Plasma development in the early phase of vacuum surface flashover

The primary physical mechanism responsible for charge-carrier amplification, in a developing surface discharge, has eluded conclusive identification for decades. This paper describes the results of experiments to directly detect charge-carriers, above the dielectric surface, within the developing discharge. Free electrons are detected by measuring the deflection of a laser beam, focused to a 20 /spl mu/m 1/e diameter, with an angular sensitivity of 0.18 mVspl mu/rad and a risetime of 6 ns. The estimated detection threshold for electrons in the developing discharge is 10/sup 16/ cm/sup -3/ to 10/sup 17/ cm/sup -3/. A streak camera is used to gather spatial information regarding luminous processes with a maximum resolution of 25 /spl mu/m and 0.6 ns. Current measurements have a sub-nanosecond response time and a detection threshold of 100 mA. Laser deflection measurements demonstrate the rapid development of a particle gradient, generally within 10 /spl mu/m of the surface near the cathode and in the range of 75 to 175 /spl mu/m from the surface near the anode, during the developing discharge. Streak camera measurements demonstrate the formation of an intense, visible emission, 25 to 50 /spl mu/m in diameter, located near the insulator surface, during the formation of the discharge. These results imply that charge-carrier amplification occurs above the surface of the insulator, in a region of neutral particles desorbed or otherwise ejected from the insulator surface.<
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