Spherical converging shock waves are produced in a conventional shock tube with a circular cross-section, where the shock is generated, focused and reflected. The shock tube is equipped with a special fast-opening valve (opening time <2ms), which offers an accurate control of the initial pressure ratio between the driver and the driven gas sections. This allows one to collect a significantly higher resolution experimental data for studying the focusing effect, compared to a common way of initiating the flow by bursting membranes. The resulting plane shock wave is transformed into the shape of a spherical cap by means of a smoothly convergent cross-section. The wall shape in the transformation section is designed to gradually change the form of the shock wave until it approaches a spherical shape. Thereafter, the shock enters a conical section where it converges towards the apex of the cone. Numerical calculations with the axisymmetric Euler equations show that the spherical form is only slightly dependent on the initial Mach number of the plane shock within the range of up to Mach 5.5, and is preserved to a close vicinity of the focal point. The strength of the shock is significantly increased during the focusing up to Mach numbers of ~30 at the final stage of spherical convergence. The test gas is heated to very high temperatures as a result of shock convergence and emits a bright light flash at the tip of the test section. The light radiation is collected by optical fibers mounted at the tip of the convergence chamber and investigated by photometric and spectroscopic measurements. Experiments are performed with argon as the test gas and with different initial Mach numbers. The radiation of the shock-heated argon closely resembles blackbody radiation. Fits to the experimental data result in apparent blackbody temperatures in argon of up to 30000K. The efficient amplification of the shock wave strength and the related temperature development are studied as a function of initial shock wave Mach number and test gas pressure.
