Diagnosing the electron density of plasma amplifiers using XUV vortex beams

In this article, we study computationally the wavefront of high-order harmonics (HOH) carrying orbital angular momentum (OAM) after amplification in inhomogeneous Ni-like krypton plasmas. The results clearly show that electron density gradients induce curvature in the constant phase locus of the initial wavefront. Thus, it is expected that its value, along with the electron density, might be recovered from the perturbation. Two methods have been developed for analyzing the resulting phase curvature: one based on the identification and polynomial fitting of this locus, the so-called phase jump lines (PJLs), and the other on the decomposition into modified Laguerre-Gaussian modes. The results of these two methods provide valuable information about the plasma, such as the density profile, gradient direction, group velocity, and qualitative data on the electron density and gradient values. These results highlight the potential of using the wavefront of HOH carrying OAM as diagnostic tools for applications like inertial confinement fusion (ICF), laboratory astrophysics, or filamentation and lasing from filaments.