High temperature plasma is produced in a controlled thermonuclear reactor and a plasma beam is directed therefrom through an expander having a decreasing intensity magnetic field to provide an ion beam in which the ions have substantially only translational kinetic energy. The ions of the beam are then directed, by application of an E X B field, into an offset collector system having collectors arranged in a zig-zag pattern while the electrons are separated from the ions. The collectors situated in an E X B field have progressively lower retarding potentials applied thereto so that ions having a kinetic energy exceeding that of a particular stage are collected in successively lower energy groups to produce an electrical current therein. An advantage is obtained in that ions in a beam having a wide energy spread are collected at an electrode where there is a low energy differential between the effective retarding potential of the particular collector electrode and the group of energetic ions collected thereat making for efficient conversion of the kinetic energy of the particles into electrical energy.