Fluctuation of laser induced X-rays from electron beam and plasma

摘要

There has been a significant progress in the X-ray generation technologies for various laboratory and production applications by laser-Compton scheme and laser produced plasma. Femtosecond X-ray generation was performed by Compton scattering through interaction between a 3-ps electron beam and 100-fs laser photons in a 90 degree scattering configuration. The X-ray energy and pulse duration were estimated as 2.3keV and 280fs from the observed electron and laser beam parameters. The fluctuation of the X-ray output was measured as 25% (1σ) during 30-min operation, and analyzed as a function of fluctuations of parameters like timing between the electron and laser beam, electron beam charge, laser beam energy, pulse widths, and spatial stability. Further reduction of the fluctuation is possible by improvement of component stability. All optical synchronization was proposed and component technologies are under development. The novel scheme employs optical control of the RF phase of the linear accelerator and femtosecond lasers by optical locking of lasing between different laser oscillators. Two mode-locked Ti:sapphire lasers of different wavelengths were precisely synchronized by a simple feedback system employing sum frequency generation. The measured timing jitter between two lasers was 28fs. The optical locking technology enables to achieve 3% (1 σ) X-ray fluctuation with improvements of other parameters. Clean EUV light source is strongly demanded for next generation lithography, with 13.5nm, 2% bw, 100W at clean focus and high pulse stability of less than l%(3σ). The most promising approach is to employ a liquid Xe jet of 10-100 μm diameter as the plasma source with a high repetition rate solid state laser. The laser pulse energy usually fluctuates more than a few % and the liquid jet fluctuates of the position up to 10% of the diameter. Higher repetition rate plasma generation causes acoustic instability inside the liquid jet and limits the stable operation. All these obstacles prevent to achieve the desired EUV stability. Discussion will be given on these issues and the analysis of the spatial fluctuation is shown on the available pulse to pulse stability. The architecture by multiple-pulse averaging is evaluated to achieve a clean EUV light source with required stability.