The topic of this thesis is the study of basic plasma transport and chemistry in the diffusion chamber of the ARIS (Applied Radio-frequency Ion Source). The ARIS experiment has been designed as a two chamber system, with a source (high density plasma region) and a diffusion region (plasma expansion region). The boundary conditions on the diffusion chamber and the neutral pressure determine the plasma parameters. Careful control over the boundaries give us some control on the plasma volume chemistry key element: the electron energy distribution function(EEDF). We explore the possibility of a magnetised boundary in the the diffusion chamber region by means of a multi-cusp magnetic array. A multi-cusp produces an effective field close to the boundary with low penetration on the plasma bulk. The diffusion chamber gains a sub-region defined by the magnetic field penetration region. Experimental measurements of the electron energy distribution function with a single Langmuir probe are compared with double probe results. Also the negative ion density (H- ) is measured in Hydrogen plasmas. Volume production of H_ requires special conditions of the EEDF and molecular Hydrogen vibrationally excited states to maximise the negative ion density. Results are compared for magnetised and non magnetised boundaries in the diffusion chamber. Results show th a t the magnetic field improve the plasma confinement (higher plasma density) and reduces the average kinetic electron energy (or Te) by actually changing the EEDF shape. It is proposed that this effects are the reason for the improvement on volume production of H- in a magnetised case in the diffusion chamber while the state of the population of vibrationally excited Hydrogen molecules is not clear. Finally, particle models complement the experimental work by providing some insight on single particle interaction with a multi-cusp magnetic field and self-consistent plasma interaction with a cylindrical Langmuir probe Also a global model developed by R Zorat is used to complement and understand the experimental results.