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Helicon sources coupled to diffusion chambers provide a new method for better controlling plasma processing. It is, however, important to understand the effect of diffusion on the plasma evolving from the source to the wafer situated in the chamber. The diffusion of a 300 W argon plasma from a 13.56 MHz (6 cm diam, 20 cm long) cylindrical magnetic field free source into a 30 cm diam, 20 cm long chamber has been experimentally and theoretically investigated. A mobile electrostatic energy analyzer was used to measure the electron temperature, plasma potential, plasma density, and ion energy distribution functions (EDFs) along the common axis of the source and chamber. An analytical model based on experimental results has been developed. Although there is a potential difference of around 40 V between the source and the substrate table, charge exchange collisions associated with the plasma expansion reduce the average energy of the ions impinging on a grounded probe at the bottom of the chamber to about 3 eV. At 1 μbar the number of collisions is too low to cool all of the ions accelerated in the field and a tail remains in the ion distribution function. Although not measured, hot neutrals should also be generated as a consequence of charge exchange collisions and will impinge on the substrate.