Apparatus including a chamber and a coil system for converting a field-generating current into a RF magnetic field in the chamber when the chamber contains an ionized gas which interacts with the RF magnetic field to create a plasma. The plasma is contained within a cylindrical region enclosed by the chamber, which region has a longitudinal center axis, and the region is considered to be made up of a plurality of annular zones concentric with the center axis and disposed at respectively different distances from the center axis. The coil system is composed of: a plurality of individual coils each positioned and dimensioned to produce a RF magnetic field which predominantly influences a respective annular zone.