A radiation source for generation of extreme ultraviolet radiation or use in high resolution lithography includes a plasma formation site where fuel is contacted by a radiation beam to form a plasma (12) generating EUV radiation. A mirrored collector (10) collects and reflects the EUV radiation generated at a first focus towards a second focus (F2). A contamination barrier (20) is positioned such the periphery of the contamination barrier does not occlude more than 50% of the solid angle subtended by the mirror at the second focus, such that EUV radiation reflected by the collector mirror is not excessively attenuated by passing through the contamination barrier. The contamination barrier serves to trap fuel material such as ions, atoms, molecules or nanodroplets from the plasma to prevent their deposition onto the collector mirror where they reduce the mirror's effective lifetime. Gas extraction ports may be provided near the plasma formation site to suppress diffusion of fuel debris and contamination towards the collector mirror.