Patent Number: 
Section: claims

1. A system, comprising:a) an electrostatic accelerator unit including an array of spatially separated charged particle emitters, each emitter having an electrostatic potential difference with respect to an immediately adjacent emitter in the array and each emitter producing an charged particle beamlet, whereby the array of charged particle emitters produces a corresponding array of charged particle beamlets having different energies, wherein the charged particle beamlets are converged laterally as one energy-modulated direct current (DC) charged particle beam at an output of the accelerator unit; andb) an optical-frequency modulator for longitudinally modulating the energy-modulated DC charged particle beam from the output of the accelerator unit with infrared radiation, thereby forming modulated beamlets, wherein electrons in the modulated beamlets are bunched longitudinally to form a bunched energy-modulated DC charged particle beam. 2. The system of claim 1, wherein the charged particle emitters are electron emitters, each charged particle beamlet is an electron beamlet, and the charged particle beam is an electron beam. 3. The system of claim 2, wherein the emitters are DC electron guns. 4. The system of claim 2, further comprising an undulator or other free-electron radiation device located at a point of optimum bunching of the modulated beamlets to generate a coherent radiation output from modulation of the bunched energy-modulated DC electron beam. 5. The system of claim 2, each of the electron beamlets is accelerated to about 100 KeV from the respective emitter in the array. 6. The system of claim 2, wherein the optical-frequency modulator is an inverse free electron laser. 7. The system of claim 1, wherein the array of charged particle emitters is placed at a mid-plane of a half-chicane to spatially overlap the array of charged particle beamlets at an exit of the half-chicane. 8. The system of claim 1, wherein the electric potential difference between the charged particle emitters in the array is adjustable. 9. The system of claim 1, wherein one or more of the charged particle emitters in the array are configured to be selectively switched on or off. 10. The system of claim 1 further comprising a bunch compressor placed along an electron propagation pathway between the optical frequency modulator and the undulator to shorten a travel distance of the charged particle beam downstream of the optical frequency modulator to bunch. 11. The system of claim 1, wherein the optical-frequency modulator includes a laser. 12. The system of claim 10, wherein the laser is a Nd:YAG laser. 13. The system of claim 1, further comprising a charged particle collector configured to collect an output electron beam from the system at a potential slightly below that of the potential used to accelerate the charged particle beamlets. 14. The system of claim 1, wherein the accelerator unit includes a half chicane having two bending magnets configured to bend the charged particle beamlets from the emitters in the array so as to laterally converge the charged particle beamlets to form the electron beam output from the accelerator unit. 15. A method of generating EUV radiation, comprising:emitting an array of spatially separated beamlets, wherein each beamlet is produced by a corresponding array of electron emitters, wherein each electron emitter is at an electrostatic potential difference with respect to an immediately adjacent emitter in the array, whereby the array of electron emitters produces a corresponding array of electron beamlets having different energies;converging the beamlets laterally to form an energy modulated direct current electron beam;modulating the beamlets in the energy modulated direct current electron beam longitudinally with infrared radiation to form a modulated beam;bunching electrons in the modulated beam longitudinally to formed a bunched energy-modulated electron beam; andmodulating the bunched energy-modulated electron beam with an undulator to generate a coherent radiation output. 16. The method of claim 15 further comprising a step of compressing the bunched energy-modulated electron beam laterally to correct electron dispersion. 17. The method of claim 15, further comprising adjusting a wavelength of the coherent radiate output by adjusting the electrostatic potential difference between adjacent emitters in the array. 18. The method of claim 15, further comprising selectively switching one or more of the electron emitters on or off so as to adjust a pulse period of the coherent output radiation. 19. The method of claim 15, wherein the array of electron emitters is placed at a mid-plane of a half-chicane to spatially overlap the array of electron beamlets at an exit of the chicane thereby converging the beamlets laterally. 20. A method of generating a bunched particle beam, comprising:emitting an array of spatially separated beamlets, wherein each beamlet is produced by a corresponding array of charged particle emitters, wherein each emitter is at an electrostatic potential difference with respect to an immediately adjacent emitter in the array, whereby the array of emitters produces a corresponding array of charged particle beamlets having different energies;converging the beamlets laterally to form an energy modulated direct current charged particle beam;modulating the beamlets in the energy modulated direct current charged particle beam longitudinally with infrared radiation to form a modulated beam; andbunching charged particles in the modulated beam longitudinally to formed a bunched energy-modulated charged particle beam.