The corona and breakdown characteristics of pure N
2 and mixture (SF
6/N
2), in high-field needle-to-plane gaps with short gap length, were examined under positive polarity with various wave fronts of microseconds. Although a small addition of SF
6 gas to N
2 gas showed a slight increase in the corona onset voltage, the breakdown voltage was extremely high compared with that of pure N
2 gas. The breakdown voltage increased on increasing the wave fronts in the region of μs and reduced on further increase of wave fronts. In order to make clear the mechanism of the corona initiated breakdown, the corona properties under various pulse waveforms were observed with an optical technique such as an image intensifier tube. In the high nonuniform field under the pulse voltage of μs region, it was found that corona stabilization is enhanced effectively by a small addition of SF
6 gas to N
2 gas and' that the development of the corona is dependent on the wavefront time of the pulse voltage.
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