Abstract: Internal and external magnetic screens made of magnetic permeable material are added between the discharge chamber and the internal and external sources of magnetic field, respectively. A longitudinal gap is maintained between the screens and their respective internal and external poles, that does not exceed half the distance between the internal and external poles. The exit end part of the internal magnetic screen is placed closer to the middle point of the accelerating channel than the internal pole. The walls of the exit end part of the discharge chamber are constructed with an increased thickness, and extend beyond the planes that the poles lay. The magnetic screens can be located with a gap relative to the magnetic path if connected by a bridge between the screens. The discharge chamber, the anode, and the magnetic system are symmetrically designed relative to two mutually perpendicular longitudinal planes.