Patent Number: 
Section: claims

1. An optical hollow waveguide assembly, comprising:an optical hollow waveguide configured to guide illumination light, comprising:a tubular main body with a continuous waveguide cavity having an illumination light inlet and an illumination light outlet,wherein a cavity inner wall of the waveguide cavity is configured to reflect the illumination light under grazing incidence,a gas source that has a fluid connection to the waveguide cavity,wherein the gas source is configured to introduce gas into the waveguide cavity, anda chemically reducing environment over the cavity inner wall,wherein the chemically reducing environment comprises radicals produced in the gas under action of the illumination light, and the atmosphere is selected to decontaminate the waveguide cavity. 2. The waveguide assembly according to claim 1, wherein the gas source has a fluid connection to the waveguide cavity at the illumination light inlet and/or at the illumination light outlet. 3. The waveguide assembly according to claim 1, wherein the gas source has a fluid connection to the waveguide cavity via at least one gas inlet that opens into the waveguide cavity between the illumination light inlet and the illumination light outlet. 4. The waveguide assembly according to claim 3, wherein an opening of the at least one gas inlet tapers conically toward the waveguide cavity. 5. The waveguide assembly according to claim 3 wherein an opening cross section of the gas inlet into the waveguide cavity is less than 1 mm2. 6. The waveguide assembly according to claim 1, further comprising at least one positive pressure portion of the waveguide cavity, into which at least one gas inlet of the fluid connection to the gas source opens, wherein the at least one positive pressure portion is connected to a space surrounding the hollow waveguide through at least one pressure reduction opening such that, when gas is supplied via the gas source to the at least one positive pressure portion, a pressure (p1, p2) in the pressure reduction opening is greater than a pressure (pext) of the surrounding space. 7. The waveguide assembly according to claim 6, further comprising a further positive pressure portion, wherein the one positive pressure portion is arranged in proximity to the illumination light inlet and the further positive pressure portion is arranged in proximity to the illumination light outlet. 8. An illumination optical unit comprising a hollow waveguide assembly as claimed in claim 1. 9. An illumination system comprising an illumination optical unit as claimed in claim 8 and a light source. 10. An inspection apparatus comprising an illumination system according to claim 9 and configured to receive an object for inspection. 11. The optical hollow waveguide assembly of claim 1, wherein the gas comprises molecular hydrogen. 12. The optical hollow waveguide assembly of claim 1, wherein the illumination light comprises Extreme Ultraviolet (EUV) illumination. 13. The optical hollow waveguide assembly of claim 1, wherein the radicals comprise hydrogen radicals. 14. The optical hollow waveguide assembly of claim 1, wherein the atmosphere comprises a positive pressure atmosphere. 15. A method comprising:introducing a gas into an optical hollow waveguide comprising a cavity inner wall of the waveguide cavity configured to reflect illumination light under grazing incidence to guide the illumination light, wherein introducing the gas comprises introducing gas into the cavity from a gas source having a fluid connection with the cavity;transmitting the illumination light through the cavity;forming a reducing atmosphere including radicals in the cavity, wherein forming the reducing atmosphere comprises causing the illumination light to act on the gas to form the radicals; anddecontaminating the cavity under action of the reducing atmosphere. 16. The method of claim 15, wherein transmitting the illumination light through the cavity comprises transmitting Extreme Ultraviolet (EUV) illumination. 17. The method of claim 15, wherein causing the illumination light to act on the gas to form the radicals comprises causing EUV light to act on a gas comprising molecular hydrogen to form hydrogen radicals. 18. The method of claim 15, wherein decontaminating the cavity under action of the reducing atmosphere comprises:reacting the radicals with carbon particles on the inner walls of the cavity to form hydrocarbons; andblowing the hydrocarbons out of the cavity by positive pressure of the reducing atmosphere. 19. The method of claim 15, wherein introducing the gas into the optical hollow waveguide comprises introducing the gas via at least one gas inlet that opens into the waveguide cavity between an illumination light inlet and an illumination light outlet. 20. The method of claim 19, wherein introducing the gas via the at least one gas inlet comprises introducing the gas via an opening of the at least one gas inlet that tapers conically toward the waveguide cavity.