High Energy Gain of Trapped Electrons in a Tapered, Diffraction-Dominated Inverse-Free-Electron Laser

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 ${\mathrm{CO}}_2$ laser beam in a 50 cm long undulator strongly tapered in period and field amplitude. The Rayleigh range of the laser, $\sim 1.8\mathrm{cm}$, is much shorter than the undulator length yielding a diffraction-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 first half of the undulator at a gradient of $70\mathrm{MeV}/\mathrm{m}$ and that the structure in the measured energy spectrum arises because of higher harmonic IFEL interaction in the second half of the undulator.

Figure 1

Layout of the Neptune IFEL experiment.

Figure 2

IFEL output energy versus measured delay on the streak camera between the reference $e$-beam pulse and the ${\mathrm{CO}}_2$ laser. Also shown is a typical streak of the ${\mathrm{CO}}_2$ laser pulse.

Figure 3

IFEL output energy versus laser focus position. As the laser focus is moved towards the undulator center, the laser intensity at the magnet entrance drops below the trapping threshold.

Figure 4

Single-shot energy spectrum. The simulated spectrum is normalized to the same area as the measured one.

Figure 5

IFEL output energy and fraction of trapped particles versus input $e$-beam energy for 300 GW input laser power (a) and versus driving laser power for 14.5 MeV input energy (b). The lines representTREDI simulation results.

Figure 6

Simulations of the IFEL experiment. (a) Maximum-energy particle evolution along the undulator. The first and second harmonic resonant curved are also shown. (b) Longitudinal phase space at 45 cm with the energy modulation due to the second harmonic interaction.