Hollow Cathode Plasma Contactor Research Articles

Analysis of Effect of Ground Experiment Environment on Plasma Contactor Performance

The large difference in the working performance of a hollow cathode plasma contactor between the ground and actual on-orbit environments will cause difficulties in selecting a ground-test schemes to fully simulate the contactor real on-orbit characteristics. In this study, the effect of the simulated anode size and structure, the simulated anode surface state, the flow rate of background working gas, and the background plasma density on the emission characteristics and plume structure of the contactor are studied. Three self-made simulated anodes of different sizes and structures are applied in the ground experiments. The change of anode surface state (particularly the ability of absorbing electrons) is realized by dividing the self-made simulated anodes into four double-separated regions and alternatively electrically isolating them. An additional gas channel and an auxiliary contactor are used to create background working gas and a low-Earth-orbit plasma atmosphere, respectively. The voltage–current curves as well as the plasma parameter distributions at the contactor exit and in the far-field regions are determined under different regimes and working conditions. The relationship between the contactor’s emission characteristics and plume structure is clarified. The experimental results could provide useful information for instructing the contactor design and developing a real on-orbit experiment plan.

Lorentz Force Characteristics of a Bare Electrodynamic Tether System with a Hollow Cathode

The electrodynamic tether (EDT) is a type of propulsion system that uses the geomagnetic field and ionospheric plasma and has the potential to conduct a space-debris removal mission without consuming a large amount of propellant. To understand the dynamic properties of the bare EDT system, an orbital dynamic model based on a detailed environmental space model and the real discharge characteristics of a hollow cathode plasma contactor (HCPC) was built. By numerical simulation, the differences in the bare tether performance caused by various orbital conditions and HCPC voltage models (at constant or various voltages) were compared and discussed. The results suggest that dynamic distinctions generated by the two bias voltage models increased as the latitude increased from 0° to 60°.

Ion Emission From a Positively Biased Hollow Cathode Plasma

Hollow cathode plasma contactors have an extensive history as spacecraft neutralizers, but questions remain on whether the technology can neutralize significant positive charging in tenuous space plasmas. Simulations and a representative semianalytical model (which we term the ion emission model) predict effective neutralization in these scenarios as ion current is emitted from the surface of the quasi-neutral contactor plasma according to the space-charge limit. This article focuses on experimental plasma measurements of a hollow cathode biased to several positive voltages with respect to a surrounding vacuum chamber in order to validate the ion emission model. Particular attention is paid to the emitted ions (those reaching the chamber wall) and to the ion-rich sheath region where the ion emission process takes place. Retarding potential analyzer (RPA) measurements were performed to understand ion flow velocity and direction as these parameters relate directly to the current predicted by the ion emission model. Planar probe measurements were performed at the chamber wall to compare local emission currents to model predictions. Evidence of collisions within the plasma (particularly charge exchange collisions) and simple models predicting ion energetics are presented. These experiments suggest that ion emission from the contactor plasma acts according to the ion emission model and adds to the physical understanding of spacecraft neutralization using hollow cathode plasma contactors as it may occur in tenuous space plasmas.

Discharge characteristics of cathodic hollow-cathode plasma contactors and related similarity to ground simulations

This paper focuses on the main factors affecting the electron emission current–bias voltage (I–V) characteristics of cathodic hollow-cathode contactors and similarity conditions that should be satisfied in ground-based simulation experiments. These issues are of crucial importance for ensuring the reliability and credibility of the I–V data obtained in ground experiments, improving the I–V characteristics, and for reducing the mass flow consumption and dependence on the space environment. In this study, a 1-D spherically-symmetric model is first applied and compared with experimental data. We explore the influence of ion current, electron temperature, ion energy at the contactor exit, and environmental plasma density on the I–V curves, potential structure, and size of the plume area. The influence of the diameter of the vacuum chamber on the experimental I–V characteristics is investigated and the necessary conditions required for the diameter of the vacuum chamber is proposed. Moreover, an analysis is conducted of the effect of the background plasma source on the experimental I–V curves. The results suggest that the I–V behavior is sensitive to the exit ion current and flow rates. Another model is further employed to predict the exit ion-beam current for different net electron-emission currents and consumption flow rates at a given clamping current. The calculations show that a single-cathode contactor presents an extremely low utilization ratio. The exit ion current and utilization ratio should be effectively enhanced to improve the I–V characteristics.

Surface Charging Controlling of the Chinese Space Station with Hollow Cathode Plasma Contactor

A highly charged manned spacecraft threatens the life of an astronaut and extravehicular activity, which can be effectively reduced by controlling the spacecraft surface charging. In this article, the controlling of surface charging on Chinese Space Station (CSS) is investigated, and a method to reduce the negative potential to the CSS is the emission electron with a hollow cathode plasma contactor. The analysis is obtained that the high voltage (HV) solar array of the CSS collecting electron current can reach 4.5 A, which can be eliminated by emitting an adequate electron current on the CSS. The theoretical analysis and experimental results are addressed, when the minimum xenon flow rate of the hollow cathode is 4.0 sccm, the emission electron current can neutralize the collected electron current, which ensures that the potential of the CSS can be controlled in a range of less than 21 V, satisfied with safety voltage. The results can provide a significant reference value to define a flow rate to the potential controlling programme for CSS.

A method of controlling floating potential for space station based on ion current magnification

The charging events of the International Space Station have been a focus of concern in recent years. Hollow cathode plasma contactors were applied to control the floating potential for their high emission currents of amps order. The in flight measurements showed that the floating potential was much smaller than predicted originally, and the actual emitted currents by the plasma contactors were well below amps level. In such case, a simpler method to control the floating potential may be applicable and suggested in the paper. By application of a metal sphere, which is biased at a high negative voltage relative to the structure and used as an ion current collector, the electron charging current of the structure can be effectively neutralized and the structure׳s floating potential can be controlled nearly to zero. The mechanism is introduced and the constraint condition for the system is investigated. Finally, a range of parameters for design are presented. Typically, for 100V high voltage array, a sphere of r=0.3–0.7m and biased at 500–200V is enough. For its simplicity, the method has obvious advantages, such as no requirement of working gas, high reliability and long life, etc.

CIV Experiments on ATLAS‐1

A test of the Critical Ionization Velocity (CIV) theory was made with neutral xenon releases from the Space Experiments with Particle Accelerators (SEPAC) hollow cathode plasma contactor onboard the Shuttle Orbiter Atlantis during the ATLAS‐1 mission. The gas velocity perpendicular to the Earth's magnetic field was essentially the orbital velocity (7.5 km/s), and thus it exceeded the CIV for xenon. The releases were observed with onboard instrumentation. A factor of 60 enhancement was seen in the Langmuir probe current. Calculations confirmed that release conditions generally satisfied criteria for CIV and predicted a maximum factor of 20 increase in plasma density. Thus, CIV effects were likely to have occurred during the ATLAS‐1 experiments.

Electron collection enhancement arising from neutral gas jets on a charged vehicle in the ionosphere

Observations of current collection enhancements due to cold nitrogen gas control jet emissions from a highly charged, isolated rocket payload in the ionosphere have been made during the cooperative high altitude rocket gun experiment (CHARGE) 2 using an electrically tethered mother/daughter payload system. The current collection enhancement was observed on a platform (daughter payload) located 100 to 400 m away from the main payload firing an energetic electron beam (mother payload). We interpret these results in terms of an electrical discharge forming in close proximity to the daughter vehicle during the short periods of gas emission. The results indicate that it is possible to enhance the electron current collection capability of positively charged vehicles by means of deliberate neutral gas releases into an otherwise undisturbed space plasma. Our results are also compared with recent laboratory observations of hollow cathode plasma contactors operating in the “ignited” mode.