Ultra-fast breakdown at pressures below one atmosphere

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2006-08

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Abstract

Ultra-fast gaseous breakdown is an important phenomenon in pulsed power related to ultra wideband systems, plasma limiters, and ultra fast switches. Recent advances in digitizers and pulser technology has allowed for pulses with sub-nanosecond properties to be examined in detail. This thesis examines voltage pulses in quasi homogenous fields with rise times less then 200 ps and pulse widths less then 300 ps. The breakdown is examined in pressures from high vacuum to 600 torr in argon and air. E/N values range from the order of 103 to 106 Td, and are typically above the threshold for the generation of runaway electrons. Also discussed is the design and characterization of a transmission line system including a pulse shaping lens intended to limit wave distortion at a coaxial to biconical geometry. Finite element numerical simulations have been used to explain the behavior of the system.

Diagnostic methods include waveform analysis from fast capacitive voltage dividers, X-Ray detection through a scintillator photomultiplier combination, and optical analysis through fast streak camera imaging. Parameters for the breakdown are established through modeling of the gap. X Ray emissions point to the role of runaway electrons in the breakdown. Electron energy at the anode is roughly determined for various pressure ranges. Streak camera imaging is used to show channel distribution and structure and its dependence on pressure. Results show breakdowns with development times too fast to be explained by standard breakdown mechanics indicating the importance of fast electrons in the event.

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