EFFECTS OF ARGON ION INJECTIONS IN THE PLASMASPHERE S. A. Curtis and J. M. Grebowsky Laboratory for Planetary Atmospheres NASA/Goddard Space Flight Center. Greenbelt, Maryland 20771 In lifting massive space power system payloads from low earth orbit to geosynchronous earth orbit. Cargo Orbit Transfer Vehicles (COTV) using ion propulsion will inject energetic beams of argon ions into the plasmasphere. The argon ion beams have a fast velocity V, < V^ the Alfven velocity of the plasmasphere medium and V, » V , the thermal velocity of the plasmasphere ions. The relationship of the beam velocity to these characteristic velocities as a function of radial distance in the plasmasphere is shown in Figure 1 for positions near the earth's equatorial plane. The Alfven speeds are shown for the Chiu et al model plasmasphere and the average and l^w Alfven speeds are calculated from OGO-5 observations analyzed by Chen et al . As can be seen, the Chiu et al model gives an upper bound to the Alfven speeds. The average OGO-5 Alfven speeds give the best indication of the Alfven speeds which are of the order of the beam speed throughout most of the plasmasphere whose outer boundary is between 4 and 6 earth radii (R ). Hence V^ < V^. The thermal speeds in Figure 1 are taken from the Chiu et af model. In this case discrepancies between observations and the model are unimportant since V^ » always. The spread velocity of the beam perpendicular to its direction of propagation is AV, 0.4 V^. Thus the exhaust of the COTV's may be described as a fast, rapidly diverging ion beam. Due to these beam characteristics, the numerous potential plasma instabilities which could take energy from the beam and hence stop it are ineffective. This is due to the fact that the beam and background plasma - parameters change sufficiently rapidly as not to^allow amplification of instability generated waves to significant amplitudes . Another beam stopping mechanism which models the fast ion beam as a slowly moving ion cloud with V^ « V h and V^ « V^ is not applicable given the relationship of V, to V^ and shown in Figure 1. In addition, to this inconsistency the ion cloud model assumes the beam plasma can be regarding as infinitely conducting. This frozen fiel^l line concept is not applicable here since a realistic model of the beam plasma which accounts for both the initial plasma turbulence and that generated by the low amplitude plasma wave turbulence carried with the beam gives rapid diffusion times t =X^/Djl* as shown in Figure 2. Note that the beam Debye length and Dj^* the anomalous diffusion coefficient associated with the plasma turbulence. The currents resulting from the turbulence induced anomalous resistivity are insufficient to short out the polarization electric field. Despite the limitations on beam stopping mechanisms caused by the beam velocity characteristics and its finite conductivity, not all of the beam plasma "escapes the plasmasphere. Since the polarization electric field imposed at the thruster to allow cross field propagation of the beam is nonuniform over the sheath of the beam, the plasma in this sheath is lost and deposited on local field lines. This beam sheath loss model results in a deposition of argon ions and hence energy in the plasmasphere which is much less than that in models which call for ion clouds or plasma instabilities to rapidly stop the beam. In Figure 3, a comparison is given of the cumulative fractional mass loss of an ion beam injected at 1.5 R for the ion cloud and the ion beam sheath loss process. The ion cloud process yields total deposition very rapidly whereas all but a few percent of the beam in the ion beam sheath loss process escapes. In Figure 4 the integrated difference of these two deposition models is shown for the construction of one SPS. The ion cloud process gives better than an order of magnitude greater energy and number density perturbation
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