For bunch currents up to 200 mA it is possible to make a technical layout of a linac which allows operation in a non-space charge dominated regime. As a consequence almost no rms emittance increase is obtained in all three planes and the production of halo particles is reduced. To examine the diffusion process of particles moving from the core into the halo, Monte Carlo simulations with a large number of macroparticles are necessary. Single particle properties are obtained from the simulations and compared with analytical models. For high current proton linacs, based on a technical layout, results are presented including mismatch and field errors in magnets and rf systems.