High-repetition-rate, all-reflective optical guiding and electron acceleration in helium using an off-axis axicon

TL;DR

This work demonstrates high-repetition-rate, all-reflective laser guiding and electron acceleration in helium using an off-axis reflective axicon. By forming OFI plasma channels and employing self-waveguiding, the authors achieve stable guiding at 3.3~Hz and multi-GeV electron energies up to $5~\mathrm{GeV}$ at 0.2~Hz with a single-compressor setup, avoiding modifications to the main laser system. The combination of post-compressor beam splitting and reflective optics preserves pulse duration and phase quality while enabling efficient channel formation and guiding. The results highlight a practical path toward high-average-power plasma accelerators and compact radiation sources compatible with existing user facilities and safety considerations, particularly through the use of helium as a safer gas, and open avenues for future high-repetition-rate, plasma-based instruments.

Abstract

We present recent results on high-power guiding and laser wakefield acceleration (LWFA) in the ELBA beamline at ELI Beamlines, using the L3-HAPLS laser system (13 J, 30 fs, 0.2 Hz). By employing self-waveguiding in a 20 cm plasma channel in helium, we achieved stable acceleration of electron beams to energies approaching 5 GeV. A novel all-reflective optical setup, including an off-axis reflective axicon, enabled efficient acceleration at 0.2 Hz and guiding at repetition rates up to 3.3 Hz. This compact single laser, single compressor implementation of plasma channels for electron acceleration stabilizes electron pointing and enhances energy gain without requiring modifications to the laser system, paving the way for broader adoption of the technology across user facilities.

Figures (7)

• Figure 1: Experimental setup overview. The laser beam enters the auxiliary chamber from the left and is reflected by mirror M1. Two pick-off mirrors split small portions of the pulse to form the channel-forming and probe beams, while the main pulse continues as the LWFA drive beam. The channel-forming beam is routed through the auxiliary chamber and directed into the interaction chamber toward the off-axis axicon (OAA) positioned above the gas jet SN200, where it generates the plasma channel. The probe beam is guided through its delay line and diagnostic path, and the drive beam is focused by the off-axis parabola (OAP) into the interaction region for laser wakefield acceleration. The high-power focal-spot and guided-beam diagnostics are shown on the right side of the interaction chamber and include a mirror with a central aperture (HM1), an uncoated wedge (W3), an imaging lens (L5), and a mirror (SM1) that directs the attenuated pulse out of the chamber to CMOS cameras. The green beam path indicates a separate nanosecond laser system used in independent experiments to test alternative injection mechanisms. A detailed description of the setup is provided in the Experimental setup section below.
• Figure 2: Schematic layout of the interaction chamber setup for the self-waveguided LWFA experiment. (a) The final section of the optical system is shown, starting from mirror BM3, where the channel-forming (Bessel) beam enters the chamber, and including all subsequent mirrors BM3–BM7, the periscope, attenuator (half-wave plate, $\lambda /2$, and thin-film polarizer, TFP), and delay-line mirrors leading to the off-axis axicon (OAA) positioned above the 20 cm gas jet SN200. The LWFA drive beam passes through the axicon’s central aperture and interacts with the plasma channel formed by the Bessel beam, while the probe beam is directed from its apodization aperture via mirror PM3 to the telescope (lenses L1 and L2) with an SHG BBO crystal, then through a delay line toward the interferometer for diagnostics. (b) A zoomed-in view of the target area, showing the spatial overlap of the channel-forming (Bessel) beam, LWFA drive beam, and probe beam above the gas jet. A detailed description of the setup is provided in the Experimental setup section.
• Figure 3: Simulations of laser beam wavefront splitting and focal spot. (a) Wavefront of the LWFA driver beam after the pick-offs at the off-axis parabola (OAP). (b) Wavefront of the channel-forming beam after 12 m of free-space propagation at the axicon surface. (c) Focal spot of the LWFA driver beam (a) on target.
• Figure 4: Laser beam measurements. (a) Low-power focal-spot image and corresponding analysis. (b) Laser pulse characterization using SPIDER measurement.
• Figure 5: Guiding overview illustrating the interaction between the LWFA drive beam and the channel-forming beam, with the plasma column (yellow) highlighted between the focal plane and the waveguide exit plane above a 20 cm gas jet. (a) High-power focal-spot diagnostic image taken through the gas sheet during active guiding, used for online alignment monitoring. (b) High-power focal spot in vacuum, recorded using the focal-spot diagnostic camera. (c) Bessel beam focal spot, measured with a CMOS camera with $5\times$ magnification objective. (d) Image of a guided mode exiting 20 cm plasma waveguide acquired by the guided-mode diagnostic camera when the driver beam is successfully coupled into the waveguide. (e) Zoomed-out high-power focal spot image, same as in (b) for comparison with (f) Guided-mode exiting 3 cm plasma waveguide using short 3 cm gas jet (see Figure \ref{['fig:hrrguide']}), showing leaky modes Shrock2024 surrounding the guided beam. (g) Zoomed-out guided mode exiting 20 cm plasma waveguide, same as in (d). (h) Guided-mode image recorded when the driver beam misses the waveguide entrance due to laser-pointing jitter. (i) Top-view fluorescence image of the helium plasma channel generated by the Bessel beam. All images are individually normalized.