Abstract: Apparatus wherein a partially ionized mass of gas is enclosed by a rotationally symmetric, axially elongate housing and set in rotation by a force which arises as a result of a substantially axial magnetic field constant in time and an electric current having a non-zero vector component in the radial direction, whereby positively charged ions originating from a centralized axial gas discharge arc region and moving at a high speed of rotation set the whole mass of gas in rotation by pulse transfer, said housing being lined on the inside with annular wall electrodes segmented in non-azimuthal direction and arranged against each other; the inside wall of said housing at an extremity thereof curving toward the center axis and approaching a centrally mounted electrode of opposite polarity, the wall electrodes being connected with variable resistors which are adjusted so that the voltage applied to the wall electrodes increases blockwise from a minimum value at a chosen point on the axis of the housing to the grid-