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 μm 1/e diameter, with an angular sensitivity of 0.18 mV/μrad and a risetime of 6 ns. The estimated detection threshold for electrons in the developing discharge is 10¹⁶ cm^-3 to 10¹⁷ cm^-3. A streak camera is used to gather spatial information regarding luminous processes with a maximum resolution of 25 μm 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 μm of the surface near the cathode and in the range of 75 to 175 μm from the surface near the anode, during the developing discharge. Streak camera measurements demonstrate the formation of an intense, visible emission, 25 to 50 μm 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