Space Charge To safely generate electrical power in space, designers
of the ISS had to keep in mind that low-Earth orbit is filled
with a charged plasma.
Before the ISS, electric power systems on U.S. space vehicles operated at the nominal 28 volts DC inherited from the aircraft industry. At such low voltages, plasma interactions in low-Earth orbit (LEO) are negligible and were not a consideration in spacecraft design. High-power systems such as those on the International Space Station operate at higher voltages to reduce power loss and system weight. But they suffer the drawback of interacting with the ionospheric plasma in several different ways:
1. Conducting surfaces whose electrical potential is highly negative with respect to the plasma undergo arcing. Such arcing not only damages the material but also results in current disruptions, significant electro-magnetic interference, and large discontinuous changes in the array potential. Further, inbound ions, accelerated by the high fields, will cause sputtering from surfaces with which they impact.
2. Solar arrays or other surfaces whose charge is biased positively with respect to the plasma collect electrons from the plasma, resulting in a parasitic loss to the power system. Since the mass of an electron is much less than an ion, the magnitude of current density is much greater for surfaces with positive bias. At bias potentials in the 200-volt range, sheath formation and secondary electron emission from the surface causes the entire surrounding surface, normally an insulator, to behave as if it were a conductor. This effect, called "snapover," results in large current collection from even a very small exposed area.
3. Currents collected by biased surfaces also significantly affect the potentials at which different parts of the spacecraft will "float." (See "Floating Potential" section below.) Because of their large mass and low mobility, ions collected by negatively biased surfaces result in a relatively small plasma current density. Lighter weight electrons, on the other hand, are readily collected by positively biased surfaces. Ram and wake effects around the moving spacecraft further complicate the picture. The worst situations occur when the spacecraft power system uses a negative ground, so that large surfaces are negative and collect slow moving ions to balance the current from electron collection. In this arrangement, parts of the spacecraft will be biased with respect to the ionosphere to a level very near the maximum voltage used on the solar arrays.
This charging of spacecraft as they float in the plasma of space, if not controlled, can lead in some cases to mission degradation or worse. Loss of contact with some geosynchronous (GEO) satellites has been attributed to arcing caused by differential charging of the vehicle. There is also the problem of floating potential in low-Earth orbit (LEO), as with the space station.
In the GEO space environment, the major interaction of concern is differential charging of various parts of a spacecraft, leading to high electric fields and arcing between spacecraft components. In GEO, the ambient plasma thermal current is insufficient to discharge spacecraft surfaces rapidly. Although there are many sources of charging, most of the arcs are the result of charging during solar substorm events. During these solar storms, the Earth's geomagnetic tail, laden with particles from a sudden solar wind transient, undergoes field line breakage and reconnection, accelerating electrons to thousands of electron-volts (a unit of energy). These electrons charge insulating spacecraft surfaces, increasing electric fields to adjacent conductors beyond a breakdown level, leading to arcing. The arc currents travel through spacecraft conductors to upset electronic components, induce spurious signals, and other ill effects. It is common with GEO craft to coat all outside surfaces with conducting materials to eliminate differential charging.
Spacecraft surfaces charge up to (or "float" at) potentials that result in no net current collection from the plasma. If a spacecraft has no exposed conductive surfaces, it will float within a few volts of the surrounding plasma potential ("plasma ground"). Typically, a spacecraft will charge negatively in order to collect enough of the less mobile heavy ions. If conductors of similar area but at different voltages are exposed to the plasma, a rule of thumb is that the most negative surfaces will float negative of the plasma about 90 percent of the total voltage difference between the surfaces.
In addition to the surface voltage, a spacecraft's floating potential also depends on how it is grounded to the power system. For example, a spacecraft grounded to the negative end of a solar array (160 volts DC end to end) will float at about 140 volts negative from plasma ground, which is where the negative end of the array will float. If grounded to the positive end of the solar array, the spacecraft will float close to plasma ground.
Most of the adverse effects caused by spacecraft charging in LEO (i.e., sputtering, contamination, arcing) depend on the charging level -- the "floating potential" -- of the spacecraft. As the floating potential increases more negatively (greater than 30 volts negative of plasma ground), the severity of these effects also increases.
A Solution: The Plasma Contactor
The grounding of the space station's photovoltaic arrays (the negative end of the arrays are grounded to the structure) place the ISS at large negative electrical potentials (-160 volts DC) relative to the ambient space plasma when the arrays are producing power.
In order to avoid sputtering and to lessen other undesired results of this grounding scheme, plasma contactor devices in the hull emit a steady stream of plasma from the station to raise its potential to within a few volts of the ambient plasma.
The plasma contactor acts as an electrical ground rod to connect the space station structure to the local environment and harmlessly dissipate the structure charges. GRC engineers designed, manufactured, and tested the hollow cathode assembly, which performs this function by converting a small supply of gas into ions and electrons and discharging this stream to space. The stream carries with it the excess electrons that created the surface charge.
A problem to be solved is the need to work on parts of the ISS solar arrays, such as faulty gimbals, in sunlight while they are operating. If the plasma contactors in the hull are turned off, says Sheila Bailey, the ISS surface charge floats up to the "open circuit voltage" of the arrays -- which is 260 volts DC! A solution is being researched now, she reports.
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