Abstract: This invention relates to a method of cooling an immersed induction coil, such as an ionising coil used in a vacuum processing chamber. The coil 10 may comprise a coiled hollow tube 11 through which coolant can flow when pumped by a pump 13. A valve 16 is provided so that air can be passed through the tube in either direction and the direction of flow is controlled by a controller 19, which acts on the valve 16. The rate of switching of the valve 16 can be determined by a process condition or by monitoring the temperature of the downstream coolant and/or the coil.