The analysis of ion data from retarding potential analyzers (RPA's) is generally done under the planar approximation, which assumes that the grid transparency is constant with angle of incidence and that all ions reaching the plane of the collectors are collected. These approximations are not valid for situations in which the ion thermal velocity is comparable to the vehicle velocity, causing ions to enter the RPA with high average transverse velocity. To investigate these effects, we calculate current-voltage curves for H+ at 4000° K, taking into account the finite collector size and the variation of grid transparency with angle. These curves are then analyzed under the planar approximation. The results show that only small errors in temperature and density are introduced for an RPA with typical dimensions;and that even when the density error is substantial for non-typical dimensions, the temperature error re¬mains minimal. This last fact may be useful for designing a non-typical RPA which is optimized to operate in unusual conditions.
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