Patent Number: 
Section: claims

1. A radiation system comprising:a fuel emitter configured to provide fuel to a plasma formation region;a laser configured to provide a laser beam at the plasma formation region incident on the fuel to generate a radiation emitting plasma; anda reflective or transmissive device configured to receive radiation emitted by the radiation emitting plasma and to reflect or transmit at least some of the received radiation along a desired path, wherein the reflective or transmissive device comprises:a body configured to reflect or transmit the at least some of the received radiation; andsecondary electron emission (SEE) material arranged relative to the body such as to emit secondary electrons in response to the at least some of the received radiation, thereby to clean material from a surface of the reflective or transmissive device. 2. The radiation system of claim 1, wherein:the SEE material comprises an SEE layer that at least partly covers a surface of the body;the body comprises a layered structure that is configured such that reflection of the at least some of the received radiation by the layered structure causes a standing wave to be formed in the reflective or transmissive device; andthe layered structure and the SEE layer are arranged such that in operation a maximum of the standing wave is formed in the SEE layer. 3. The radiation system of claim 2, wherein the layered structure comprises a multi-layer mirror. 4. The radiation system of claim 1, wherein the SEE material comprises a cap layer that at least partly covers a surface of the body. 5. The radiation system of claim 1, wherein the SEE material is located at or near an edge of the reflective or transmissive device. 6. The radiation system of claim 5, wherein:the body comprises a layered structure configured to reflect the at least some of the received radiation, andat least some of the SEE material located at or near the edge of the reflective or transmissive device is beyond a boundary of the layered structure. 7. The radiation system of claim 1, further comprising a component comprising SEE material positioned adjacent to the reflective or transmissive device. 8. The radiation system of claim 1, wherein:the reflective or transmissive device is a collector, andthe SEE material comprises a coating on a rim of the collector. 9. The radiation system of claim 1, wherein, for received radiation having a wavelength in a range of 10 nm to 100 nm, the SEE material has a secondary electron yield (SEY) of greater than 0.01 electron per photon, greater than 0.05 electrons per photon, or greater than 0.1 electrons per photon. 10. The radiation system of claim 1, wherein the SEE material has a band gap energy of greater than 1 eV, greater than 3 eV, or greater than 5 eV. 11. The radiation system of claim 1, wherein the SEE material has an electron affinity less than 4 eV, less than 1 eV, or less than 0 eV. 12. The radiation system of claim 1, wherein the SEE material comprises an insulator or semiconductor. 13. The radiation system of claim 1, wherein the SEE material comprises at least one of diamond, H-terminated diamond, ZrO, ZrN, TiO2, Al2O3, or quartz. 14. The radiation system of claim 1, wherein the radiation comprises EUV radiation. 15. The radiation system of claim 1, wherein the reflective or transmissive device is configured to collect EUV radiation. 16. The radiation system of claim 1, wherein the material to be cleaned from material from the surface of the reflective or transmissive device comprises tin. 17. The radiation system of claim 1, further comprising a source of hydrogen for providing hydrogen gas to a surface of the reflective or transmissive device. 18. A reflective and/or transmissive optical device for receiving radiation and reflecting and/or transmitting at least some of the received radiation, the reflective and/or transmissive optical device comprising:a body configured to reflect and/or transmit the at least some of the received radiation; andsecondary electron emission (SEE) material arranged relative to the body such as to emit secondary electrons in response to the at least some of the received radiation, thereby to clean material from a surface of the reflective and/or transmissive optical device. 19. A radiation source comprising:a fuel emitter configured to provide fuel to a plasma formation region, wherein plasma is formed in operation when the fuel is subject to laser radiation; anda reflective and/or transmissive device configured to receive radiation emitted by the plasma and to reflect and/or transmit at least some of the received radiation along a desired path, wherein the reflective and/or transmissive device comprises:a body configured to reflect and/or transmit the at least some of the received radiation; andsecondary electron emission (SEE) material arranged relative to the body such as to emit secondary electrons in response to the at least some of the received radiation, thereby to clean material from a surface of the reflective and/or transmissive device. 20. A lithographic system comprising:a lithographic apparatus configured to project a pattern from a patterning device onto a substrate, anda radiation system configured to provide radiation to the lithographic apparatus, the radiation system comprising:a fuel emitter configured to provide fuel to a plasma formation region;a laser configured to provide a laser beam at the plasma formation region incident on the fuel to generate a radiation emitting plasma that produces the radiation; anda reflective or transmissive device configured to receive the radiation and to reflect or transmit at least some of the received radiation along a desired path, wherein the reflective or transmissive device comprises:a body configured to reflect or transmit the at least some of the received radiation; andsecondary electron emission (SEE) material arranged relative to the body such as to emit secondary electrons in response to the at least some of the received radiation, thereby to clean material from a surface of the reflective or transmissive device. 21. A method comprising:providing fuel to a plasma formation region;providing a laser beam at the plasma formation region incident on the fuel to generate a radiation emitting plasma;receiving, by a reflective and/or transmissive device, radiation emitted by the plasma;reflecting or transmitting along a desired path, by a body of the reflective and/or transmissive device, at least some of the received radiation; andemitting secondary electrons from secondary electron emission (SEE) material in response to the at least some of the received radiation thereby to clean material from a surface of the reflective and/or transmissive device.