We measured dc discharge inception voltage Vi in air, N/sub 2/ and He gases for various kinds of electrode configurations by changing vacuum range from 0.1 MPa to 0.1 Pa. By comparison with the uniform field characteristics, we investigated the discharge inception phenomena for non-uniform field in low and medium vacuum. At the same time, we observed the discharge type varying with gas pressure. Discussion was made on the discharge characteristics such as the polarity effect of Vi, and the comparison of Vi among the three kinds of gases with varying the field non-uniformity. In order to quantitatively discuss the changes of discharge type obtained above, we introduced the image processing method. We newly proposed four shape parameters to characterize the discharge type. Based on these analyses, the discharge type in air was successfully classified into three categories with two boundary pressures 200 and 2000 Pa over the pressure range from 1.3/spl times/10/sup 4/ to 27 Pa. We also analyzed discharge type in He gas in the same way as above.
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