Demonstration of Cascaded Optical Inverse Free-Electron Laser Accelerator

We report on a proof-of-principle demonstration of a two-stage cascaded optical inverse free-electron laser (IFEL) accelerator in which an electron beam is accelerated by a strong laser pulse after being packed into optical microbunches by a weaker initial laser pulse. We show experimentally that injection of precisely prepared optical microbunches into an IFEL allows net acceleration or deceleration of the beam, depending on the relative phase of the two laser pulses. The experimental results are in excellent agreement with simulation. The demonstrated technique holds great promise to significantly improve the beam quality of IFELs and may have a strong impact on emerging laser accelerators driven by high-power optical lasers.

Figure 1

Initial and final beam longitudinal phase space in an IFEL for an unmodulated beam [(a), (b)] and a prebunched beam [(c), (d)]. The corresponding beam current [in (a), (c)] and energy [in (b), (d)] distributions are illustrated with magenta and blue lines, respectively. The horizontal axis is the beam longitudinal position normalized to the laser wavelength, and the vertical axis is the particle’s energy deviation with respect to the reference particle normalized to the rms slice energy spread of the beam. The maximal energy change is assumed to be 30 times larger than the beam energy spread.

Figure 2

Schematic layout of the cascaded optical IFEL accelerator experiment at SLAC’s NLCTA.

Figure 3

Measured beam energy distribution: (a) with both lasers off; (b) with only the first laser on; (c) with only the second laser on; (d) net acceleration with both lasers on; (e) net deceleration with both lasers on. The beam intensity in (a) and (b) is reduced by a factor of 7 to avoid saturation using the same scaling as (c), (d), and (e).

Figure 4

Beam energy distributions in experiment (a) and simulation (b): with both lasers off (black solid lines); with only the first laser on (blue circles); with only the second laser on (red squares); net acceleration with both lasers on (green dashed lines); (e) net deceleration with both lasers on (magenta dashed-dotted lines).

Figure 5

Simulated beam longitudinal phase space at the entrance to (a) and exit of a cascaded IFEL [(b),(c), (d)]: (b) with only the second laser on; (c) net acceleration with both lasers on; (d) net deceleration with both lasers on.