Vacuum Surface Flashover of Insulators for Microsecond Pulses

Summary form only given. A critical component to a high energy pulsed power system is the insulator. The insulator's function is to standoff high voltages between two electrodes of opposite polarity. Although the intrinsic dielectric strength of the insulator is rather high, relatively low electric fields -several times less-is withstood by insulator surfaces that is exposed to vacuum. To investigate surface flashover for the widely used truncated conical insulator configuration, a vacuum chamber was fabricated where we installed a positive 45 degree conventional HD polyethylene insulator in the electrode spacing (nominal 1.0 cm). With applied 100 kV pulses of 5 microsecond duration the insulator held off the applied voltage -but experienced flashover if a source of plasma/electrons in the form of a small piece of velvet (1.0 mm dia.) was introduced in the vicinity of the insulator on either of the electrode's surfaces. Plasma expansion velocities of 1.4 to 2.7 cm/microseconds are inferred from voltage collapse and current drawn signals. It appears that the actual flashover occurs when the plasma that is launched from the cathode reaches either the insulator or the anode electrode. The breakdown mechanism and improvements in the insulator design strictly speaking can be made independently with the more advanced computational means. We also report on the progress that has been made with our PIC code modeling and its agreement with our experimental observation.