Modeling and Simulation of Electrical Breakdown in DC for Dielectric-Loaded Systems with Non-Orthogonal Boundaries Including the Effects of Space-Charge and Gaseous Collisions

Improved modeling of angled-dielectric insulation in high-voltage/nsystems is described for use in particle-in-cell (PIC) simulations./nTreatment of non-orthogonal boundaries is a significant challenge in/nmodeling angled-dielectric flashover, and conditions on boundaries are/ndeveloped to maintain uniform truncation error in discretized space/nacross the dielectric angles studied. Extensive effort was expended in/nisolating particular operating regimes to illustrate fundamental/nphenomenological surface effects that drive the discharges studied/nherein; consequently, this document focuses on the phenomenology of/ntwo specific dielectric angles at 6.12° for multiplicative breakdown/n(the so-called single-surface multipactor) and 22.9° for a/nnon-multiplicative discharge that evolves into a dark current at/nsteady state./nPhenomenological results for simulations in vacuum through "ultra-low/npressures" on the order of a few hundred mTorr are presented. A/nmultipactor front forms via net emission of electrons from impact on/nthe dielectric surface, where emission leads to saturated field/nconditions in the wake of the front, producing a well-defined/nforward-peaked wave. A treatment of the gain and saturation/ncharacteristics is presented, isolating the surface electric fields as/nthe driving contributor to both metrics. Physical models include/noftenneglected effects such as space-charge, dielectric-surface/ncharging, and particle distributions in energy and space. For the/ndischarges treated in this study, breakdown voltages of the typical/nPaschen form are not applicable, since multiplicative conditions are/ndriven primarily by surface effects./nPhenomenological results are also presented for simulations at low/npressure (~ 1Torr), which is shown to be a transitional limit where/nvolume effects become appreciable compared to surface effects. A/ncoupling between space charge and surface charge is shown to lead to/noscillatory effects in otherwise DC discharges. Surface multipactor/nleads to increased ionization and space charge, and the ensuing/nspace-charge momentum alters what would have been a steady-state/nsaturation as in the case of vacuum-like discharges. Models for/ndiffusive outgassed species are developed and implemented, extending/nthe capabilities of the PIC suite./nThe overarching theme of this study is to communicate the dependence/nof multiplicative discharges dominated by surface effects on/nnear-surface electric field conditions. It is shown through various/nexamples from vacuum through low pressures, and in diffusive gases,/nthat single-surface multipactor conditions can be expressed solely in/nterms of the dielectric surface field angles. This treatment lays the/nfoundation for a novel extension of RF breakdown susceptibility theory/n[1] to the DC regime, grounding breakdown susceptibility to the/nwell-established fundamentals on secondary emission [2, 3]. This/ntheory shows that breakdown characteristics can be modeled in an/na-priori framework, hence the lack of a Paschen-type curve./nFinally, the effect of the seed source on discharge characteristics is/nalso studied. A comparison between a constant-waveform source, a/nFowler-Nordheim source, and an application of a modified source based/non theoretical treatment from [4] are presented, showing that the seed/nis a necessary but insufficient condition for surface flashover, where/nthe dominant contributor is the configuration of the surface fields/ndownstream of the seed source. While the seed can influence upstream/nconditions to alter the injected current, the gain characteristics of/nthe downstream region are still well described by the framework/ndeveloped in the remainder of this document.

Advisor: Leung, Ka-Ngo
Verboncoeur, John P