Demonstration of Cascaded Modulator-Chicane Microbunching of a Relativistic Electron Beam

We present results of an experiment showing the first successful demonstration of a cascaded microbunching scheme. Two modulator-chicane prebunchers arranged in series and a high power mid-IR laser seed are used to modulate a 52 MeV electron beam into a train of sharp microbunches phase locked to the external drive laser. This configuration is shown to greatly improve matching of the beam into the small longitudinal phase space acceptance of short-wavelength accelerators. We demonstrate trapping of nearly all (96%) of the electrons in a strongly tapered inverse free-electron laser accelerator, with an order-of-magnitude reduction in injection losses compared to the classical single-buncher scheme. These results represent a critical advance in laser-based longitudinal phase space manipulations and find application in high gradient advanced acceleration as well as in high peak and average power coherent radiation sources.

Figure 1

(a) Experiment beam line layout. (b)–(e) Electron beam phase space after first modulator, first chicane, second modulator, second chicane from analytical model. (f) Phase space after IFEL acceleration.

Figure 2

(a) Optimal double-buncher parameter values maximizing trapping. (b) double-buncher and single-buncher (dashed) injection losses (yellow) and coherent form factor (blue). Experimental values corresponding to $A_2=20$ are also shown as points on the graphs.

Figure 3

Hall probe measurements of upstream (left) and downstream (right) bunchers, varying chicane current and chicane gap.

Figure 4

Single shot experimental data (dots) showing $f_T>55\mathrm{MeV}$ (Blue) and $f_T>77\mathrm{MeV}$ (red) varying downstream prebuncher chicane gap (a) and upstream prebuncher chicane current (b) compared with gpt simulations with lower and upper lines corresponding to 70 and 100 GW laser power, respectively. (c) Single shot experimental data after double-buncher optimization (dots) showing $f_T>55\mathrm{MeV}$ (blue) and $f_T>77\mathrm{MeV}$ (red) vs laser power compared with gpt simulation with experimental and ideal parameters.

Figure 5

(a)–(e) Single shot raw spectrometer images with no laser seed, no bunchers, downstream buncher only, both bunchers, and gpt simulation. (Bottom) Single shot projections showing electron beam energy distribution $(1/N)(dN/dy)$ vs $E$ and ten shot average of double-buncher spectrum.