Femtosecond Visualization of Laser-Induced Optical Relativistic Electron Microbunches

It has long been known that lasers can interact with relativistic electrons in magnetic undulators to imprint sinusoidal modulations that can be used to slice electrons into microbunches equally separated at the laser wavelength. Here we report on the first direct measurement of laser-induced microbunching of a relativistic electron beam with femtosecond resolution in the time domain. Using a modified zero-phasing technique to map the electron beam’s temporal structures into the energy space, we show that this method can be used to directly quantify the time and spectral content of coherent current modulations imprinted on the beam for harmonic and multicolor lasing applications in accelerator-based light sources.

Figure 1

Layout of the experiment at SLAC’s NLCTA.

Figure 2

Longitudinal phase space for maximizing bunching at high harmonic (a) and suppressing the bunching at the fundamental (b); corresponding beam current distribution (c) and bunching factors (d). In (a), (b), and (c) the horizontal axis is the beam longitudinal position normalized to the laser wavelength and the vertical axis is particle’s energy deviation with respect to the reference particle normalized to the rms slice energy spread of the beam.

Figure 3

Measured optical microbunches equally separated at $2.4\mu \mathrm{m}$ (a) and the corresponding current distribution (b) for optimal laser energy modulation; optical microbunches produced at two separated regions (c) and the corresponding current distribution (d) with an increased laser energy modulation amplitude which overbunches the beam that interacts with the center of the laser; optical microbunches measured with improved temporal resolution with optimal energy modulation (e) and significantly larger energy modulation (f) and the corresponding Fourier transform (g). Because the beam is longer than the laser, only the part of the beam that interacts with the laser is shown in (a) and (c); in (e) and (f) only the part of the beams that experience almost constant modulation are shown.

Figure 4

Fourier transform of the current distributions in Fig. 3 (a) and Fig. 3 (b), and the corresponding Wigner functions of Fig. 3 (c) and Fig. 3 (d).

Figure 5

Spectrograms of measured bunching for different initial $e$-beam chirps of (a) ${\varphi }_{\text{rf}}=1°$, (b) ${\varphi }_{\text{rf}}=0°$, (c) ${\varphi }_{\text{rf}}=-3°$, (d) ${\varphi }_{\text{rf}}=-7°$, (e) ${\varphi }_{\text{rf}}=-9°$, and (f) ${\varphi }_{\text{rf}}=-11°$.