A radiation source unit is provided that includes an anode and a cathode that are configured and arranged to create a discharge in a substance in a space between said anode and cathode and to form a plasma so as to generate electromagnetic radiation. The substance may comprise xenon, indium, lithium, tin or any suitable material. To improve conversion efficiency, the source unit may be constructed to have a low inductance, and operated with a minimum of plasma. To, for example, improve heat dissipation, a fluid circulation system can be created within the source volume and a wick by using a fluid in both its vapor and liquid states. To, for example, prevent contamination from entering a lithographic projection apparatus, the source unit can be constructed to minimize the production of contamination, and a trap can be employed to capture the contamination without interfering with the emitted radiation.