碩士生: 黃怡龍
畢業年分: 2023年7月
論文名稱: 六硼化鑭無加熱器中空陰極放電電壓特性研究(中文) / Investigation of discharge voltage characteristics in lanthanum hexaboride heaterless hollow cathode (英文)
中文摘要:
霍爾推進器的運作需要穩定電子源以完成燃料氣體的電離與離子束的中和，因此中空陰極常被用作電子來源。傳統中空陰極需仰賴加熱器將發射體加熱至工作溫度以釋放電子，但因長時間熱循環造成加熱器材料疲乏，常導致加熱器損壞，陰極壽命受限。為解決此問題，無加熱器中空陰極的概念被提出，利用高電壓與大流量觸發電弧放電加熱發射體，不僅避免加熱器失效，亦具備重量減輕、能耗降低與設計簡化等優勢。
中空陰極運作可分為「點模式」與「羽流模式」。點模式為穩定運作狀態，保持器出口處可見明亮電漿球；羽流模式則伴隨電壓與電流大幅振盪及高能粒子生成，易導致結構侵蝕與壽命縮短。本研究以陽極放電壓與保持器浮動電壓峰對峰值大於5 V 作為陰極進入羽流模式的判定標準，並對所設計的無加熱器中空陰極進行性能測試。
實驗結果顯示，當流量低於 5 sccm 時，放電電壓產生顯著振盪，顯示進入羽流模式；而在 7 sccm、放電電流 4–7 A 條件下，電壓振盪幅度低於標準，仍保持於點模式。不同背景壓力下的比較（8.1×10⁻⁴ 與 3×10⁻⁴ torr）發現，過高的壓力會使模式轉換消失，無法藉由振盪判定最佳參數。此外，施加軸向磁場使放電電壓下降，推測為磁場降低電子跨場傳輸能力，增加陽極接收電流的有效表面積。
本研究驗證了無加熱器中空陰極之可行性，並對其模式轉換判定與操作參數影響進行分析，對霍爾推進器中空陰極設計與壽命提升具有參考價值。
英文摘要:
The operation of Hall thrusters requires a stable electron source to ionize propellant gases and neutralize accelerated ion beams. Traditionally, hollow cathodes serve this purpose, consisting of a cathode tube, emitter, keeper, and heater. The heater raises the emitter to operating temperature to release electrons; however, repeated thermal cycling leads to heater fatigue, resulting in failure or short circuits. This issue often limits the cathode’s lifetime before the emitter is depleted. To address this, heaterless hollow cathodes have been proposed, in which high-voltage and high-flow arc discharges heat the emitter. This approach avoids heater degradation and offers advantages such as reduced weight, lower energy consumption, and simplified design.
Hollow cathodes operate in two regimes: spot mode and plume mode. Spot mode is characterized by stable operation with a bright plasma spot at the keeper orifice, whereas plume mode corresponds to instability with widespread plasma, large current–voltage oscillations, and the generation of energetic particles, leading to erosion and reduced lifetime. In this study, mode transition was defined by the peak-to-peak voltage between the anode discharge voltage and keeper floating potential exceeding 5 V, and performance of the designed heaterless hollow cathode was evaluated.
Experimental results showed that at flow rates below 5 sccm, discharge voltage exhibited strong oscillations, indicating transition into plume mode. At 7 sccm with discharge currents between 4–7 A, oscillation amplitudes remained below the threshold, maintaining spot mode. Tests under different background pressures (8.1×10⁻⁴ and 3×10⁻⁴ torr) revealed that excessive pressure suppressed mode transitions, preventing identification of optimal flow and current conditions. Applying an axial magnetic field reduced discharge voltage, likely due to suppressed electron cross-field transport and an increase in effective anode current collection area.
This study demonstrates the feasibility of heaterless hollow cathodes and clarifies the effects of operating parameters and mode transitions, providing valuable insights for improving hollow cathode design and extending the lifetime of Hall thrusters.
