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abstract:
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Energy gain of trapped electrons in excess of 20 MeV has been demonstrated in an Inverse-Free-
Electron-Laser (IFEL) accelerator experiment. A 14.5 MeV electron beam is copropagated with a
400 GW CO2 laser beam in a 50 cm long undulator strongly tapered in period and ¯eld amplitude.
The Rayleigh range of the laser, » 1.8 cm, is much shorter than the undulator length yielding
a di®raction-dominated interaction. Experimental results on the dependence of the acceleration
on injection energy, laser focus position, and laser power are discussed. Simulations, in good
agreement with the experimental data, show that most of the energy gain occurs in the ¯rst half of
the undulator at a gradient of 70 MeV/m and that the structure in the measured energy spectrum
arises because of higher harmonic IFEL interaction in the second half of the undulator.
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