A rectangular magnetron placed at the back of a rectangular target to intensify the plasma in a sputter reactor configured for sputtering target material onto a rectangular panel. The magnetron has asize only somewhat less than that of the target and is scanned in the two perpendicular directions of the target with a scan length of, for example, about 100 mm for a 2 m target. The scan may followa double-Z pattern along two links parallel to a target side and the two connecting diagonals. The magnetron includes a closed plasma loop formed in a convolute shape, for example, a rectangularized helix with an inner pole of nearly constant width extending along a single path and having one magnetic polarity completely surrounded by an outer pole having the opposed polarity.