Patent Number: 
Section: claims

1. An optical collector (15) for collecting extreme ultraviolet radiation or EUV light (18) generated at a central EUV production site (20), the collector (15) comprising:a reflective shell (25) including means for compensating thermally induced deformations of the reflective shell (25), wherein the reflective shell (25) is of near ellipsoidal shape and axisymmetric with respect to an axis (30);a support structure (24) supporting the reflective shell (25), such that a cooling channel (29) is established between a back side of the reflective shell (25) and the support structure (24), wherein the reflective shell (25) has a thickness, such that convective heat transfer between the back side of the reflective shell (25) and a cooling medium (26) flowing through the cooling channel (29) dominates the process of removing heat from the reflective shell (25) with respect to heat conduction, and wherein the cooling channel (29) is funnel-shaped with respect to the axis (30); anda cooling circuit (33) connected to the cooling channel (29) to supply a cooling medium (26) to the cooling channel (29) with a controlled coolant pressure and/or mass flow and/or temperature. 2. An optical collector according to claim 1, wherein the cooling channel (29) is connected to the cooling circuit (33) through a plurality of inlet ports (27) and exit ports (32). 3. An optical collector according to claim 2, wherein volutes (28, 31) are provided between the inlet ports (27) and the cooling channel (29) and the exit ports (32) and the cooling channel (29). 4. An optical collector according to claim 1, wherein the cooling medium (26) enters the cooling channel (29) near the axis (30) and exits the cooling channel (29) far from the axis (30). 5. An optical collector according to claim 1, wherein flow disturbing means (36) are provided at predetermined locations within the cooling channel (29). 6. An optical collector according to claim 5, wherein the flow disturbing means comprise a plurality of obstacles which are mounted on a side of the cooling channel (29) opposite to the back side of the reflective shell (25) and/or on the back side of the reflective shell (25). 7. An optical collector according to claim 1, wherein the cooling circuit (29) is a closed circuit comprising a heat exchanging means (34), a compressor (35) and a control valve (41), and a control (40) for controlling the compressor (35) and/or the control valve (41) and/or the heat exchanging means (34). 8. An optical collector according to claim 6, wherein the obstacles comprise a plurality of turbulators (36), which are mounted on a side of the cooling channel (29) opposite to the back side of the reflective shell (25) and/or on the back side of the reflective shell (25). 9. A EUV source (10) comprising:a target delivery system (17), which emits a chain of droplets (19) of a target material, a high power drive laser (12), which ignites the target material at a EUV production site (20); andan optical collector (15), which collects the EUV light (18) generated at the EUV production site (20), wherein the optical collector (15) is a collector according to claim 1. 10. A method for operating an optical collector for collecting extreme ultraviolet radiation or EUV light (18) generated at a central EUV production site (20), comprising: compensating thermally induced deformations of a reflective shell (25) using pressure and/or mass flow and/or temperature of a cooling medium (26) flowing through a cooling channel (29) to compensate for thermally induced deformations of the reflective shell (25), wherein the reflective shell (25) is of near ellipsoidal shape and axisymmetric with respect to an axis (30), and the cooling channel (29) is funnel-shaped with respect to the axis (30), wherein the collector comprises: a reflective shell (25), a support structure (24) supporting the reflective shell (25), such that a cooling channel (29) is established between a back side of the reflective shell (25) and the support structure (24), wherein the reflective shell (25) has a thickness, such that convective heat transfer between the back side of the reflective shell (25) and a cooling medium (26) flowing through the cooling channel (29) dominates the process of removing heat from the reflective shell (25) with respect to heat conduction; and a cooling circuit (33) connected to the cooling channel (29) to supply a cooling medium (26) to the cooling channel (29) with a controlled coolant pressure and/or mass flow and/or temperature. 11. The method according to claim 10, wherein the pressure and/or the mass flow and/or the temperature of the cooling medium (26) is controlled in dependence of an input signal (42) being characteristic of a deformation of the reflective shell (25). 12. The method according to claim 10, wherein a gas is used as the cooling medium. 13. The method according to claim 12, wherein the gas is one of the gases including hydrogen, helium, argon, neon, krypton, xenon, chlorine, nitrogen, fluorine, bromine, and iodine, or a mixture of two or more of said gases.