Electron cyclotron resonance (ECR) is achieved in a source chamber of a size which is non-resonant with respect to propagation of the microwave power within the chamber. The microwaves are delivered into the chamber via a waveguide and window so that breakdown occurs initially only in a region in the vicinity of the window. A dielectric coupler between the waveguide and the window has a larger end and a smaller end and is filled with a dielectric material. The magnetic field generator for stimulation the electron resonance in the chamber includes a pair of conductive current carrying coils coaxial with each other and with an axis of the chamber, the coils being arranged in a Helmholtz configuration. The waveguide includes a microwave stub tuner for tuning the propagation and absorption of the microwave power in the plasma within the chamber to control the location and shape of the region in which the plasma is formed. A conduit provides a path for delivery of the plasma from one end of the chamber and is sized to be non-resonant with respect to propagation of the microwave energy. The magnetic field is controlled to cause the formation of the plasma with high absorption of microwave power occurring substantially within a columnar region centered on an axis of the chamber. The chamber has an insulative lining.