相較於霍爾推進器等連續式電力推進系統，真空電弧推進器 (Vacuum Arc Thruster, VAT) 所需操作功率較低。此外，由於真空電弧推進器之陰極可直接作為推進劑來源，因此能有效降低整體系統之體積與重量，使其特別適合應用於立方衛星等小型衛星之長時間任務。真空電弧推進器主要由絕緣層與電極所組成，因此電極材料之選擇與絕緣層結構設計，將直接影響推進器之操作特性與性能表現。André 等人於真空電弧無觸發點火研究中指出，於陶瓷絕緣層表面塗佈一層薄石墨膜，可有效降低所需之崩潰電壓，此方法稱為無觸發絕緣層。然而，雖然此無觸發結構能降低初始崩潰電壓，但隨著石墨層因燒蝕而逐漸消耗，崩潰電壓將再次上升。此外，若石墨層過厚，則可能造成電極間短路，使推進器操作壽命受到嚴重限制。為了解決此一關鍵瓶頸，Zaplab 過去提出一種創新的多層絕緣層結構，此設計不僅改善傳統真空電弧推進器之壽命問題，同時亦可進一步降低崩潰電壓。
顧名思義，多層絕緣層係由不同材料之薄片交錯堆疊並黏合而成，而本研究中所使用之材料為石墨片與聚四氟乙烯片(polytetrafluoroethylene, PTFE）。此結構降低崩潰電壓的主要機制，來自於三結點 (triple junction) 概念之導入。當傳統表面塗佈石墨膜之陶瓷絕緣層施加電壓於兩電極之間時，其電場分布相對均勻；相較之下，多層絕緣層因不同介電常數材料於界面處交錯排列，使局部電場產生扭曲與集中，此現象即為三結點效應。藉由多層絕緣層之導入，推進器整體壽命亦可大幅延長，原因在於整個絕緣層本體可隨操作過程逐漸作為推進劑被消耗。
除了絕緣層本身之影響外，本研究亦深入探討再沉積層之形成與演化，因其厚度與元素分布將顯著影響推進器性能。在真空電弧推進器點火過程中，陰極與絕緣層會同時發生燒蝕，使絕緣層表面逐漸累積沉積層。此沉積層之結構形貌將深刻影響崩潰電壓，而其空間擴展行為亦與電極和絕緣層之間的幾何深度差具有直接關聯。因此，本研究特別針對此幾何深度差所造成之影響進行分析。實驗結果顯示，不同陰極材料之熱物理性質，會對沉積層微觀結構形貌及崩潰電壓之長期演化產生決定性影響。
在共平面幾何結構下，由於不存在狹窄通道限制，燒蝕產物可沿視線方向較順暢地傳輸，使關鍵三結點放電區域不易受到燒蝕沉積物阻塞，因此能維持極為穩定的長期崩潰電壓。相較之下，在具有狹窄通道之內縮幾何結構中，純紅銅陰極因具有較高沸點，其材料侵蝕過程主要受熔池中液態金屬巨觀顆粒之機械噴濺所主導，導致通道內形成巨觀粗糙且呈顆粒狀之沉積結構。於後期操作過程中，這些微尺度顆粒突起會誘發強烈之局部場發射效應，並顯著提高場增強因子。此形貌轉變使真空崩潰更容易於較低崩潰電壓下發生，因此其崩潰電壓曲線中出現大量低電壓數據點，且明顯低於黃銅電極之結果。
相反地，黃銅陰極因富含低沸點且具高飽和蒸氣壓之揮發性鋅元素，其侵蝕過程主要受劇烈體積蒸發所主導。金屬原子蒸氣會優先於狹窄通道內凝結，並形成平坦、高度均勻且呈片狀連續結構之沉積層。由於此極為平滑之表面缺乏可誘發局部場發射之微尺度突起，因此黃銅電極在長期操作後將完全喪失原先由尖端效應所帶來的降壓優勢，最終導致其崩潰電壓呈現嚴重且階梯式上升之趨勢。

Vacuum Arc Thrusters (VATs) offer a compelling alternative to continuous electric propulsion systems, such as Hall-effect thrusters, due to their significantly lower power requirements. By utilizing the cathode material itself as the propellant, VATs minimize both system volume and dry mass, making them exceptionally well-suited for the propulsion needs of small satellites, including CubeSats. Given that a VAT's architecture is fundamentally defined by its electrode geometry and insulating configuration, these components exert a decisive influence on thruster performance. Building upon the seminal work by André et al., which demonstrated that a thin graphite coating on a ceramic insulator can facilitate triggerless ignition by lowering the breakdown voltage, recent developments have addressed the inherent limitations of this "triggerless" approach. Specifically, while such configurations reduce initial breakdown voltages, the subsequent ablation-driven depletion of the graphite layer leads to a progressive increase in breakdown voltage; furthermore, excessive coating thickness poses a risk of electrode short-circuiting, thereby limiting operational lifetime.
To mitigate these bottlenecks, this study investigates an innovative multi-layer insulator design composed of alternating layers of heterogeneous materials specifically graphite and polytetrafluoroethylene (PTFE). This configuration leverages the "triple junction" effect: unlike conventional coated ceramics that establish a relatively uniform electric field, the dielectric mismatch at the interfaces of the multi-layer structure induces local field distortion and concentration. This mechanism not only suppresses the breakdown voltage but also extends the operational lifetime by allowing the entire bulk of the insulating material to be progressively consumed as propellant.
Beyond the insulator's architecture, this work provides an in-depth investigation into the evolution of the redeposited coating layer, as its thickness and elemental distribution are primary drivers of thruster performance. During VAT operation, the simultaneous ablation of the cathode and insulator results in a continuous accumulation of material on the insulator’s surface. We demonstrate that the structural morphology of this deposition, and its subsequent impact on breakdown voltage is heavily modulated by the geometric recession depth between the electrode and the insulator.
Experimental results reveal that the distinct thermophysical properties of various cathode materials dictate the microstructural evolution of the deposit and the long-term stability of the breakdown voltage. Under a co-planar geometry, where the absence of narrow channels allows for high line-of-sight transport of the exhaust plume, the critical triple junction discharge zone remains unobstructed by ablated species, ensuring an exceptionally stable breakdown voltage over time. In contrast, in recessed configurations characterized by narrow channels, a pure copper cathode owing to its high boiling point, undergoes an erosion process dominated by the mechanical ejection of liquid metal macroparticles from the melt pool. This results in a macroscopically rough, granular deposition structure within the channel. During late-stage operation, these microscale protrusions trigger intense localized field emission, increasing the field enhancement factor and facilitating vacuum breakdown at significantly lower voltages than those observed with brass electrodes. Conversely, a brass cathode, characterized by high concentrations of volatile zinc with low boiling points and high vapor pressure, undergoes an erosion phase dominated by violent bulk vaporization. The resulting atomic vapor preferentially condenses within the narrow channels to form a smooth, continuous, flake-like structure. Lacking the microscale protrusions necessary to induce localized field emission, the brass electrode loses its voltage-suppression advantage during long-term operation, leading to a severe, step-like escalation in breakdown voltage.

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