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An ongoing CO2 laser upgrade contributed to the traditionally successful ATF IFEL research program by affording a record breaking experiment wherein a 100 MeV/m accelerating gradient and a 50 MeV per stage electron acceleration (previous best result is 20 MeV) have been recently achieved. This is the first time a strongly tapered helical undulator (Figure 1) has been built. Tapering varies the resonant energy, allowing high-gradient acceleration. The helical geometry coupled with a circularly polarized laser offers near continuous acceleration as the electrons follow a helical trajectory along the undulator axis resulting in more than twice the gradient of a planar undulator.
We note that using electron micro-bunching before the main acceleration stage might considerably improve the process with the production of practically meaningful mono-energetic beams. This was demonstrated earlier in the ATF STELLA experiment. The goal of the Rubicon IFEL experiment (Figure 2), viz., is to achieve an energy gain and an accelerating gradient larger than what is possible with conventional RF accelerator technology. This will pave the way for applications, such as a portable driver for inverse Compton sources and FELs. Researchers are contemplating plans for 1-GeV class IFELs based on an helical undulator and a 100 TW laser.
Duris, J. Helical IFEL Acceleration at ATF. ATF Newsletter, Oct 2013