Phase Space Dynamics of a Plasma Wakefield Dechirper for Energy Spread Reduction

Plasma-based accelerators have made impressive progress in recent years. However, the beam energy spread obtained in these accelerators is still at the $\sim 1\%$ level, nearly one order of magnitude larger than what is needed for challenging applications like coherent light sources or colliders. In plasma accelerators, the beam energy spread is mainly dominated by its energy chirp (longitudinally correlated energy spread). Here we demonstrate that when an initially chirped electron beam from a linac with a proper current profile is sent through a low-density plasma structure, the self-wake of the beam can significantly reduce its energy chirp and the overall energy spread. The resolution-limited energy spectrum measurements show at least a threefold reduction of the beam energy spread from 1.28% to 0.41% FWHM with a dechirping strength of $\sim 1(\mathrm{MV}/\mathrm{m})/(\mathrm{mm}\mathrm{pC})$. Refined time-resolved phase space measurements, combined with high-fidelity three-dimensional particle-in-cell simulations, further indicate the real energy spread after the dechirper is only about 0.13% (FWHM), a factor of 10 reduction of the initial energy spread.

Figure 1

(a) Schematic diagram of the PD. (b) The beam longitudinal phase space at the entrance of the PD. (c) The beam current profile (green line) and the on-axis longitudinal wakefield $E_z$ excited by the beam in the PD (black line). (d) The final beam longitudinal phase space.

Figure 2

(a) Three current profiles for electron beams with the same total charge $Q$, peak density $n_b$, and transverse size ${\sigma }_r$; here, $\xi =ct-z$ and $\xi <0$ corresponds to the beam head. Panels (b) and (c) show $E_z$ $(r=0)/I_b$ versus $\xi$ and $E_z$ $(\xi =0)/I_b$ versus $r$ for these three current profiles, respectively. Here, $E_z$ is normalized to the plasma wave-breaking limit $m{c}^2{k}_p/e$ and $I_b=2\pi {\sigma }_r^2{n}_{b}ec\approx 0.2(n_b/n_p)\mathrm{kA}$. The dechirping strength $S_d$ is calculated using the linear fit to $E_z$ between points A and B. (d) Evolution of the beam FWHM energy spread $\mathrm{\Delta }W$ (normalized to $\mathrm{\Delta }{W}_i$) versus $d_p{I}_b$ for these three current profiles.

Figure 3

(a) Schematic layout of the plasma dechirping experiment. (b) The beam waist profile. (c) Measured beam longitudinal phase space. (d) Deconvoluted beam current profile. The shaded regions correspond to the standard deviation of 20 consecutive shots. (e) Longitudinal distribution of plasma density through off-line measurement, where the shaded regions correspond to the standard deviation of 10 consecutive shots.

Figure 4

(a)–(d) Beam energy spectrum distributions on Screen3 with plasma off, $P_g=0.5\mathrm{MPa}$, $P_g=1\mathrm{MPa}$, and $P_g=2\mathrm{MPa}$, respectively. (e) Beam energy spectra derived from the images in the left-hand panel. For the plasma-off case, the shaded regions correspond to the standard deviation of 20 consecutive shots.

Figure 5

(a)–(l) Beam longitudinal phase space distributions [left (middle) column, the experimental (simulated) results recorded on Screen3; right column, the simulated results at the exit of the PD]. The top row of images is for plasma off, the second, third, and bottom rows are for plasma on with $P_g=0.5\mathrm{MPa}(n_p=1\times {10}^{14}{\mathrm{cm}}^{-3})$, $P_g=1\mathrm{MPa}(n_p=2\times {10}^{14}{\mathrm{cm}}^{-3})$, and $P_g=2\mathrm{MPa}(n_p=4\times {10}^{14}{\mathrm{cm}}^{-3})$, respectively. (m) The energy spectra of above cases. For the measured plasma-off case (solid blue line), the shaded regions correspond to the standard deviation of 20 consecutive shots.