Axially resolved measurements of plasma parameters were performed by two Langmuir probes moving in parallel from the exit of the driver (where the plasma is generated) up to the extraction region neighbourhood of the IPP RF negative hydrogen ion source prototype. At the driver exit, the plasma parameters show an unexpected inhomogeneity in the presence of the magnetic field: a cold and dense plasma flows out of the top part of the driver while a hot and low density plasma flows from the bottom part. A local relation between the top and bottom parameters is derived from the conservation of the energy flux.