Description
Plasma wakefield accelerators driven by ultra-intense laser pulses (LWFA) or relativistic particle beams (PWFA) have emerged as a promising route toward compact next-generation accelerators. Precise characterization of the plasma wave is essential for understanding and optimizing these schemes. Optical shadowgraphy techniques already enable direct imaging of plasma density perturbations and wakefield structures [1]. Despite these advances, the sensitivity of conventional shadowgraphy limits its application to high plasma densities, typically in the $10^{18}-10^{19}$ $\mathrm{cm}^{-3}$ range.
Here, we introduce a dark-field or Schlieren shadowgraphy diagnostic that greatly enhances the sensitivity of optical shadowgraphy and enables probing ultra-low plasma densities down to the $10^{16}$ $\mathrm{cm}^{-3}$ range, where plasma waves are invisible to conventional shadowgraphy. Furthermore, we show with this new diagnostic that we clearly observe for the first time several detailed features of the plasma accelerator: (i) the position of the laser driver that typically appears as the brightest signal, (ii) the ion cavities with the plasma electrons circulating around, and (iii) the plasma electron crossing points at the back of the cavities, characterized by particularly high plasma densities and strong gradients, yielding a strong diagnostic signal. The method was experimentally tested first in Salle Jaune at LOA and more recently during campaigns at the Apollon (3 PW) facility. Our study further demonstrates the capability of this diagnostic to track the temporal evolution and damping of the wakefield with a relaxation time estimated to be on the order of 10 ps after the laser passage in good agreement with numerical simulations, together with the formation and expansion of the associated ion channel. This high-contrast, non-intrusive diagnostic thus provides a powerful tool for investigating and optimizing plasma-based particle accelerators.
Finally, double gas-jet targets combining a low-density stage followed by a higher-density region are considered for optimizing ultra-bright gamma-ray production aimed at probing strong-field quantum electrodynamics processes such as nonlinear Breit–Wheeler pair production [2]. Dark-field shadowgraphy proves particularly well suited to characterize laser propagation and wakefield generation in such complex density profiles.
[1] M. F. Gilljohann et al., Phys. Rev. X 9, 011046 (2019).
[2] Matheron et al., Phys. Rev. Research 7, L032011 (2025).