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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS2008

Wear Mechanisms in Electron Sources for Ion Propulsion, 2: Discharge Hollow Cathode

The wear of the keeper electrode in discharge hollow cathodes is a major impediment to the implementation of ion propulsion onboard long-duration space science missions. The development of a predictive theoretical model for hollow cathode keeper life has long been sought, but its realization has been hindered by the complexities associated with the physics of the partially ionized gas and the associated erosion mechanisms in these devices. Thus, although several wear mechanisms have been hypothesized, a quantitative explanation of life test erosion profiles has remained incomplete. A two-dimensional model of the partially ionized gas in a discharge cathode has been developed and applied to understand the mechanisms that drove the erosion of the keeper in two long-duration life tests of a 30-cm ion thruster. An extensive set of comparisons between predictions by the numerical simulations and measurements of the plasma properties and of the erosion patterns is presented. It is found that the near-plume plasma oscillations, predicted by theory and observed by experiment, effectively enhance the resistivity of the plasma as well as the energy of ions striking the keeper.

hollow cathode↗
NASA NTRS2014

Operational Status of the International Space Station Plasma Contactor Hollow Cathode Assemblies from July 2011 to May 2013

The International Space Station has onboard two Aerojet Rocketdyne developed plasma contactor units that perform the function of charge control. The plasma contactor units contain NASA Glenn Research Center developed hollow cathode assemblies. NASA Glenn Research Center monitors the onorbit operation of the flight hollow cathode assemblies. As of May 31, 2013, HCA.001-F has been ignited and operated 123 times and has accumulated 8072 hours of operation, whereas, HCA.003-F has been ignited and operated 112 times and has accumulated 9664 hours of operation. Monitored hollow cathode ignition times and anode voltage magnitudes indicate that they continue to operate nominally.

International Space Station↗
NASA NTRS2013

Temporal Fluctuations in a 100-A LaB6 Hollow Cathode

The temporal fluctuations in the near plume of a 100-Amperes LaB6 (Lanthanum hexaboride) hollow cathode are experimentally investigated. At high currents, turbulent oscillations may contribute to two of the anomalous processes of hollow cathode operation - anomalous resistivity and the production of energetic ions. A detailed study of the properties of the oscillations in a high current cathode is necessary to determine the impact of the fluctuation spectrum on these two effects. In this investigation, a probe array is employed to measure the amplitude and dispersion of axial modes in the plume while a retarding potential analyzer yields estimates of the radial ion energy distribution. The onset of the ion acoustic turbulence (IAT) is observed at high current values, and the character of the turbulent spectrum is shown to agree with weak turbulent theory: the amplitude of the spectrum decreases with flow rate but increases with discharge current. Estimates of the anomalous collision frequency based on experimental observations indicate that the IAT collision frequency can exceed the classical collision frequency at sufficiently high discharge current densities. Additionally, the onset of the IAT is shown to be correlated with the appearance of a high energy ion tail and that the energy in this tail is comparable to the energy in the experimentally-observed IAT.

production of ions↗
NASA NTRS2008

Tungsten and Barium Transport in the Internal Plasma of Hollow Cathodes

The effect of tungsten erosion, transport and redeposition on the operation of dispenser hollow cathodes was investigated in detailed examinations of the discharge cathode inserts from an 8200 hour and a 30,352 hour ion engine wear test. Erosion and subsequent re-deposition of tungsten in the electron emission zone at the downstream end of the insert reduces the porosity of the tungsten matrix, preventing the flow of barium from the interior. This inhibits the interfacial reactions of the barium-calcium-aluminate impregnant with the tungsten in the pores. A numerical model of barium transport in the internal xenon discharge plasma shows that the barium required to reduce the work function in the emission zone can be supplied from upstream through the gas phase. Barium that flows out of the pores of the tungsten insert is rapidly ionized in the xenon discharge and pushedback to the emitter surface by the electric field and drag from the xenon ion flow. Thisbarium ion flux is sufficient to maintain a barium surface coverage at the downstream endgreater than 0.6, even if local barium production at that point is inhibited by tungsten deposits. The model also shows that the neutral barium pressure exceeds the equilibrium vapor pressure of the impregnant decomposition reaction over much of the insert length,so the reactions are suppressed. Only a small region upstream of the zone blocked by tungsten deposits is active and supplies the required barium. These results indicate that hollowcathode failure models based on barium depletion rates in vacuum dispenser cathodes are very conservative.

Hollow cathodes↗
NASA NTRS2010

Barium Depletion in the NSTAR Discharge Cathode After 30,000 Hours of Operation

Dispenser hollow cathodes rely on a consumable supply of barium released by impregnant materials in the pores of a tungsten matrix to maintain a low work function surface. Examinations of cathode inserts from long duration ion engine tests show deposits of tungsten at the downstream end that appear to block the flow of barium from the interior. In addition, a numerical model of barium transport in the insert plasma indicates that the barium partial pressure in the insert may exceed the equilibrium vapor pressure of the dominant barium-producing reaction, and it was postulated previously that this would suppress barium loss in the upstream part of the insert. New measurements of the depth of barium depletion from a cathode insert operated for 30,352 hours reveal that barium loss is confined to a narrow region near the downstream end, confirming this hypothesis.

plasma transport↗
NASA NTRSDate not supplied

Effects of Environmental Exposure on Barium Oxide Cathode Operation

Hollow cathode assemblies (HCA) are typically designed around a cylindrical emitter for the electron source. Barium oxide (BaO) emitters are one type of emitter commonly used, which consists of a porous tungsten matrix impregnated with a mixture of barium oxide, calcium oxide, and alumina. Barium oxide emitters are susceptible to poisoning by a variety of substances, including oxygen, water vapor, carbon dioxide, and air. Poisoning can occur both during operation and exposure during storage. Water vapor is the primary contaminant of concern with ground exposure, and has been historically controlled by appropriate storage and handling procedures. Over the past several years, NASA Glenn Research Center (GRC)has been testing BaO cathodes to observe the effects on cathode operation after extended environmental exposures

electric propulsion↗
NASA NTRS1965

Titanium diaphragm makes excellent amplitron cathode support

Cathode support structure designed around a titanium diaphragm prevents radial misalignment between the cathode and anode in amplitrons. The titanium exhibits low thermal conductivity, tolerates lateral thermal expansion of the cathode, and is a poor primary and secondary emission medium.

Teich, W. W.↗
NASA NTRS1972

Effects of the hollow cathode discharge on throttled ion thruster performance.

An experimental study performed on a 20 cm electron bombardment thruster shows substantial improvements in throttled performance can be achieved if cathode discharge region diameter and length, cathode flow rate and baffle aperture area are reduced simultaneously. This improvement is related to a thruster operating mode in which arc current does not vary significantly with arc voltage. In this mode it is possible to operate the thruster at a high arc voltage and a correspondingly high primary electron energy without the penalty of a prohibitively high arc power. Langmuir probe data indicate the arc current is limited by the cathode region plasma potential and electron density which are both nearly invariant with arc voltage.

Wilbur, P. J.↗
NASA NTRS1972

Parametric study of mercury hollow cathode neutralizers.

A parametric investigation of mercury hollow cathode neutralizers was carried out in a bell jar over a range in bias currents up to 2 A. The parameters of particular interest were the mercury flowrate, the bias voltage, and the neutralizer cathode temperature. Three cathode orifices and several keeper geometries were tested. The bell jar tests compared well with neutralizer run with active thrusters. The effects of these parameters on the neutralizer stability and lifetime capability as well as the effects on the total thrust subsystem efficiency are discussed.

Fitzgerald, D. J.↗
NASA NTRS1972

Experimental investigation of a throttlable 15 cm hollow cathode ion thruster

The use of dished high perveance grids on a 15 cm modified SERT 2 thruster is shown to facilitate throttled operation over a beam current range from 60 to 600 mA. Effects of increasing the radial component of the magnetic field in the main discharge chamber and decreasing the dimensions of the cathode discharge region are examined and found to degrade performance to the extent that primary electrons are forced in toward the center-line of the thruster. Studies of the baffle aperture region of two thrusters indicate that the electric potential gradient vector is perpendicular to the local magnetic field lines when the thruster is operating properly. The correlation between the shape of the ion beam current density and that of the ion density at the screen grid within the thruster is shown to be 94%. Additional experimental studies on maximum propellant utilization, plasma ion production cost, neutral density in the cathode discharge region, double ion production in hollow cathode thrusters and thermal flow meter performance are discussed.

Wilbur, P. J.↗
NASA NTRS1973

A 13000-hour test of a mercury hollow cathode

A mercury-fed hollow cathode was tested for 12,979 hours in a bell jar at SERT 2 neutralizer operating conditions. The net electron current drawn to a collector was 0.25 ampere at average collector voltages between 21.8 and 36.7 volts. The mercury flow rate was varied from 5.6 to 30.8 equivalent milliamperes to give stable operation at the desired electrode voltages and currents. Variations with time in the neutralizer discharge characteristics were observed and hypothesized to be related to changes in the cathode orifice dimensions and the availability of electron emissive material. A facility failure caused abnormal test conditions for the last 876 hours and led to the cathode heater failure which concluded the test.

Rawlin, V. K.↗
NASA NTRS1973

Casting copper to tungsten for high power arc lamp cathodes

A method for making 400-kW arc lamp cathodes is described. The cathodes are made by casting a 1.75-in. diameter copper body onto a thoriated tungsten insert. The addition of 0.5-percent nickel to the copper prevents voids from forming at the copper-tungsten interface. Cathodes made by this process have withstood more than 110 hours of operation in a 400-kW arc lamp.

Will, H. A.↗
NASA NTRS1973

SERT 2 hollow cathode multiple restarts in space

Future missions, both station keeping and primary electric propulsion, will require multiple thrust restarts after periods of inactivity from a few hours to over one year. Although not a part of the original SERT 2 (Space Electric Rocket Test) flight objective, the opportunity to demonstrate multiple cathode restarts in space became available following completion of thruster running. Both neutralizer and main cathodes of each flight thruster were restarted repeatedly following storage periods up to 490 days. No deterioration of cathode heaters was noted nor was any change required in starting voltages or currents.

Kerslake, W. R.↗
NASA NTRS1973

The development of a multi-cathode electron gun

A multicathode electron gun, capable of automatically interchanging cathode structures of a microwave electron tube upon command was developed. This technique extends the life of a microwave electron tube into the range of fifty to one hundred years. The concept consists of mounting a number of cathode structures on a rotating platform. Rotation of the platform moves the different cathodes into operating position. The component tests, construction and operation of the gun are described.

Roberts, L. A.↗
NASA NTRS1973

SERT II hollow cathode multiple restarts in space

Future missions, both station keeping and primary electric propulsion, will require multiple thruster restarts after periods of inactivity from a few hours to over one year. Although not a part of the original SERT II (Space Electric Rocket Test) flight objective, the opportunity to demonstrate multiple cathode restarts in space became available following completion of thruster running. Both neutralizer and main cathodes of each flight thruster have been restarted repeatedly following storage periods up to 490 days. No deterioration of cathode heaters has been noted nor has any change been required in starting voltages or currents.

Kerslake, W. R.↗
NASA NTRS1974

Cold cathodes for sealed-off CO2 lasers

Cold cathodes for CO2 lasers should possess a very low sputtering rate, have a minimum of negative ions in the sputtering products, and consume no oxygen. These conditions can be satisfied by selecting cathode materials from those elements of group I in the periodic table which form semiconducting oxide layers and then exposing these elements to a reducing gas mixture. Experimental results show that silver, internally oxidized silver-copper alloys, and pure copper satisfy most of these requirements in He-CO2-CO-Xe gas mixtures. Infrared CO and CO2 gas analysis performed on simple gas discharge tubes using such cathodes indicate that laser lives of 10,000 hr can be expected.

Hochuli, U. E.↗
NASA NTRS1975

Hollow cathode characteristics in high current discharges

Current and potential distributions have been measured in various large hollow cathodes for currents from 0.9 to 7 kA and mass flows from 0.001 to 16 g/sec. For a variety of cavity diameters and shapes, all operated at 7 kA and 4 g/sec, the cathode current attaches over 0.6 cm of the cavity near the open end, where the electric field is less than 10 V/cm. However, for a fixed cathode configuration, the current conduction pattern is sensitive to both the current and the mass flows. For a given current, a range of mass flows is determined for maximum current penetration into the cavity, with the extent of penetration increasing as current decreases. Spectroscopic and photographic studies of AII radiance and electron densities within the cavity indicate a corresponding distribution of highly conducting plasma.

Krishnan, M.↗