Apparatus for and method of controlling formation of droplets used to generate EUV radiation. The droplet source includes a fluid exiting an nozzle and a sub-system having an electro-actuatable element producing a disturbance in the fluid. The droplet source produces a stream that breaks down into droplets that in turn coalesce into larger droplets as they progress towards the irradiation region. The electro-actuatable element is driven by a control signal having a sine wave component and a square wave component. Various parameters such as a phase difference between the sine wave component and the square wave component are measured and controlled to minimize the formation of noncoalesced satellite droplets in the stream.