Vacuum Gap Breakdown For Pulsed Power Liners, From Amperes To Megaamperes
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Vacuum Gap Breakdown For Pulsed Power Liners, From Amperes To Megaamperes

  • Author(s): Cordaro, Samuel
  • Advisor(s): Krasheninnikov, Sergei;
  • Bott-Suzuki, Simon C
  • et al.
Abstract

Vacuum gap breakdown at high voltages, in the range of tens of kilovolts to megavolts, occurs in many systems where high-current conduction is required. Several standard geometries have been extensively investigated under both pulsed and D.C. conditions and are generally well understood and documented. To date, little detailed analysis of explosive emission in coaxial geometry has been performed, although this geometry is a common feature of high-energy devices including vacuum transmission lines, switch systems, and fusion devices. Of specific interest is the evolution of the current density and magnetic field in space and time along the electrodes and how it effects plasma formation in the gap. As current rises, plasma channels form in the gap as pathways to travel to the load. If the channel formation is asymmetric in distribution about the azimuth, the resulting current could be non-uniform, leading to early time scale instabilities in the load. Experiments were performed on machines with currents ranging from amperes to megaamperes to investigate implications of asymmetric breakdown distribution and how these effects scale, via the use of magnetic field probe array around the load to measure the magnetic field evolution, and thus current density distribution, in time and space. Results show a strong link between asymmetric plasma formation and current distribution in the load, which scales from ampere to megaampere experiments. Fowler-Nordheim analysis at 240 amperes for gap sizes >150μm, shows geometry was the driving factor in breakdown initiation and distribution, while at gap sizes <100μm surface profile played the dominant role. Through electroplating and dip-coating metals of varied work functions, it was found that at 1 megaampere the surface profile of the material dominates in driving the formation of breakdown channels in a vacuum gap, and in the case of a graphite coating a uniform gap and current distribution was achieved. With an external axial magnetic field at 1 megaampere, it was found that the asymmetric distribution of breakdown could be mitigated, and uniformity of gap plasma could be achieved. Experimental results show while asymmetric breakdown does lead to asymmetric current in the load, effective and mitigation is possible.

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