A cylindrical magnetron sputtering apparatus includes a rotating cylindrical sputtering target, a non-rotating magnet structure within the cylindrical sputtering target and at least one non-rotating trim magnet adjacent an end of the magnet structure. The trim magnets are manipulated during operation of the apparatus to alter a magnetic field produced by the magnet structure within the cylindrical sputtering cathode. As a result the shape of a racetrack discharge plasma formed at an end of the sputtering target is altered such that the formation of an erosion groove is avoided.