statement of the fact that for these large lengths the force due to deflecting the ions has become small compared to the direct ram and reflected force. In their interacting limit they obtained drag coefficients as high as 10 as well as a significant coefficient of lift. It is easy to see that the interacting limit will give the maximum ion drag on the array since it suggests that the drag on parts of the array where the bias is low will be affected by the highly biased parts. Noninteracting Limit The key assumption of the interacting model is that all the sheaths overlap. However for a typical space station solar cell which is 8 X 8 cm with a 2 mm interconnector the aspect ratio of conductor length to dielectric length is 0.03. Only for very large potentials would one expect that the sheath from the conductor would overlap the dielectric and reach to the next conductor. This motivates a model which assumes that the sheaths do not overlap so that each segment of the array can be treated independently of the other segments. We shall call this the noninteracting limit. The noninteracting limit will give a useful lower bound on the force on a solar array. In the non-interacting limit the force on an array composed of ns segments and with total area Jarray is where d is the unit vector in the drag direction (parallel to the ion flow velocity) and I is the unit vector in the lift direction (perpendicular to the ion flow velocity). The key assumption that leads to the noninteracting limit is that the force on each segment of the array depends only on the potential of the conductor in that segment and not on the potential on the nearby conductors. The physical model is schematically indicated in Fig. 1. The sheath boundary can be defined physically by the condition that efy/\ w,F^rbital = 1. This is the potential at which the kinetic energy of the ions is just equal to their potential energy. As indicated in Fig. 1 the noninteracting limit will be true as long as the sheath boundaries from neighbouring conductors do not overlap and as long as the incoming ion flow can reach a given segment without passing through the sheath of any neighbouring segments. This will put a limit on the angle of attack for which the noninteracting model will be valid since it is clear that particles striking the array at very small angles of attack will pass through the sheaths of several conductors before hitting the array. Therefore for small angles of attack the force on any given segment will depend on the potentials on the other segments. Hence we have to know the ion flow field over the whole solar array which has many conductors and dielectrics.
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