Plasma Contactor Research Articles

An Universal System to De-Orbit Satellites at End of Life

A 3-year Project financed by the European Commission is aimed at developing a universal system to de-orbit satellites at their end of life, as a fundamental contribution to limit the increase of debris in the Space environment. The operational system involves a conductive tapetether left bare to establish anodic contact with the ambient plasma as a giant Langmuir probe. The Project will size the three disparate dimensions of a tape for a selected de-orbit mission and determine scaling laws to allow system design for a general mission. Starting at the second year, mission selection is carried out while developing numerical codes to implement control laws on tether dynamics in/off the orbital plane; performing numerical simulations and plasma chamber measurements on tether-plasma interaction; and completing design of subsystems: electronejecting plasma contactor, power module, interface elements, deployment mechanism, and tether-tape/end-mass. This will be followed by subsystems manufacturing and by currentcollection, free-fall, and hypervelocity impact tests.

A Proposed Two-Stage Two-Tether Scientific Mission at Jupiter

A two-stage mission to place a spacecraft (SC) below the Jovian radiation belts, using a spinning bare tether with plasma contactors at both ends to provide propulsion and power, is proposed. Capture by Lorentz drag on the tether, at the periapsis of a barely hyperbolic equatorial orbit, is followed by a sequence of orbits at near-constant periapsis, drag finally bringing the SC down to a circular orbit below the halo ring. Although increasing both tether heating and bowing, retrograde motion can substantially reduce accumulated dose as compared with prograde motion, at equal tether-to-SC mass ratio. In the second stage, the tether is cut to a segment one order of magnitude smaller, with a single plasma contactor, making the SC to slowly spiral inward over several months while generating large onboard power, which would allow multiple scientific applications, including in situ study of Jovian grains, auroral sounding of upper atmosphere, and space- and time-resolved observations of surface and subsurface.

Comparison of hollow cathode discharge plasma configurations

Hollow cathodes used in plasma contactor and electric propulsion devices provide electrons for sustaining plasma discharges and enabling plasma bridge neutralization. Life tests show erosion on hollow cathodes exposed to the plasma environment produced in the region downstream of these devices. To explain the observed erosion, plasma flow field measurements are presented for hollow cathode generated plasmas using both directly immersed probes and remotely located plasma diagnostics. Measurements on two cathode discharge configurations are presented: (1) an open, no magnetic field configuration and (2) a setup simulating the discharge chamber environment of an ion thruster. In the open cathode configuration, large amplitude plasma potential oscillations, ranging from 20 to 85 V within a 34 V discharge, were observed using a fast response emissive probe. These oscillations were observed over a dc potential profile that included a well-defined potential hill structure. A remotely located electrostatic analyzer (ESA) was used to measure the energy of ions produced within the plasma, and energies were detected that met, and in some cases exceeded, the peak oscillatory plasma potentials detected by the emissive probe. In the ion thruster discharge chamber configuration, plasma potentials from the emissive probe again agreed with ion energies recorded by the remotely located ESA; however, much lower ion energies were detected compared with the open configuration. A simplified ion-transit model that uses temporal and spatial plasma property measurements is presented and used to predict far-field plasma streaming properties. Comparisons between the model and remote measurements are presented.

The radiation impedance of electrodynamic tethers in a polar Jovian orbit

Juno, the second mission in the NASA New Frontiers Program, will both be a polar Jovian orbiter, and use solar arrays for power, moving away from previous use of radioisotope power systems (RPSs) in spite of the weak solar light reaching Jupiter. The power generation at Jupiter is critical, and a conductive tether could be an alternative source of power. A current-carrying tether orbiting in a magnetized ionosphere/plasmasphere will radiate waves. A magnitude of interest for both power generation and signal emission is the wave impedance. Jupiter has the strongest magnetic field in the Solar Planetary System and its plasma density is low everywhere. This leads to an electron plasma frequency smaller than the electron cyclotron frequency, and a high Alfven velocity. Unlike the low Earth orbit (LEO) case, the electron skin depth and the characteristic size of plasma contactors affect the Alfven impedance.

Floating bare tether as upper atmosphere probe

Use of a conductive bare tape electrically floating in low Earth orbit as an effective electron beam source to produce artificial auroral effects, free of problems that mar standard beams, is considered. Ambient ions impacting the tape with keV energies over most of its length liberate secondary electrons that race down the magnetic field, excite neutrals in the E layer, and result in auroral emissions. The tether would operate with both a power supply and a plasma contactor off at nighttime; power and contactor would be on at daytime for reboost. Tomographic analysis of auroral emissions from the footprint of the beam, as observed from the spacecraft, can provide density profiles of dominant neutral species in the E layer. A characteristic tether system, at altitude ∼300 km and moderate orbital inclination, would involve an aluminum tape with a length of ∼20 km, a width of ∼15 mm, and a thickness of ∼0.2 mm for a full‐system mass around 1200 kg, with two thirds going into the power subsystem.

Ground-Based Experiment of Electric Breakdown of Spacecraft Surface Insulator in an Ambient Plasma Environment

In the future, LEO spacecraft will be larger and higher powered. Because of the balance of the leakage currents through the ambient space plasma, their main conductive body will have a higher negative potential without plasma contactor operation. When the spacecraft operates with a higher voltage, more intensive arcing is suspected to occur on the surface. In this study, a ground-based experiment was carried out to understand the arcing phenomenon and to examine the influences of the ambient space plasma on the arcing process. Simulating plasmas were generated by electron-cyclotron-resonance discharge. When arcing occurred on negatively biased anodized-aluminum-sample (AAS) plates in the plasma environment, the time variations in the arc current and bias voltage were measured. The arc-spot diameter was also measured. The single-arcing characteristics showed that both the peak arc current and the total charge emitted by arcing increased with the initial-charging voltage and neutral-particle number density. The diameter of the arc spot increased with the initial-charging voltage although it was almost constant regardless of the neutral-particle density. The repetitive-arcing characteristics showed that the arc rate gradually increased with the arcing number. Lots of overlapping arc spot were observed after 1000 arcings, and its number increased with the arcing number. The influences of the initial-charging energy on arcing characteristics were also examined by widely changing the capacitance and the initial-charging voltage. The arc-spot diameter intensively increased with the initial-charging energy, and the fitting line could be evaluated. Accordingly, a high-voltage operation of the LEO spacecraft might bring a drastic degradation of AAS by arcing, depending on the ambient plasma conditions and spacecraft capacitance

Solid and grid sphere current collection in view of the tethered satellite system TSS 1 and TSS 1R mission results

[1] Passive end body plasma contactors have been operationally validated in space and have been shown to provide a simple, effective, and robust means of current collection at the positive terminal of an electrodynamic tether system. A grid sphere has been suggested as a possible end body since it potentially has distinct advantages compared to a solid sphere, including a lower neutral dynamic drag and a higher current-to-mass ratio. This paper estimates the maximum current collected by a grid sphere, taking into account its orbital motion and ion production inside the grid sphere. We first review the data from the tethered satellite system (TSS 1) and the TSS 1R flights, formulate a model for current collection by a solid sphere, and suggest how to incorporate it into the grid sphere current collection estimate. Then we calculate the potential distribution inside the grid sphere and the potential distribution outside the solid sphere for the same system parameters. Finally, we estimate the maximum current collected by a grid sphere depending on its transparency.

Survey of DSCS-III B-7 differential surface charging

An analysis of differential charging between dielectric surface materials and the frame of a DSCS-III geosynchronous spacecraft is presented. Charging levels measured by surface potential monitors (SPMs) covered with samples of Kapton and Astroquartz have been recorded for one half of a solar cycle. Both seasonal and solar cycle effects are seen in the daily peak levels of the SPM voltages, with local maxima occurring near the equinoxes and a general trend increasing as solar max is approached. Charge neutralization by an onboard Xe plasma contactor was demonstrated to be effective throughout the mission, with a mean voltage reduction of 86% for Astroquartz and 74% for Kapton. Though a statistical analysis shows a general correlation between the fluence of charging electrons with SPM voltages, the event-specific correlation contains enough variance to cast doubt on the usefulness of an electron sensor as a differential charging alarm. We have found that a Kapton-covered SPM may be better suited than an electron sensor as a differential charging alarm.

Ground-based experiments of a spacecraft plasma contactor using a hollow cathode

In the future, a plasma contactor using a hollow cathode will be applied to charging mitigation of large structures and electron emitter/collector of electrodynamic tether systems in space. In this study, ground-based experiments on the plasma contactor were carried out to examine basic operational characteristics. In electron emission, the same operational performance as that only with a simple assembly of the hollow cathode was obtained in the simulating low-earth-orbit (LEO) plasma. In electron collection, the excellent effect of collection current enhanced in the simulating LEO plasma existed. Furthermore, the mechanism of electron collection by the plasma contactor was examined from plasma parameter distributions near the plasma contactor. As a result, an intensive interaction between the contactor plasma and the simulating LEO plasma was observed.

宇宙機表面のアーキングに関する地上実験

In the future, LEO spacecraft will be larger and higher powered. Because of the balance of currents through ambient space plasma, their main conductive body will have a higher negative potential without plasma contactor operation. When spacecraft operate with a higher voltage, more intensive arcing is suspected to occur on their surface. In this study, ground-based experiment was carried out to understand the arcing phenomenon and to examine influences of ambient space plasma on the arcing process. Simulating plasmas were generated by electron cyclotron resonance discharge. When arcing occurred on anodized aluminum sample (AAS) plates negatively biased by a capacitor in the plasma environment, the time variations in arc current and bias voltage were measured. Arc spot diameter was also measured. The experimental results showed that both the peak arc current and the total charge emitted by arcing increased with initial charging voltage and neutral particle number density. The diameter of arc spots increased with initial charging voltage although it was almost constant regardless of neutral particle density. Accordingly, high voltage operation of LEO spacecraft might bring drastic degradation of AAS by arcing depending on ambient plasma conditions.

Calibrating and deriving physical parameters using plasma contactor data from the international space station

The International Space Station (ISS) regularly passes through the southern auroral oval south of Australia. The ISS has two plasma contactors that emit the electron currents needed to balance electron collection by surfaces such as the lattice of bare rods on the solar array masts. These electron currents exceed 0.1 A at times. The largest currents are observed in the auroral oval south of Australia. On the space station, the solar array 40 m long masts each have over 400 m of stainless steel tensioning rods. When subject to orbital v × B· l induced potentials, the rods collect substantial currents from the ionosphere. Maximum v × B· l potentials are generated near the magnetic poles. The plasma contactor emission current can be converted to an estimate of plasma density and calibrated using Floating Potential Probe (FPP) and other data. These measurements show that the plasma density in the nighttime auroral ionosphere is frequently several times that predicted by the International Reference Ionosphere (IRI)-90 and IRI2001 models.

One-Dimensional Hollow Cathode Model

Hollow cathodes with barium-rich inserts are used in ion engines as electron sources both in the discharge chamber and to neutralize the ion beam. Future deep space missions will require hollow cathode lifetimes in excessof the 28,000 h demonstrated during the International Space Station plasma contactor hollow cathode life test. A hollow cathode model is developed based on the observation that xenon ion mobility is diffusion limited due to resonant charge exchange reactions. Application of the model to the orifice region shows that the resultant ion generation profile correlates with previously reported orifice erosion. Modeling of the insert region shows that vapor-phase barium atoms are ionized almost immediately and electric fields accelerate the ions upstream from the emission zone.

エレクトロダイナミックテザーへの適用に関するプラズマコンタクタの電子放出・収集特性

Plasma contactors are proposed as making good electrical contact between spacecraft and ambient ionospheric plasma. Especially, it has been studied as the one of key components of electrodynamic tether for electron emitter and collector. In this study, the experiments were conducted to understand the plasma contacting process. Hollow cathodes were used for plasma contactor to establish a low impedance electrical contact. The results show the simultaneous operation of the plasma contactors for current collection and emission from and to an ambient plasma at the current levels of the order of five ampere. We obtained a performance curve, potential profile and the appearance of a bright luminous cloud that typically extends some tens of centimeters from the collector and confirmed closed current loop formation via simulated plasma between the emitter and the collector.

The Vasimr Engine: Concept Development, Recent Accomplishments and Future Plans

The development of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) was initiated in the late 1970s to address a critical requirement for fast, high-power interplanetary space transportation. While not being a fusion rocket, it nevertheless borrows heavily from that technology and takes advantage of the natural topology of open-ended magnetic systems. In addition to its high power density and high exhaust velocity, VASIMR is capable of “constant power throttling” a feature, which allows in-flight mission-optimization of thrust and specific impulse to enhance performance and reduce trip time. A NASA-led, research team, involving industry, academia and government facilities is pursuing the development of this concept in the United States. The technology can be validated, in the near term, in venues such as the International Space Station, where it can also serve as both a drag compensation device and a plasma contactor for the orbital facility. Other near-Earth applications in the commercial and scientific satellite sectors are also envisioned. This presentation covers the evolution of the VASIMR concept to its present status, as well as recent accomplishments in our understanding of the physics. Approaches and collaborative programs addressing the major technical challenges will also be presented.

Dynamic model of a plasma structure with an intermediate double-layer, formed outside an anodic plasma contactor

The plasma structure around an electron-collecting, plasma contactor consists of two quasineutral regions separated by an intermediate, free double layer(DL). A dynamic model of this three-region structure is here established, based in a linear perturbation of the complete steady solution. Time-dependent conditions at the mobile DL are obtained; these consist of Bohm and Langmuir laws, jump conditions on the accelerated species and barrier conditions on the confined ones. Boundary conditions at the contactor surface and at the undisturbed, ambient plasma are analyzed in detail. The dynamic response to perturbations in plasma emission is the combination of a quasisteady electron mode and several ion–electron modes, with different characteristics in the core and presheath. Mode interaction takes place through the DL and the electric potential. At large frequencies, the DL displacement is restricted by ambient ions, which tend to be rigid. The stability of the steady solution is studied through a nonlocal dispersion relation. It is found that this plasma structure does not develop the radial, ion–electron (macro)instability, and, in particular, the Buneman instability.

Spacecraft Potential Control by the Plasma Source Instrument on the POLAR Satellite

This increasingly recognized that the low-energy core plasma is a critically important part of magnetospheric plasma transport, yet this plasma cannot be accurately measured from spacecraft at potentials much different from that of the ambient plasma. In low-density regions such as the polar cap and lobes, spacecraft charge positively, excluding core ions from the spacecraft and accelerating core electrons so much that their velocities cannot be measured with any accuracy. In regions of high electron pressure and temperature, spacecraft charge negatively, excluding the ambient core electrons and accelerating the core ions so much that their velocity cannot be accurately measured. Plasma contactors have been used on a number of spacecraft operating in low-plasma-density regions to prevent charging of spacecraft to high potentials, particularly when exposed to high fluxes of energetic particles. This concern has prompted extensive studies by NASA for use of plasma contractors on the international space station where solar arrays may significantly affect the spacecraft potential.

Monte Carlo Simulation of Neutral Xenon Flows in Electric Propulsion Devices

Numerical simulations of neutral xenon flow in three different electric propulsion configurations are presented. Flows through an ion thruster, a plasma contactor, and a Hall thruster are considered. The computations are performed using a Monte Carlo method. The ion thruster configuration chosen for study is the UK-10 engine. Comparisons of the simulations are made with a simple analytical model that has been used in previous computations of backflow from ion thrusters. Both sets of theoretical results are compared with number density measurements. The Monte Carlo simulations provide closer agreement with the experimental data and the analytical model is found to overpredict density in the backflow plume region. The Monte Carlo simulation of the plasma contactor plume also compares well with experimental measurements. For the Hall thruster, the simulations reveal detailed structures of the fluid flow in the annual anode chamber. Comparison with a measured data point for velocity indicates that the experiment is compromised by a relatively high facility back pressure. Inclusion of the back pressure in the simulation leads to excellent agreement with the datum.

Enhanced electrodynamic tether currents due to electron emission from a neutral gas discharge: Results from the TSS‐1R Mission

During the reflight of the first electrodynamic Tethered Satellite System (TSS‐1R) mission, the unplanned separation of the tether at the Orbiter end resulted in the highest tether current during the mission. In the moments just prior to the tether separation with 19.7 km of tether deployed and a generated electromotive force (EMF) of 3482 V, currents reaching approximately 0.97 A were shunted through the tether to the Orbiter electrical ground, which was in contact with the ionosphere primarily through its main engine surfaces. This current level was nearly twice as large as observed during any nominal operating period. As the failure point of the tether entered into the ambient plasma, the current increased to 1.1 A and maintained this level even after the break for approximately 75 s. The principal surprise in these results was that the broken end of the tether, with only a few short strands of copper wire, could support higher currents than the much larger Orbiter conducting surface areas. Analysis of possible current enhancement mechanisms revealed that only a gas‐enhanced electrical discharge, providing an electron emission source, was plausible. Ground plasma chamber tests confirmed this analysis. The TSS‐1R results thus represent the highest electron current emission from a neutral plasma source yet demonstrated in a space plasma. This is of interest for current collection processes in general and plasma contactor development in particular.

Of Plasma Contactor Performance

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

Current–voltage response of anodic plasma contactors with external ionization

A spherical, steady-state model that considers the combined influence of plasma emission and external ionization of neutral gas on the operation of electron-collecting contactors is presented. Ionization by both Maxwellian and streaming electrons is investigated. A core (or quasineutral cloud) and a no-core mode are considered and the transition conditions from one to another are derived. The collected current is determined in terms of the main plasma and contactor parameters. The current-voltage response reproduces the ignited regime observed in experiments. The transition to the ignited regime and the transition to the core mode are not equivalent. It is shown that ionization by the electron beam is the main driver of ignition. The Langmuir law for the double layer causes a self-regulation of the core that affects the collection of current and leads to a multiplicity of solutions. Different neutral gas profiles are analyzed. It is uncertain whether ignition is reached when the neutral gas is provided by the contactor exclusively. The appearance of nonmonotonic potential profiles for low plasma emissions is noted.