Patent Number: 
Section: claims

1. A radiation source comprising:a radiation emitter configured to emit radiation;a collector configured to collect radiation emitted by the radiation emitter; andan outlet configured to introduce a cooled gas into the radiation source,wherein the cooled gas suppresses an amount of debris generated by the radiation emitter from contacting or entering the collector. 2. The radiation source of claim 1, wherein the cooled gas is arranged to be cooled prior to introduction into the radiation source. 3. The radiation source of claim 1, wherein the cooled gas is arranged to be introduced into the radiation source in a compressed state. 4. The radiation source of claim 3, wherein the gas is arranged to be introduced as a pressurized liquid that evaporates to form the cooled gas. 5. The radiation source of claim 3, further comprising a nebulizer for nebulizing the liquid. 6. The radiation source of claim 1, wherein the outlet is configured to introduce the cooled gas at a location in-between the radiation emitter and the collector. 7. The radiation source of claim 1, wherein the outlet is configured to introduce the cooled gas at a location in-between the radiation emitter and an exit aperture of the source. 8. The radiation source of claim 1, further comprising one or more further outlets, the one or more further outlets being arranged to introduce a cooled gas into the radiation source at a different location within the radiation source. 9. The radiation source of claim 1, wherein the cooled gas is introduced at a temperature below an ambient temperature. 10. The radiation source of claim 1, wherein the cooled gas is arranged to serve as one or more of:a buffer gas;a gas for cooling a component of the radiation source; anda gas for cleaning a component of the radiation source. 11. A radiation source arrangement comprising:a radiation emitter configured to emit radiation;a collector configured to collect radiation emitted by the radiation emitter;an outlet configured to introduce a cooled gas into the radiation source; anda source of the cooled gas, pressurized gas or liquefied gas in connection with the outlet,wherein the cooled gas suppresses an amount of debris generated by the radiation emitter from contacting or entering the collector. 12. A lithographic apparatus comprising:a radiation source comprising:a radiation emitter configured to emit radiation;a collector configured to collect radiation emitted by the radiation emitter; andan outlet configured to introduce a cooled gas into the radiation source,wherein the cooled gas suppresses an amount of debris generated by the radiation emitter from contacting or entering the collector;a patterning device configured to pattern the radiation; anda projection system configured to direct the patterned radiation onto a substrate. 13. A method comprising:operating a radiation source comprising:a radiation emitter configured to emit radiation;a collector configured to collect radiation emitted by the radiation emitter; andan outlet configured to introduce a cooled gas into the radiation source;introducing the cooled gas into the radiation source via the outlet of the radiation source,wherein the cooled gas suppresses an amount of debris generated by the radiation emitter from contacting or entering the collector. 14. The method of claim 13, wherein the cooled gas is introduced to cool a part of the radiation source. 15. The radiation source of claim 1, wherein the cooled gas is introduced at a temperature that is substantially the same as a boiling point of the gas.