Provided is an apparatus for and method of controlling formation of droplets (102a, b) used to generate EUV radiation that comprise an arrangement producing a laser beam directed to an irradiation region and a droplet source. The droplet source (92) includes a fluid exiting an nozzle (98) and a sub-system having an electro-actuatable element (104) producing a disturbance in the fluid (96). The droplet source produces a stream (100) 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 hybrid waveform that controls the droplet generation/coalescence process. Also disclosed is a method of determining the transfer function for the nozzle.