In the last few years, systematic studies on radiation hydrodynamics in the X-ray confining cavity and a fuel capsule have attained remarkable progress. This makes it possible to analyze quantitatively the energy transfer processes from laser to the fusion capsule and find uniform irradiation conditions of the fusion capsule driven by thermal X rays. As a result, reproducible and stable implosions were achieved. Throughout implosion experiments with the Gekko XII blue laser system (351 nm, kJ, 0.8 ns), good agreement of implosion has been obtained between the experiment and numerical simulations, assuming perfectly spherical symmetry, up to a radial convergence ratio of 15. Described are particularly the issues of (1) energy transfer processes from laser to a fuel capsule and conditions for uniform irradiation, (2) properties of the X-ray propagation through aluminum heated by X-ray radiation, and (3) dependence of the convergence ratio of Ri/Rf (where Ri and Rf are the initial and final radii) of the capsule on the initial fill pressure of D–T gas and its influence on the core parameters and fusion products to evaluate implosion sphericity.