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A scheme to create coherent light with orbital angular momentum (OAM) using Free Electron Laser (FEL) at NLCTA is proposed. An 795 nm light co-propagating with relativistic unmodulated electron beam is fed through a helical undulator tuned to the second harmonic of the laser, which acts as a pre-buncher that helically micro-bunches the beam, modulating it in energy. The energy modulation is transferred to helical density modulation by propagating through a longitudinally dispersive section, such as a chicane. Finally the helical density 3-D modulated electron beam is sent through a second undulator resonant at light’s fundamental frequency, causing the electron beam to radiate OAM light. NLCTA facility has everything to make this experiment, including a planar undulator tuned to the fundamental frequency, except for a helical pre-bunching undulator, which can be easily constructed and installed to generate OAM light at NLCTA. According to simulations generated with Mathematica 7 and Genesis 1.3 a 3 period long pre-buncher will be enough to get out 140 MW of laser power from a seeded 10 MW, after transversing a 1.5 m long planar radiator using electron beam generated by NLCTA.
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