[en] it is shown in the work that mica crystals bent over 100 mm vadius sphere can be successfully applied in the experiments on the dense plasma diagnostics by X-ray spectral methods. High spectral and spatial resolution realized in the experiment allows one to apply these crystals in quite fine experiments, e.g. for measuring the transition extension parameters in the X-ray range and determining plasma parameters under 10²²-10²³ cm^-3 densities and 0.5-5 keV temperatures