Stable and High-Quality Electron Beams from Staged Laser and Plasma Wakefield Accelerators

We present experimental results on a plasma wakefield accelerator (PWFA) driven by high-current electron beams from a laser wakefield accelerator (LWFA). In this staged setup stable and high-quality (low-divergence and low energy spread) electron beams are generated at an optically generated hydrodynamic shock in the PWFA. The energy stability of the beams produced by that arrangement in the PWFA stage is comparable to both single-stage laser accelerators and plasma wakefield accelerators driven by conventional accelerators. Simulations support that the intrinsic insensitivity of PWFAs to driver energy fluctuations can be exploited to overcome stability limitations of state-of-the-art laser wakefield accelerators when adding a PWFA stage. Furthermore, we demonstrate the generation of electron bunches with energy spread and divergence superior to single-stage LWFAs, resulting in bunches with dense phase space and an angular-spectral charge density beyond the initial drive beam parameters. These results unambiguously show that staged LWFA-PWFA can help to tailor the electron-beam quality for certain applications and to reduce the influence of fluctuating laser drivers on the electron-beam stability. This encourages further development of this new class of staged wakefield acceleration as a viable scheme toward compact, high-quality electron beam sources.

Popular Summary

Particle accelerators have enabled many scientific discoveries over the last 100 years. In recent decades, plasma-based accelerators, driven by either powerful laser pulses or dense particle beams, have complemented accelerator research. While these novel accelerators promise a dramatic reduction in size, they still face challenges regarding the stability and quality of the generated electron bunches. To help solve those problems, we present the somewhat counterintuitive result that these parameters can be significantly improved by staging two plasma accelerators.

In a hybrid approach, one laser-driven wakefield accelerator generates high-current electron bunches. These bunches drive a subsequent particle-driven wakefield accelerator, where “witness bunches” of electrons are internally injected and accelerated using a robust optical injection scheme. Thus, the hybrid approach combines the advantages of the two complementary driver types for plasma-based accelerators. In particular, it benefits from the availability of high-current bunches from laser-driven plasma accelerators and from the higher stability and lower emittance that can be achieved in one that is driven by a particle beam.

Our experimental results show that a hybrid accelerator can outperform pure laser-driven acceleration in terms of stability and spectral-spatial charge density of the generated electron bunches. This performance enhancement is of immediate interest for high-impact applications such as plasma-based free-electron lasers. Current plasma-based accelerators have recently proven that they can enable the onset of free-electron lasing. It is expected that free-electron lasers can benefit greatly from the electron quality improvements presented in this work.

Figure 1

Experimental setup for staged LWFA-PWFA with internal injection in the PWFA stage. (a) Schematic of the double-jet setup for staged LWFA-PWFA experiments. In the first plasma, the drive pulse propagating along the $z$ axis generates LWFA electrons via shock-front-assisted ionization injection. A $25\text{−}\mu \mathrm{m}$-thick polyimide tape after the first jet prevents the laser from ionizing the second jet for the first set of experiments. An astigmatic laser focus oriented perpendicular to the wakefield axis (b) heats the plasma in the PWFA stage a few nanoseconds before the electron driver arrives. A pair of planar plasma density shocks evolves. The second shock provides the down ramp for the injection of witness electrons into the PWFA stage. A few-cycle probe was used to image the laser-driven plasma wave in the LWFA stage (c) and the electron-driven plasma wave in the PWFA stage (d). Typical spectra (e) of the electron beam from the LWFA stage, the spent drive beam after the PWFA stage without injection, and an internally injected witness beam are shown. Without injector laser beam only a broadband background of decelerated LWFA electrons is formed in the PWFA stage. With injector beam the witness is the defined peak on top of this background at around 70 MeV.

Figure 2

Experimental data on stable plasma wakefield acceleration. Top: output spectra of 20 consecutive shots with the HOFI-generated shock and preionized plasma in the second jet. Bottom: electron spectra with injector laser beam (blue). This is compared to spectra without plasma (orange) and with plasma, but no injector (green) in the PWFA stage. The strong charge and energy loss of the driver in the green case and the injection of a high-charge witness in the blue case are evident. Dashed lines and red shaded areas indicate the mean of the energy and its standard deviation of driver and witness bunches, respectively. For reference, angle-resolved spectra of the drive beam alone and after the PWFA stage but without injector can be found in Fig. 1.

Figure 3

PIC simulations on stable staged acceleration. (a) Modeled density distribution and evolution of driver and witness energy in the PWFA stage. (b) Angular-resolved witness spectrum for the experimental driver parameters. (c) Set of 100 simulations with randomized variation of driver charge and energy with standard deviation as in the experiment. For comparison the spectra of an earlier simulation step [gray shaded in (a)] are shown where the energy gain resembles the experimental outcome. Panels (d) and (f) show the spectral charge density of the witness for parameter scans of the driver energy and charge. Given that driver depletion can be neglected (here driver energy $>200\mathrm{MeV}$), the witness energy does barely depend on the driver energy. Panel (f) shows the spectral charge density of the witness bunch (color scale and black crosses) as a function of the driver charge. With increasing driver charge the witness charge increases (e), the witness spectrum broadens, and its mean energy slightly decreases. For comparison the energy gain of a test bunch in an unloaded wakefield with equal driver is plotted (gray scale and black dots). In both scans the experimental working point is indicated by the orange shaded area. The simulation outcome in (d) and (f) is normalized to the experimental working point in (b). For comparison the experimental standard deviation of the witness energy is plotted in blue. Note that the experimental result is normalized independently.

Figure 4

Increase of angular-spectral charge density in the PWFA stage. (a) Typical spectrum of a low-divergence LWFA-generated drive beam with 640 pC bunch charge in the high-energy feature and an average divergence of 0.41 mrad (rms of super-Gaussian fit), leading to an angular-spectral charge density of $5\mathrm{pC}/(\mathrm{MeV}\mu \mathrm{sr})$. (b) Spectrum after the PWFA stage with optimized beam loading for high charge density of the witness beam. Because of the lower divergence of 0.28 mrad (rms of super-Gaussian fit) of the 30 pC witness beam, its angular-spectral charge density is 40% higher than the driver [$7\mathrm{pC}/(\mathrm{MeV}\mu \mathrm{sr})$]. (c) Typical LWFA-driver spectrum for the high-divergence case (1.2 mrad, rms of super-Gaussian fit). Using this beam with a charge of 400 pC and an angular-spectral charge density of only $0.4\mathrm{pC}/(\mathrm{MeV}\mu \mathrm{sr})$, a witness beam (d) with 0.22 mrad rms divergence, 20 pC charge, and an angular-spectral charge density of $6\mathrm{pC}/(\mathrm{MeV}\mu \mathrm{sr})$ is generated.