Precise control of the polarization of X-ray lasers is crucial in broad applications such as ultrafast-physics experiments and material characterization. Existing X-ray polarization converters, however, are mainly suited for low-power conditions and usually suffer from either large size or low conversion efficiency. Here we propose a compact and efficient scheme for polarization conversion of high-power, high-intensity femtosecond X-ray lasers, based on linear total internal reflection at the interface of the vacuum and solid-density plasma plate. Particle-in-cell simulations show that although the reflectivity is affected by the density oscillations of the surface plasma waves that are inevitably excited, the single-pass reflectivity can still exceed 95% and approach 100% for a broad range of laser parameters. Beyond its high conversion efficiency and damage resistance, this method offers dynamic tunability and enables ultrafast polarization switching (sub-ps), positioning it as a compact and innovative solution for polarization control in high-power X-ray laser systems.
