Browsing by Subject "Runaway electrons"
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Item Investigation of sub-nanosecond breakdown through experimental and computational methods(Texas Tech University, 2008-08) Chaparro, Jordan E.; Krompholz, Hermann G.; Neuber, Andreas A.; Hatfield, Lynn H.Sub-nanosecond breakdown, at sub-atmospheric pressures, is governed by significantly different physics when compared to standard breakdown processes. Applied field risetimes of 100s of ps combined with high peak amplitudes and short gap spacing allows for overvoltage to develop in the gap greatly exceeding static breakdown conditions. These conditions lead to a significant portion of electrons in the runaway mode and highly inhomogeneous charge distributions that greatly affect the scaling relationships for the discharge. The continued progression of pulsed power applications to shorter time scales makes a full understanding of such discharges necessary for the future development of devices relying on ultrafast, high voltage pulses. Insights into the physical background of sub-nanosecond breakdown are provided in this dissertation through both empirical analysis and numerical modeling. The modeling of the discharge is implemented through a customized particle-in-cell code combined with Monte-Carlo methods for simulating particle collisions. The results of the model show reasonable agreement to experimental results across the full range of test parameters. Additional insights into physical mechanisms that are not easily empirically measured are provided.Item Ultra-fast breakdown at pressures below one atmosphere(2006-08) Chaparro, Jordan E.; Krompholz, Hermann G.; Neuber, Andreas A.; Hatfield, Lynn L.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.