The results in Table 1 depend on the momentum accommodation coefficient an. In Table I ern=0.7 cos 0, was used for both the neutrals and ions to the conductor and the dielectric. In Fig. 10 we show the ion drag coefficient as a function of the normal accommodation coefficient crn = cr cos 6, of the conductor and the dielectric for P=— 90. As 0 changes only the reflection component changes. At high voltage, the change of o of the conductor gives a larger change than the change of a of the dielectric since the total force is dominated by the reflection on the conductor and the ram-scattering component. At the maximum cr=O (complete reflection), cd is as much as two times that at <7= 1 (complete accommodation). In reality, the accommodation coefficient crn depends on the ion species, surface material, incident energy and incident angle. The data of the momentum accommodation coefficient for O+ are limited. Boring & Humphris [5] showed the momentum accommodation of O+ was independent of the surface material since O+ interacted with the adsorbed gas layer. If we use their data cr=0.8~0.95 for /?= —90, it leads to low cd. But at high voltage the physics of momentum accommodation become complicated because we have to take into account sputtering of the surface material. Conclusion We have examined the enhanced ion drag on a highly negatively biased solar array identified in Ref. [2]. We have shown that the drag found in Ref. [2] is an overestimate of the drag and occurs because all the conductors are taken as interacting. We have formulated a model where the conductors do not interact and examined its predictions and validity with PIC simulations. We find drag coefficients lower than found before
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