Abstract: In a magnetron cathode sputter assembly for coating objects within an evacuated chamber, a disc of cathode material to be sputtered is supported over an annular magnet structure for producing crossed magnetic and electric fields over the sputter surface of the cathode disc to be sputtered for enhancing the glow discharge and the sputtering rate. The magnet structure is contained within a chamber in a hollow cylindrical receiving member and a clamp serves to clamp the cathode sputter disc over the open end of the receiving chamber. Coolant is circulated through the receiving chamber in heat exchanging relation with the cathode disc and with the magnet structure for cooling same in use. An internally flanged cylindrical shield structure is disposed overlying the outer peripheral edge of the cathode sputter disc and, in a preferred embodiment, the shield is permitted to operate by self biasing at a floating potential intermediate the anode and cathode potential.