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We study the dynamics of the gas flow discontinuities after pulse ionization of a half space in front of a flat shock wave moving in a channel. Pulse volumetric electric discharge initiated in the vicinity of the shock concentrates in front of the shock and heats the gas there. The heating is shown to be very rapid. We use the shadow imaging technique and a high speed camera to study the flow pattern evolution after the discharge. The pattern consists of two shocks separated by a contact surface. This structure corresponds to the classical Riemann problem formulation. Based on the observed pattern, we estimate the amount of discharge energy converted to heat during the discharge time: the rate of temperature increase is in the order of several degrees K per nanosecond.

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Most of the established methods of visualizing gas flows are suited for studying steady and quasisteady processes with characteristic times greater than a millisecond. Rapid interactions of gasdynamic disturbances and discontinuities are visualized primarily by conventional shadow and holographic interference methods, which have serious limitations with respect to sensitivity and range of gas density.Several investigations [1, 2] have employed an electric discharge to visualize steady gas flows with shock waves. The local intensity of luminescence of the gas in the region of the discharge is related to the gasdynamic parameters of the flow and the parameters of the electric field [3]. This problem is most complex near the front of a shock wave.Studies of the effect of ionization on the propagation of shock waves have shown that the velocity, amplitude, and structure of the waves change in glow, high-frequency, and superhigh-frequency discharges [4, 5]. These changes are connected both with heating of the gas and with nonequilibrium excitation of its molecules. Thus, significant changes are introduced when a gas flow is visualized by means of a steady discharge. To be precise, we should therefore state that a gas flow ionized by a discharge is being visualized.Some of the limitations attending the use of an electric discharge to visualize gasdynamic processes can be eliminated if a highly uniform impulsive discharge is created within the test volume and the period of luminescence is much shorter than the characteristic gasdynamic times. Such conditions have been realized on a STDO (shock-tube --discharge --optics) unit, which is a combination of a shock tube and a special discharge chamber [6].The internal cross section of the tube is 24 x 48 mm. In the tests we conducted, the Math number of the shock waves M = 1.1-6, the period of supersonic flow was 200-400/~sec, and the initial pressure in the slipstream ranged up to 50 kPa. A discharge scheme employing plasma electrodes used in laser technology [7] generated the space charge in the working chamber (which changes into the shock-tube channel). A high degree of spatial uniformity of the discharge was ensured by pre-ionization ultraviolet irradiation from plasma electrodes --plates sliding over the surface of dielectrics located on the top and bottom walls of the chamber flush with the shock-tube channel. The uniformity of the discharge was checked by taking photographs at the ends of the chamber and by continuously observing the discharge visually through the side walls, made of quartz glass. The criterion for uniform luminescence of the air in the region of the discharge was the presence of a uniform gas (stationary or two-dimensional flow) within the density range from 8 g/m 3 to 0.2 kg/m 3. The presence of discontinuities in the air flow led to nonuniform ionization of the test volume. The sliding discharge was initiated unevenly when the discontinuities were adjacent to the plasma electrodes. The ionization coefficient ~ is a nonlinear function of E/N, and ...