential mechanisms are reviewed and critically assessed. The importance of solute uptake and hydraulic conductance is advocated by some recent studies, but the evidence is indirect and conclusions remain controversial. For most growing plant cells with substantial turgor pressure, it appears that reduction in cell turgor pressure, as a consequence of wall relaxation, serves as the major initiator and control point for plant cell enlargement. Two views of wall relaxation as a viscoelastic or a chemorheological process are compared and distinguished.

Non-NASA Center↗
NASA NTRS2000

A Dust Grain Photoemission Experiment

A laboratory experiment has been developed at Marshall Space Flight Center to study the interaction of micron-sized particles with plasmas and FUV radiation. The intent is to investigate the conditions under which particles of various compositions and sizes become charged, or discharged, while exposed to an electron beam and/or UV radiation. This experiment uses a unique laboratory where a single charged micron size particle is suspended in a quadrupole trap and then subjected to a controlled environment. Tests are performed using different materials and sizes, ranging from 10 microns to 1 micron, to determine the particle's charge while being subjected to an electron beam and /or UV radiation. The focus of this presentation will be on preliminary results from UV photoemission tests, but past results from electron beam, secondary electron emission tests will also be highlighted. A monochromator is used to spectrally resolve UV in the 120 nm to 300 nm range. This enables photoemission measurements as a function of wavelength. Electron beam tests are conducted using I to 3 micron sized aluminum oxide particles subjected to energies between 100 eV to 3 KeV. It was found that for both positive and negative particles the potential tended toward neutrality over time with possible equilibrium potentials between -0.8 Volts and 0.8 Volts.

Venturini, C. C.↗
NASA NTRS1995

Parasitic current collection by PASP Plus solar arrays

Solar cells at potentials positive with respect to a surrounding plasma collect electrons. Current is collected by the exposed high voltage surfaces: the interconnects and the sides of the solar cells. This current is a drain on the array power that can be significant for high-power arrays. In addition, this current influences the current balance that determines the floating potential of the spacecraft. One of the objectives of the Air Force (PL/GPS) PASP Plus (Photovoltaic Array Space Power Plus Diagnostics) experiment is an improved understanding fo parasitic current collection. We have done computer modeling of parasitic current collection and have examined current collection flight data from the first year of operations. Prior to the flight we did computer modeling to improve our understanding of the physical processes that control parasitic current collection. At high potentials, the current rapidly rises due to a phenomenon called snapover. Under snapover conditions, the equilibrium potential distribution across the dielectric surface is such that part of the area is at potentials greater than the first crossover of the secondary yield curve. Therefore, each incident electron generates more than one secondary electron. The net effect is that the high potential area and the collecting area increase. We did two-dimensional calculations for the various geometries to be flown. The calculations span the space of anticipated plasma conditions, applied potential, and material parameters. We used the calculations and early flight data to develop an analytic formula for the dependence of the current on the primary problem variables. The analytic formula was incorporated into the EPSAT computer code. EPSAT allows us to easily extend the results to other conditions. PASP Plus is the principal experiment integrated onto the Advanced Photovoltaic and Electronics Experiments (APEX) satellite bus. The experiment is testing twelve different solar array designs. Parasitic current collection is being measured for eight of the designs under various operational and environment conditions. We examined the current collected as a function of the various parameters for the six non-concentrator designs. The results are similar to those obtained in previous experiments and predicted by the calculations. We are using the flight data to validate the analytic formula developed. The formula can be used to quantify the parasitic current collected. Anticipating the parasitic current value allows the spacecraft designer to include this interaction when developing the design.

Davis, Victoria Ann↗
NASA NTRS1981

'Bootstrap' charging of surfaces composed of multiple materials

The paper examines the charging of a checkerboard array of two materials, only one of which tends to acquire a negative potential alone, using the NASA Charging Analyzer Program (NASCAP). The influence of the charging material's field causes the otherwise 'non-charging' material to acquire a negative potential due to the suppression of its secondary emission ('bootstrap' charging). The NASCAP predictions for the equilibrium potential difference between the two materials are compared to results based on an analytical model.

Stannard, P. R.↗
NASA NTRS1989

On the stability of a class of isothermal, magnetostatic atmospheres

The stability of a class of isothermal magnetostatic atmospheres with one ignorable coordinate corresponding to a uniform gravitational field in a plane geometry is explored. Special attention is given to the stability of nonpotential equilibria that have the same magnetic field geometry as the potential equilibrium of the solution family. It is found that equilibria in which the source currents for the potential field are buried at large distances below the photospheric base are stable. Also considered is the stability of configurations in which the source currents for the potential field are located at a finite depth below the base.

Webb, G. M.↗
NASA NTRS1980

Configuration effects on satellite charging response

The response of various spacecraft configurations to a charging environment in sunlight was studied using the NASA Charging Analyzer Program code. The configuration features geometry, type of stabilization, and overall size. Results indicate that sunlight charging response is dominated by differential charging effects. Shaded insulation charges negatively result in the formation of potential barriers which suppress photoelectron emission from sunlit surfaces. Sunlight charging occurs relatively slowly: with 30 minutes of charging simulations, in none of the configurations modeled did the most negative surface cell reach half its equilibrium potential in eclipse.

Purvis, C. K.↗
NASA NTRS1981

Modification of spacecraft potentials by plasma emission

The ion engine operations on Applied Technology Satellite 6 (ATS-6) altered the charge state of the spacecraft, changing the spacecraft surface potentials with respect to the distant plasmas. Plasma emission in quiet environments (plasma temperatures below 1 keV) caused the spacecraft potential to shift from a few volts positive to a few volts negative. A net ion current is emitted in such cases. The emission of a plasma or beam in energetic environments (plasma temperatures in the 5-10-keV range) in sunlight caused larger changes. Typical equilibrium potentials for ATS-6 in such environments were on the order of a hundred volts negative, with variations in potential across the spacecraft surface of comparable magnitude. Engine operations under such conditions raised the mainframe potential to near zero volts, and discharged the differential potentials on the dielectric surfaces. Plasma emission by the plasma bridge was an effective method of discharging kilovolt potentials in eclipse.

Olsen, R. C.↗