Experimental Demonstration of Beam-Driven Wakefield Acceleration in Laser-Plasma Filament

Abstract

Self-guided femtosecond laser pulses propagating in low-pressure gas can generate plasma filaments, establishing a new framework for plasma wakefield acceleration. Unlike conventional schemes relying on mechanically confined or preformed plasma channels, this method exploits the intrinsic non-linear light-matter interaction, greatly reducing the energy required to generate plasma. This, in turn, allows to realise tunable stages, potentially operating above kHz repetition rate and with meter-scale interaction lengths and transverse sizes down to a few tens of micrometres. Moreover, the laser-plasma filament reproducibility is intrinsically higher than state-of-the-art discharge-plasmas, where the breakdown process is initiated in a stochastic and uncontrolled manner. As a result, laser-based plasma formation offers improved reliability and control over plasma parameters. Here we report a proof-of-principle experimental demonstration of beam-driven wakefield acceleration of electron bunches with an accelerating field exceeding 250 MV/m in a laser-generated plasma filament. The results are cross-checked with numerical simulation, showing an excellent agreement and providing a complete picture of the physical process. Beyond particle acceleration, the concept bridges laser filamentation physics, advanced plasma photonics and compact accelerator technologies, offering a promising route towards sustainable, high-repetition-rate plasma-based facilities.

Figures (9)

• Figure 1: Laser filamentation simulations. (a) Laser pulse non-linear propagation retrieved via envelope equation (Eq. \ref{['envelope']}) in nitrogen gas (blue continuous line) and in vacuum (blue dashed line); laser pulse non-linear propagation (blue circles) and plasma filament width (violet squares) retrieved from PIC simulation. (b) Longitudinal plasma density distribution retrieved from PIC simulations (green squares) and from transverse imaging of the filament (orange circles) as in the inset of Fig. \ref{['fig2']}.
• Figure 2: Experimental setup. Schematic layout of the SPARC_LAB linac interaction chamber arranged for the plasma filament-based acceleration experimental campaign. (a) 45$^{\circ}$, single-inlet capillary design. (b) Typical filament snapshots taken via the side imaging technique.
• Figure 3: Experimental energy spectra. Energy spectrum of driver and witness bunches in the (a) laser-off and (b) laser-on configurations, while gas is present in the capillary. Same colorbar for both plots. Solid lines: energy distribution of driver (red) and witness (green).
• Figure 4: Waterfall plot of experimental energy distributions. Energy distribution of driver and witness bunches of 100 consecutive events in the (a) laser-on and in the (b) discharge-on configuration, while gas is present in the capillary with similar initial beam parameters and the same average plasma electron density. The intensity of each plot is normalised with respect to the maximum of (a).
• Figure 5: Comparison between the filament-based PWFA and the discharge-based PWFA. Histogram of the accelerated witness energy $E_W$ in both filament-based (orange area) and discharge-based (blue area) configurations.