A radiation source for generating EUV from a stream of molten metal fuel droplets by LPP (Laser Produced Plasma) or (Dual Laser Plasma) has a fuel droplet generator arranged to provide a stream of droplets of fuel and at least one laser configured to vaporise at least some of said droplets of fuel, whereby radiation is generated. The fuel droplet generator has nozzle, fuel supply line, and reservoir, with a pumping device arranged to supply a flow of molten metal fuel from the reservoir through the fuel feed line and out of the nozzle as a stream of droplets. The fuel droplet generator has a replaceable filter assembly in the fuel feed line, arranged to filter the molten metal fuel in use, to deter nozzle blockage by solid particulate impurities in the fuel.