Patent Number: 
Section: claims

1. An ion generator comprising:a shielding shell having a first end, an opposite second end and a main body, the first end having an electron-input hole, the second end having an ion-output hole, the main body having a gas inlet configured for introducing an ionizable gas into the shielding shell;a cathode device disposed facing the electron-input hole, configured for emitting electrons into the shielding shell through the electron-input hole so as to ionize the ionizable gas thereby generating ions, the cathode device including a conductive base and at least one field emitter; andan annular anode arranged in the shielding shell, the annular anode being aligned with the ion-output hole. 2. The ion generator according to claim 1, further comprising a grid electrode arranged between the cathode device and the electron-input hole of the shield shell, the grid electrode being configured for promoting extraction of the electrons from the cathode device. 3. The ion generator according to claim 2, wherein a shielding shell is tubular and the annular anode coaxially disposed in the shielding shell. 4. The ion generator according to claim 1, wherein the field emitters is comprised of a material selected from the group consisting of carbon nanotubes, diamond, diamond-like carbon, silicon, and metal. 5. The ion generator according to claim 1, wherein the cathode device includes a secondary electron-emitting source, and the at least one field emitter faces the secondary electron-emitting source. 6. The ion generator according to claim 5, wherein the secondary electron-emitting source is comprised of copper or platinum. 7. The ion generator according to claim 5, wherein the secondary electron-emitting source includes at least one tip extending toward the electron-input hole. 8. The ion generator according to claim 1, wherein the gas inlet is configured to be adjacent to the first end of the shielding shell. 9. The ion generator according to claim 1, wherein the annular anode and the ion-output hole of the shielding shell are coaxial. 10. The ion generator according to claim 1, wherein the annular anode is misaligned with the electron-input hole of the shielding shell. 11. The ion generator according to claim 1, further comprising an aperture lens arranged on the second end of the shielding shell, the aperture lens configured for focusing the ions exiting from the ion-output hole of the shielding shell. 12. The ion generator according to claim 9, wherein a thickness of a wall of the annular anode is in a range from 0.1 millimeters to 0.5 millimeters. 13. An ion generator comprising:a field emission cathode device configured for emitting electrons therefrom; anda shell including an ionization chamber and an annular anode arranged therein, the ionization chamber being configured for receiving the electrons emitted from the field emission cathode device and an ionizable gas, the anode and the shell being configured for cooperatively forming a saddle electric field in the ionization chamber. 14. The ion generator according to claim 13, further comprising a grid electrode arranged between the cathode device and the electron-input hole of the shield shell, the grid electrode being configured for promoting extraction of the electrons from the cathode device. 15. The ion generator according to claim 13, wherein the field emission cathode device includes a conductive base and a plurality of a field emitters formed thereon, the field emitters configured for emitting the electron input into the ionization chamber of the shell. 16. The ion generator according to claim 13, wherein the field emission cathode device includes a field emitter, and a secondary electron emitter facing each other, the secondary electron emitter has a higher potential than the field emitter such that electrons emitted from the field emitter impact the secondary electron emitter to emit the electrons input in the ionization chamber of the shell. 17. The ion generator according to claim 13, wherein the annular anode is misaligned with the electron-input hole of the shielding shell. 18. The ion generator according to claim 13, wherein a thickness of a wall of the annular anode is in a range from 0.1 millimeters to 0.5 millimeters. 19. An ion generator comprising:an elongated cylindrical shell having an electron-input hole at a first end thereof, an ion-output hole at an opposite second end thereof, and a gas inlet configured for introducing an ionizable gas thereinto;an annular anode coaxially disposed within the shell, the annular anode being misaligned with the electron-input hole; anda cathode device disposed adjacent the electron-input hole, the cathode being configured for emitting electrons into the shielding shell through the electron-input hole.