Patent Number: 
Section: claims

1. A plasma radiation source comprising:an anode and a cathode defining a region in which to produce a plasma current, wherein the anode has a first surface and a first periphery and the cathode has a second surface with a second periphery which faces the first surface of the anode such that the anode and the cathode do not overlap each other; anda magnet that is a physically separate structure from the anode and cathode, the magnet disposed substantially around the anode and the cathode to externally induce a magnetic field in a discharge gap between the anode and the cathode, the magnet having a third periphery that does not overlap the first periphery of the first surface of the anode and/or the second periphery of the second surface of the cathode and the magnetic field in the region having an axial component directed along a direction of the plasma current. 2. The plasma radiation source of claim 1, further comprising a beam forming system configured to form a radiation beam from radiation emitted from the region. 3. The plasma radiation source of claim 1, wherein the magnet is configured to apply the magnetic field with a strength in a range of 10 milli-Tesla to 100 milli-Tesla in the region and wherein the anode and cathode are a solid disc or a solid ring. 4. The plasma radiation source of claim 1, wherein the direction of the plasma current is from the anode to the cathode and the magnetic field is of a sufficient strength to limit the collapse of a Z-pinch region of the plasma. 5. The plasma radiation source of claim 1, wherein the magnet is a permanent magnet and wherein the first surface of the anode is a first major annular surface and the second surface of the cathode is a second major annular surface. 6. The plasma radiation source of claim 1, wherein a plane of the magnet, the plane defined at least in part by the third periphery, does not overlap the first surface of the anode and/or the second surface of the cathode. 7. The plasma radiation source of claim 1, wherein the first surface of the anode, the second surface of the cathode and the magnet are disposed such that the first, second and third peripheries are located adjacent to each other with the third periphery between the first and second peripheries. 8. An apparatus to project a pattern from a patterning device onto a substrate, the apparatus comprising:a plasma radiation source comprising:an anode and a cathode defining a region in which to produce a plasma current, wherein the anode has a first major annular surface with a first periphery and the cathode has a second major annular surface with a second periphery which second major annular surface is facing the first major annular surface of the anode such that the anode and the cathode do not overlap each other, anda magnet that is a physically separate structure from the anode and cathode, the magnet disposed substantially around the anode and the cathode to externally induce a magnetic field in a discharge gap between the anode and the cathode, the magnet having a third periphery that does not overlap the first periphery of the first major annular surface of the anode and/or the second periphery of the second major annular surface of the cathode and the magnetic field in the region having an axial component directed along a direction of the plasma current; andan illumination system configured to form a radiation beam from radiation emitted from the region. 9. The apparatus of claim 8, wherein the magnet is configured to apply the magnetic field with a strength in a range of 10 milli-Tesla to 100 milli-Tesla in the region and wherein the anode and cathode are a solid disc or a solid ring. 10. The apparatus of claim 8, wherein the direction of the plasma current is from the anode to the cathode and the magnetic field is of a sufficient strength to limit the collapse of a Z-pinch region of the plasma. 11. The apparatus of claim 8, wherein the magnet is a permanent magnet. 12. The apparatus of claim 8, further comprising:a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam;a substrate table constructed to hold a substrate; anda projection system configured to project the patterned radiation beam onto a target portion of the substrate. 13. The apparatus of claim 8, wherein a plane of the magnet, the plane defined at least in part by the third periphery, does not overlap the first major annular surface of the anode and/or the second major annular surface of the cathode. 14. The apparatus of claim 8, wherein the first major annular surface of the anode, the second major annular surface of the cathode and the magnet are disposed such that the first, second and third peripheries are located adjacent to each other with the third periphery between the first and second peripheries. 15. A method of producing radiation, comprising:generating a plasma current in a plasma region in a direction between an anode and a cathode;applying a magnetic field in the plasma region with an axial component directed along the direction of the plasma current; andemitting radiation from the plasma region,wherein the anode has a first major annular surface with a first periphery, the cathode has a second major annular surface with a second periphery which second major annular surface faces the first major annular surface of the anode such that the anode and the cathode do not overlap each other, and a magnet, that is a physically separate structure from the anode and cathode, is disposed substantially around the anode and the cathode to externally induce the magnetic field in a discharge gap between the anode and the cathode, the magnet having a third periphery that does not overlap the first periphery of the first major annular surface of the anode and/or the second periphery of the second major annular surface of the cathode. 16. The method of claim 15, wherein the magnetic field is externally applied with a strength in a range of 10 milli-Tesla to 100 milli-Tesla in the plasma region and wherein the anode and cathode are a solid disc or a solid ring. 17. The method of claim 15, wherein the magnetic field is of a sufficient strength to limit the collapse of a Z-pinch region of the plasma, wherein the direction of the plasma current is from the anode to the cathode and wherein the magnet is a permanent magnet. 18. The method of claim 15, further comprising imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam and projecting the patterned radiation beam onto a target portion of a substrate. 19. The method of claim 15, further comprising forming a radiation beam from emitted radiation from the plasma region. 20. The method of claim 15, wherein a plane of the magnet, the plane defined at least in part by the third periphery, does not overlap the first major annular surface of the anode and/or the second major annular surface of the cathode. 21. The method of claim 15, wherein the first major annular surface of the anode, the second major annular surface of the cathode and the magnet are disposed such that the first, second and third peripheries are located adjacent to each other with the third periphery between the first and second peripheries. 22. A device manufacturing method comprising:applying a magnetic field in a plasma region with an axial component directed along the direction of a plasma current;following applying the magnetic field, generating the plasma current in the plasma region in a direction between an anode and a cathode; andusing a reflective optical element to form a radiation beam from radiation emitted from the plasma region,wherein the anode has a first major annular surface, the cathode has a second major annular surface which faces the first major annular surface of the anode such that the anode and the cathode do not overlap each other, and a magnet is disposed substantially around the anode and the cathode to externally induce the magnetic field in a discharge gap between the anode and the cathode. 23. The method of claim 22, wherein the magnetic field is externally applied with a strength in a range of 10 milli-Tesla to 100 milli-Tesla in the plasma region and wherein the anode and cathode are a solid disc or a solid ring. 24. The method of claim 22, wherein the initial magnetic field is of a sufficient strength to limit the collapse of a Z-pinch region of the plasma and the magnet is a permanent magnet. 25. The method of claim 22, further comprising imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam and projecting the patterned radiation beam onto a target portion of a substrate. 26. A plasma radiation source comprising:an anode and a cathode defining a region in which to produce a plasma current, wherein the anode is an annular anode having a first major annular surface and the cathode is an annular cathode having a second major annular surface which faces the first major annular surface of the annular anode such that the annular anode and the annular cathode do not overlap each other; anda permanent magnet or an electromagnet magnet not directly electrically connected to the anode or cathode, configured to induce an initial magnetic field of a sufficient strength in a discharge gap between the annular anode and the annular cathode such as to limit the collapse of a Z-pinch region of the plasma, wherein the magnetic field in the region has an axial component directed along a direction of the plasma current.