DISPOSITION OF SPS ELECTRIC THRUSTER EXHAUST D. H. Holze and H. B. Liemohn Boeing Aerospace Company, Seattle, Washington A methodology is presented which will allow the calculation of the total quantity of thruster argon precipitated into the earth's atmosphere. Our calculations conclude that the ions transfer to single particle orbits in tens of kilometers from the thrusters^ and that the dominant ion loss mechanism is charge exchange with thermospheric neutrals. The methodology is presented in more detail elsewhere.2 Figure 1 defines the geometry for an OTV in a circular transfer orbit with inclination i (defined by i = sin-^ (sin X/sin ip) and velocity v_. Because the inclination must be reduced to zero as the OTV approaches GEO, the thrust is not in general antiparallel to the velocity. For a dipole field it was found that the pitch angle between the thrust vector and the local geomagnetic field direction is (This expression which optimizes payload to GEO gives a maximum 3 value of 76° for r^EQ = 42,000 km.) The thruster plasma is sufficiently dense in the vicinity of its exit plane to generate its own currents that block out the geomagnetic field. As the beam and thermal plasmas expand, however, the geomagnetic field eventually takes control of the individual particles. As a first approximation, it will be assumed that most ions transfer to single particle orbits by the time the plasma dynamic pressure drops to one-tenth of the magnetic field pressure. The worst case occurs at GEO where the 920 OTV thrusters (TZOcm, 1.5 kV) produce a plasma plume extending 670 km downstream. At LEO, single particle behavior commences 1.4 km downstream from each corner of the OTV. Once thruster ions are trapped in the geomagnetic field they follow well- known orbits as pictured in Figure 2. It was found that the number density of thruster argon ions in the dipole shell between latitudes X] and X2 is The bounce period, Tn , is found from Eq. (5) by letting X2 be the mirror latitude Xm and setting X] = 0. In these equations e is the electron charge, Be is the equatorial field intensity at re, m and v are the ion mass and speed and the angle 0sis defined in Figure 2.
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