Measurements and simulations of the suprathermal tail of the energy distribution function of deuterium ions in the Joint European Torus are reported. The suprathermal deuterium ion tail was produced by Nuclear Elastic Scattering (NES) collisions between MeV energy (a) DT fusion a-particles, or (b) ICRH driven He3 minority ions, in deuterium plasmas. Measurements of the line-of-sight integrated energy distribution function of the suprathermal tail of deuterium ions, DT fusion a-particles and ICRH driven He3 ions were made using a high energy Neutral Particle Analyzer (NPA). The NES or 'knock-on' tail of the deuterium ion energy distribution function was simulated using the FPP-3D Fokker-Plank code which solves the 3-D trajectory averaged kinetic equations in JET geometry while taking into account the NES of the DT fusion a-particles and He3 projectile ions on deuterium target ions. The required input energy distribution function of ICRH driven He3 minority ions was simulated using the SELFO Monte-Carlo code which yields a self-consistent He3 ion energy distribution. Simulation of plasmas containing the ICRH driven MeV energy He3 ions revealed that the measured suprathermal tail ion density exceeded by nearly one order of magnitude that expected due to NES of He3 ions on deuterium ions. The comparison between measurement and simulation in the He3 ICRH experiments is contrasted with analogous comparison between measurements and simulation of JET plasmas in which 3.5MeV DT fusion a-particles were the projectile ions, where measurement and simulation roughly agreed. We conjecture that secondary NES processes with products of the fusion reaction D + He3 Æ p(14.7MeV) + He4 (3.6MeV), which are not included in the FPP-3D code simulations, are responsible for the observed excess knock-on deuterium ion tail in the He3 minority ICRH experiments. Work is in hand to incorporate these second-generation NES reactions into the FPP-3D code. The importance of the above considerations for a complete understanding of ITER plasma behaviour is underlined.