Computer simulations of a design for a Plasma Ion Mass Spectrometer (PIMS) head for diagnosing the edge plasma on the Joint European Torus (JET) magnetic fusion experiment are reported. An overview of the design issues is presented along with results of these fully three-dimensional ion optical calculations. By modelling the behaviour of the different impurity ions anticipated in JET, an evaluation of the proposed PIMS geometry has been carried out. The effects of misalignment of the electric and magnetic fields within the PIMS on mass resolution, and the effect of the range of ion temperature and sheath voltage expected in JET on the transmission are described.