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Space and time resolved density measurements have been performed to study an impulse voltage breakdown and the subsequent shock wave expansion in an atmospheric air. A two-wavelength interferometric t...echnique has been developed to discriminate the electron from the ion and neutral densities as the refractivities of the latters are almost independent of, but that of the former strongly dependent on, the wavelength of an electromagnetic wave. This system allows the measurement of the densities in the arc channel and the shock front regions by a single shot. A YAG (wavelength 1.064μm) and a ruby (0.6943μm) lasers have been found to be adequate for the two-wavelength sources on a Mach-Zehnder interferometer. The space and time resolutions are 50μm and 0.1μs, respectively, which are good enough for the purpose. The experiments have been performed by observing the breakdown spark of a point to point gap (spacing 5 mm) with current waveform of (1.5×40)μs with the maximum current of 65 A. At the discharge axis, the electron density reaches 1.3×10^<24>m^<-3>(almost fully ionized) at 0.4μs from the start of the discharge and decreases quickly to 4×10^<23>m^<-3> in 7μs by the rapid expansion of the arc channel, yielding a cylindrical shock wave whose Mach number is 2.2 at 0.4μs and decreases to～1 at 7μs. The electron density profiles are at the beginning steeper than, but finally approach to, the Bessel distribution.続きを見る
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