Charging of the outer surface or of the entire structure of a spacecraft in orbit can have a severe impact on the scientific output of the instruments. Typical floating potentials for magnetospheric satellites (from +1 to several tens of volts in sunlight) make it practically impossible to measure the cold (several eV) component of the ambient plasma. Effects of spacecraft charging are reduced by an entirely conductive surface of the spacecraft and by active charge neutralisation, which in the case of Cluster only deals with a positive potential. The Cluster spacecraft are instrumented with ion emitters of the liquid-metal ion-source type, which will produce indium ions at 5 to 8 keV energy. The operating principle is field evaporation of indium in the apex field of a needle. The advantages are low power consumption, compactness and high mass efficiency. The ion current will be adjusted in a feedback loop with instruments measuring the spacecraft potential (EFW and PEACE). A standalone mode is also foreseen as a back-up. The design and principles of the operation of the active spacecraft potential control instrument (ASPOC) are presented in detail. Flight experience with a similar instrument on the Geotail spacecraft is outlined.

Active Spacecraft Potential Control

A low and actively controlled electrostatic potential on the outer surfaces of a scientific spacecraft is very important for accurate measurements of cold plasma electrons and ions and the DC to low-frequency electric field. The Japanese/NASA Geotail spacecraft carries as part of its scientific payload a novel ion emitter for active control of the electrostatic potential on the surface of the spacecraft. The aim of the ion emitter is to reduce the positive surface potential which is normally encountered in the outer magnetosphere when the spacecraft is sunlit. Ion emission clamps the surface potential to near the ambient plasma potential. Without emission control, Geotail has encountered plasma conditions in the lobes of the magnetotail which resulted in surface potentials of up to about +70 V. The ion emitter proves to be able to discharge the outer surfaces of the spacecraft and is capable of keeping the surface potential stable at about +2 V. This potential is measured with respect to one of the electric field probes which are current biased and thus kept at a potential slightly above the ambient plasma potential. The instrument uses the liquid metal field ion emission principle to emit indium ions. The ion beam energy is about 6 keV and the typical total emission current amounts to about 15 µA. Neither variations in the ambient plasma conditions nor operation of two electron emitters on Geotail produce significant variations of the controlled surface potential as long as the resulting electron emission currents remain much smaller than the ion emission current. Typical results of the active potential control are shown, demonstrating the surface potential reduction and its stability over time.

Results from active spacecraft potential control on the Geotail spacecraft

A series of scientific spacecraft either in orbit or to be launched in the very near future carries ion emitters for active control of the electrostatic surface potential. The ion beam emission changes, via the associated reduction of the positive surface potential, the photoelectron sheath around the spacecraft which may result in the formation of a potential barrier. In this study, a spherically symmetric model of the photoelectron sheath around a spacecraft (with conductive surface) is analyzed. An algebraic equation is set up to describe the current equilibrium between the escaping photoelectrons, incoming ambient plasma electrons, and the artificially emitted charged particles at the surface of the spacecraft. The height of the potential barrier can be given by solving the algebraic equation, if the electrostatic surface potential of the spacecraft, the density and temperature of the ambient plasma electrons are known. The position of the potential barrier is estimated with the assumption that the electrostatic potential distribution is of a Debye form/The method has been applied to the data gained from the Geotail spacecraft at times when the ion emitter was operational. From the calculation, it is shown that the height of the potential barrier grows from 0.27(Tph/e) to 0.91(Tph/e), as the ion emitter current increases from 9.2(μA) to 38.3(μA). The maximum barrier height appears to be about 2 V, when the ion emitter operates at its maximum current about 50 μA.

Self-consistent determination of the electrostatic potential barrier due to the photoelectron sheath near a spacecraft

The payload of Equator-S was complemented by the potential control device (PCD) to stabilise the electric potential of the spacecraft with respect to the ambient plasma. Low potentials are essential for accurate measurements of the thermal plasma. The design of PCD is inherited from instruments for Geotail and Cluster and utilises liquid metal ion sources generating a beam of indium ions at several keV. The set-up of the instrument and its interaction with the plasma instruments on board is presented. When the instrument was switched on during commissioning, unexpectedly high ignition and operating voltages of some ion emitters were observed. An extensive investigation was initiated and the results, which lead to an improved design for Cluster-II, are summarised. The cause of the abnormal behaviour could be linked to surface contamination of some emitters, which will be monitored and cured by on-board procedures in future. The mission operations on Equator-S were not at all affected, because of the high redundancy built into the instrument so that a sufficient number of perfectly operating emitters were available and were turned on routinely throughout the mission. Observations of the effect of spacecraft potential control on the plasma remained limited to just one event on January 8, 1998, which is analysed in detail. It is concluded that the ion beam lead to the predicted improvement of the particle measurements even outside the low density regions of the magnetosphere where the effect of spacecraft potential control would have been much more pronounced, and that the similar instruments for the four Cluster-II spacecraft to be launched in 2000 will be very important to ensure accurate plasma data from this mission.

/pdf/spacecraft-potential-control-aboard-equator-s-as-a-test-for-3849an5yvh.pdf

Spacecraft potential control aboard Equator-S as a test for Cluster-II

Liquid-metal-ion source development for space propulsion at ARC.

An ion emission instrument has been developed to actively control the electrostatic surface potential of a spacecraft in orbit. Spacecraft surfaces tend to charge positive when their conductive surfaces are partially sunlit, and photoelectrons emitted, and the spacecraft is positioned outside of the very dense plasma region which is referred to as plasmasphere. A small, lightweight instrument with a lifetime of more than 104 h at a nominal emission current of 10 μA was the design goal. The ion emitter is based on the liquid metal ion source principle; a constant stream of liquified indium is evaporated from the tip of a needle and ionized; the ions are then extracted by a strong electric field. The size of the instrument is about 18 by 15 by 23 cm (length×width×height) and the entire instrument weighs about 1.9 kg and consumes 2.4 W of electrical power in the nominal emission mode. An instrument of this type is already operational on the Japanese Geotail spacecraft, while five more instruments are at the ...

A novel medium‐energy ion emitter for active spacecraft potential control

The conditions for measurements by particle instruments at low energies are significantly improved by stabilizing and reducing the high positive potentials sunlit spacecraft acquire when they are embedded in tenuous plasma. Among the many techniques employed for this purpose the emission of a beam of positive ions with energies from 5 to 8 keV is considered to be the most efficient approach. Its feasibility has been demonstrated by two instruments on board the spacecraft Geotail and Equator-S launched in July, 1992, and December, 1997, respectively. The ion emitter instruments onboard these spacecraft use liquid metal ion sources with indium as the charge material. Already small and lightweight due to carrying the charge material as a solid, these ion sources feature very high electrical and excellent mass efficiency. The reliability of these ion sources has been demonstrated in space by the flawless operation on board the Geotail spacecraft over six years. The second instrument on board Equator-S has been operating on a routine basis for four months throughout the active lifetime of the spacecraft. The efficiency of the method is shown by overviews and examples from the missions. It can also be demonstrated both in theory and experiment that this technique produces only few, if any, undesirable side effects. The outlook of this type of instrument is focused on the mission Cluster-II.

Spacecraft Potential Control Using Indium Ion Sources - Experience and Outlook Based on Six Years of Operation in Space

Active Spacecraft Potential Control: an ion emitter experiment for Cluster

Handle everyday research tasks with reliable, citation-backed results

Your personal Research Agent to handle research tasks with citation-backed results

Popular Tasks used by Researchers

How can I help with your research?

Meet SciSpace

Get more enhanced response by uploading the PDFs you want me to reference.

No relevant PDFs in your library

SciSpace is the AI research assistant for academics. Run systematic literature reviews on 280M+ papers, and write papers with cited sources. Trusted by 1M+ students, PhDs & researchers.

SciSpace | AI for Research

Analyze PDFs

Code & Manuscripts

Funding & Grants

Literature & Patents

Medical & Clinical Data

Systematic Review

Visualize & Present

Web & Data

Build a Google Scholar-like website for your research.

Build a website

Create charts and images for your research

Create a Chart

Write a paper for submission to a journal

Draft a manuscript

Patent Search

Design eye-catching scientific posters in minutes.

Scientific Poster Generation

Systematic Literature Review

One task is running at the moment. Your messages will be shown right after.

Drag and drop or click here to browse

Loved by <highlight>1 million+</highlight> researchers

Extract a list of specific topics and their sources from unstructured text

Topics

Compare and analyze relevant papers that matches with your search

Papers

Get insights from PDFs and bookmarked papers from your library

My library

Recent searches

Try searching for:

Catch AI-generated content in scholarly and non-scholarly content

{ai} Detector

Ai Writer

Get PDF Summaries, highlighted text explanations 

Chat with PDF

Effortlessly create in-text citations and bibliographies in APA and 2,500 other formats

Citation generator

Get explanations, summaries, and answers on academic papers

Ease up your research workflow with {scispace}'s cohort of exciting AI tools

Elevate your academic writing skills and convey your ideas the way you want

Paraphraser

Explore our range of reading and writing tools

Your file is being prepared and should be ready in a few minutes. If it's a large file, it might take a bit longer. You can close this window, and we'll email you the file when it's done.

You have reached a maximum limit of <strong>{limit}</strong> columns in the table. Remove at least <strong>1</strong> column to add or create another one.

A. Pedersen

Author Tools

Chat about Author

Papers

A novel medium‐energy ion emitter for active spacecraft potential control

Spacecraft Potential Control Using Indium Ion Sources - Experience and Outlook Based on Six Years of Operation in Space

Active Spacecraft Potential Control: an ion emitter experiment for Cluster