Laser wakefield acceleration (LWFA) is a cutting-edge approach to particle acceleration. When a high intensity short pulse laser travels through the plasma, it will generate wake fields behind the laser pulse. The background electrons can be trapped inside the wakefields and accelerated. The expected acceleration strength can reach up to 100 GeV/m, which is three orders of magnitude higher than the acceleration gradient generated by conventional radio frequency (RF) cavities. In the past two decades, the LWFA experiment has reached some extraordinary improvements. Current laser wakefield accelerators can generate a few hundred MeV electron beams with less than 1% of energy spread. And the electron beam emittance can be suppressed up to 3 mm mrad. Improving the stability of electron beams is still a big challenge for researchers all over the world. The instability of the electron could come from:

1) the instability of the laser system.

2) the instability of the plasma target.

A systematic study on the fluid instability of the supersonic nozzle via numerical simulation and experiment was published in High Power Laser Science and Engineering, vol. 11, e91. (Zhen-Zhe Lei, Yan-Jun Gu, Zhan Jin, Shingo Sato, Alexei Zhidkov, Alexandre Rondepierre, Kai Huang, Nobuhiko Nakanii, Izuru Daito, Masakai Kando, Tomonao Hosokai. Supersonic gas jet stabilization in laser–plasma acceleration[J]. High Power Laser Science and Engineering, 2023, 11(6): 06000e91)

This work discusses the role of the stilling chamber in a modified converging–diverging nozzle to dissipate the turbulence and to stabilize the gas jets. The fluid instability in supersonic gas jet originating from the nonlinear turbulence is studied and the mechanism to suppress the instability is proposed. And fluid instability suppression effect with stilling chamber is confirmed with Mach-Zehnder interferometer. At the end, the pointing stability of electron beams from stable and unstable gas jet were compared to confirm the effect of fluid instability suppression on the electron beam generation.

All the work indicates that the fluid instability in gas jet can be suppressed by adding a stilling chamber to the converging-diverging nozzle. This new design can significantly increase the electron pointing stability in the LWFA experiment.

Figure 1 (a) Sketch of the converging–diverging nozzle. (b) Schematic of the fluid dynamics simulation domains for the converging–diverging nozzle. (c) Velocity distributions (normalized to the sound speed) and streamlines inside the stilling chamber part. The subplots from left to right correspond to the non-, up-, down-, left- and right-shift cases, respectively. (d) The density profiles in the converging region, diverging region and 1 mm above the exit are compared between the up-shift and down-shift cases.