On accuracy of RPA measurements of ion fluxes

One of the IPEI (Ionospheric Plasma and Electrodynamics Instrument) payload onboard the ROCSAT-1 spacecraft is a RPA detector to measure the ionospheric plasma species and temperature. A 3-D computer model has been developed to simulate the ion and electron motions inside the RPA. The results are compared with the analytical solutions in the 1-D theoretical model. Overall agreement between the two approaches is good. However, the detailed dynamical behavior of the charged particles' motions in response to the changes in the electric potential profiles inside the RPA detector, explains the causes of the physical discrepancies between the analytical model and the computer simulation. The important discoveries in the computer model are: (a) Ion transparency can not be represented by the physical grid sizes in the detector. The alignments of the meshed-wire planes will affect the ion transparency, (b) Cut off energy for protons is not as sharp as indicated in the analytical model. The contamination in the collector current from lighter ion species is possible. (c) Electron contamination in the collector plate is noticed if the mesh sizes are larger than the electron Debye length.