In this review we call attention to basic phenomena and physical processes involved in the expansion of a plasma into a vacuum, or the expansion of a plasma into a more tenuous plasma, in particular the fact that upon the expansion, ions are accelerated and reach energies well above their thermal energy. Also, in the process of the expansion a rarefaction wave propagates into the ambient plasma, an ion front moves into the expansion volume, and discontinuities in plasma parameters occtlr. We discuss the physical processes which cause the above phenomena and point toward their possible application for the case of the distribution of ions and electrons (hence plasma potential and electric fields) in the wake region behind artificial and natural obstacles moving supersonically in a rarefied space plasma. To illustrate this, some in situ results are reexamined. Directions for future work in this area via the utilization of the Space Shuttle and laboratory work are also mentioned.1.
Abstract. The physical process of current collection by a "bare wire" electrodynamic tether in space is considered. The study uses an improved model that takes into account the resistance of the wire and the magnetic shielding induced by current flow in the tether. The plasma density, ne, electron temperature, Te, tether length, L, tether radius, r w, and the angle of the geomagnetic field to the tether (90ø-o0 were all used as parameters. It is shown, for certain tether configurations and parameter values, that magnetic shielding reduces the collected current. In general, any parametric change that increases tether current, and hence, the strength of the current-induced magnetic field relative to the strength of the electric field between the tether and the ambient plasma, will increase the shielding effect. Tether current is increased directly with tether collection area (which depends on L and rw), plasma conductivity (which depends on n• and T0, and the motional emf along the tether (which increases with L and the angle 90ø-00. It turns out that, as any of these parameters change so as to cause the overall tether current to increase, the overestimate of current that results from ignoring the magnetic shielding effect becomes correspondingly greater. Moreover, it is shown that a tether system in the thruster (or motor) mode suffers greater current reduction from magnetic shielding than does the same tether deployed in the generator mode. Finally, it is shown that, for certain tether system configurations combined with particular values of the governing plasma parameters, current-induced magnetic shielding can significantly reduce the collected current and, therefore, system efficiency. For example, in the case of an electrodynamic tether system in the thruster mode under conditions of ne= 1.67xl 06 cm -3, a=60 ø, rw=2.5 mm, and Em=34 Wkm, magnetic shielding will reduce the collected current 10% at a point L=0.65 km along the tether (4.3 A instead of 4.8 A) and this increases to more than 40% at L= 1.3km (9.6 A instead of 13.4 A).
Mission BackgroundThe Tethered Satellite System (TSS) program is a binational collaboration between NASA and the Italian Space Agency (ASI) with NASA providing the Shuttle-based deployer and tether and ASI providing a satellite especially designed for tethered deployment. Twelve science investigations (see Table 1) were supported by NASA, ASI, or the Air Force Philips Laboratory. The goals of the TSS-1R mission, which was the second flight of the TSS hardware, were to provide unique opportunities to explore (1) certain space plasma-electrodynamic processes--particularly those involved in the generation of ionospheric currents, and (2) the orbital mechanics of a gravity-gradient stabilized system of two satellites linked by a long conducting tether.TSS-1R was launched February 22, 1996 on STS-75 into a 300-km, circular orbit at 28.5 ø inclination. Satellite flyaway occurred at MEF 3/00:27 and a unique data set was obtained over the next 5 hours as the tether was deployed to a length of 19,695 m. At MET 3/05:11, during a day pass, the tether suddenly broke near the top of the deployer boom. The break resulted from a flaw in the tether insulation which allowed the ignition of a strong electrical discharge that melted the tether. The operations that had begun at satellite flyaway, however, allowed significant science to be accomplished. Instrumentation and MeasurementsThe TSS converted mechanical energy into electrical energy in a classical demonstration of Faraday's law. The configuration was such that the satellite received a positive bias and collected electrons from the ionosphere, which conducted through the tether to the orbiter where the circuit could be closed back to the ionosphere (see Fig. 1 sphere. An electrical schematic is shown in Figure 3 of Dobrowolny and Stone [1994].The TSS was instrumented to control the tether current (as described above) and diagnose the environmental space plasma properties under highly nonequilibrium conditions. The investigations, shown in Table 1
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