D. L. Cooke

TSS‐1R electron currents: Magnetic limited collection from a heated presheath

We show that currents collected by TSS-1R are consistent with Parker-Murphy magnetic limited collection from the ionosphere where the local electron temperatures are elevated as observed during the first TSS-1 mission. An analytic development of the previous Katz et al. [1994] presheath model is presented. This model accounts for the heating that occurs as electrons fall through the potentials necessary to reflect the ram ions while satisfying the Bohm sheath stability criterion. This presheath model is coupled to a sheath sized to the Parker-Murphy collection radius to predict the current collected by TSS-1R. The resultant currents are 2 to 3 times larger than for the standard Parker-Murphy collection model and are consistent with observations.

Potentials on Large Spacecraft in LEO

A system-oriented computer code is used to predict surface charging due to voltages generated within a satellite operating in the typical dense plasma environment of LEO. The use of this code is demonstrated by predicting the expansion of electric fields onto a kapton surface from a pinhole over a biased conductor in a LEO environment. The results are compared to a more-exact solution and experimental data.

High‐voltage interactions in plasma wakes: Simulation and flight measurements from the Charge Hazards and Wake Studies (CHAWS) experiment

The Charge Hazards and Wake Studies (CHAWS) flight experiment flew on the Wake Shield Facility (WSF) aboard STS-60 and STS-69. The experiment studied high-voltage current collection within the spacecraft wake. The wake-side sensor was a 45-cm-long, biasable cylindrical probe mounted on the 3.66-m-diameter WSF. Operations were performed in free flight and at various attitudes while on the shuttle orbiter remote manipulator system (RMS) arm. Preflight and postflight simulations were performed using the programs Potentials of Large Objects in the Auroral Region (POLAR) and Dynamic Plasma Analysis Code (DynaPAC) and are compared here with the flight results. Both programs perform three-dimensional, self-consistent, steady state plasma simulations. During high-voltage operations the wake-side probe collected current consistent with preflight predictions. In both the flight data and the steady state simulations the current collected has a power law dependence on the potential and has a less than linear dependence on density. Growth of the sheath beyond the WSF edge controls the high-voltage current collection, and kinematic and space charge effects both play important roles in attracting ions into the wake region. Measurements made at low voltage differ from the calculations. The preflight calculations for a pure oxygen plasma predict a collection threshold at −100 V bias. The flight data show little or no threshold, implying a source of ions not accounted for in the simulations. Possible sources for these ions are discussed.

SPEAR 3 flight analysis: Grounding by neutral gas release, and magnetic field effects on current distribution

The Space Power Experiment Aboard Rockets (SPEAR) 3 experiment was launched on March 15, 1993, to test grounding devices for negative payloads. In this paper we review two aspects of the high-altitude flight data and compare them with preflight predictions. The SPEAR 3 neutral gas release experiment studied a grounding mechanism observed on previous flights during attitude control system (ACS) firings. Preflight calculations using Paschen law physics generalized to three dimensions predicted that the high rate gas release (about one order of magnitude below normal ACS) would reduce the rocket potential to within 200–300 V of plasma ground. The flight data is well fit by a value of −225 V. Orientation relative to Earths magnetic field had no effect on the floating potential or grounding operations but had a large effect on the portion of the current collected by the boom. We compare these flight measurements with preflight calculations made with the DynaPAC computer code.

Analysis of ion densities in the vicinity of space vehicles: Ion-neutral chemical kinetics

Recent laboratory studies of ion-molecule reactions relevant to the analysis of ion densities in the vicinity of space vehicles in low-Earth orbit are discussed. The studies include new cross-section measurements of the important O++ H2O charge transfer collisions and the H3O+ -producing H2O++ H2O collisions, as well as product kinetic energy analysis of the reaction products. The charge transfer reaction rate is found to be less collision energy dependent than previously assumed, whereas the H3O+ -producting reaction rate is highest at thermal energies and decreases rapidly with increasing collision energy. Implications of the new results on previous interpretations and recent space observations are discussed.

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.

Simulation of the critical ionization velocity: Effect of using physically correct mass ratios

The concept of the “critical ionization velocity” (CIV) of a neutral gas has received much attention since it was proposed by Alfven. In the last several years, numerical simulations have been a useful method of examining the physical mechanism behind CIV. Since these simulations have been run using explicit particle-in-cell (PIC) codes, artificial mass ratios (e.g. mi/me = 100) have been employed to reduce the time scales in the simulation and the computational power required for the problem. However, the effect of the use of unphysical mass ratios on the simulation results has not been well discussed. In this study, simulations using artificial and realistic mass ratios are compared. The study employs an implicit PIC code to allow the realistic mass ratio simulations to be performed efficiently. Several numerical aspects of using implicit codes, such as the undesirable damping of upper hybrid heating, are discussed. The results indicate that when scaled appropriately, simulations using physical and unphysical mass ratios provide nearly identical results when the anomalous ionization rate is either much lower than the ion cyclotron frequency (νion/Ωi ≪ 1) or greater than the ion cyclotron frequency (νion/Ωi > 1). In the case of an anomalous ionization rate near the cyclotron frequency, the unphysical mass ratio results cannot be scaled easily to regain the results from the physical mass ratio simulation.

Upper bound estimates of anomalous ion production in space-based critical ionization velocity experiments

The critical ionization velocity (CIV) is an anomalous ionization mechanism first proposed by Alfven. Experiments have confirmed the existence of a critical velocity in laboratory experiments, but sounding rocket experiments have been ambiguous as to the existence of the critical velocity in the ionosphere. The purpose of this paper is to produce upper bound estimates of anomalous ion production in space-based experiments of the critical ionization velocity. The analysis relies on the results of implicit particle-in-cell simulations and a simple rate model to predict the number of ions produced as a neutral cloud traverses a point in space. The model assumes a point release of neutral gas in the ionosphere which is meant to represent a typical sounding rocket experiment. The results of the model suggest why strong evidence of CIV is not observed in space-based experiments. Space-based experiments require the anomalous ionization process to be initiated through seed ionization provided by charge exchange or other mechanisms. This seed ionization process is too slow in space experiments to ignite CIV. The results also indicate that some of the space based results can be accounted for by assuming a large barium-oxygen charge exchange cross section instead of invoking anomalous ionization mechanisms.

Bootstrap Surface Charging at GEO: Modeling and On-Orbit Observations From the DSCS-III B7 Satellite

We present an analysis of the charging interactivity between surrounding surface materials aboard a spacecraft at geosynchronous altitudes. In particular, bootstrap charging of a small surface may occur if is surrounded by a large negatively charged surface. Here, a negative potential barrier forms above the small surface, resulting in suppression of photo- and secondary electron emission from that surface. Additionally, the small surface experiences an enhancement of the collection of the photo- and secondary electrons emitted from the surrounding surface. This mechanism results in the charging of the small surface to higher levels than that of the patch in isolation, and in many cases the final potential will reach that of the potential of the larger surrounding surface. With this study we examine bootstrap charging behavior with model data and with data collected on orbit. We have modeled the DSCS-III B7 geosynchronous satellite with realistic geometry and spacecraft materials. Additionally, a previous study has shown that bootstrap charging has been observed on the DSCS-III B7 geosynchronous spacecraft. Both Astroquartz and Kapton cloth patches charged up to the frame potential of the satellite during periods of severe frame charging. The results of modeling bootstrap charging of a small Kapton patch floating relative to the DSCS-III frame fixed at a potential of -1,000 V show that the patch will indeed charge up negatively to match the frame potential, with the temporal increase in negative potential following an exponential time characteristic.