Effect of grid transparency and finite collector size on determining, ion temperature and density by the retarding potential analyzer

The analysis of ion data from retarding potential analyzers (RPA's) is generally performed by a standard RPA method which makes several assumptions, including the assumptions that the grid transparency is constant with angle of incidence and that all ions reaching the plane of the collector are collected. These approximations are not valid for situations in which the ion thermal velocity is large enough to be comparable to the vehicle velocity so that ions 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 by the standard RPA method. 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 nontypical dimensions, the temperature error remains minimal. This last fact may be useful for designing a non-typical RPA which is optimized to operate in unusual conditions.