Study of Hollow Cathode Plume Instabilities Onset using a Fast-Scanning Probe

Plasma instabilities play a fundamental role in determining the lifetime of hollow cathodes and, consequently, the operative life of electric thrusters. Three main categories have been identified: ionization instabilities, rotational instabilities, and turbulent ion acoustic instabilities (IAT). Ionization instabilities typically arise when the cathode operates in plume mode and decrease at lower discharge currents and mass flow rate ratios. Rotational instabilities occur in the presence of an externally applied axial magnetic field, even under spot-mode operation, while IAT develops at frequencies > 100kHz. Several studies have suggested that plasma instabilities are linked to the generation of high-energy ions, which are responsible for keeper surface erosion and thus limit cathode lifetime. This paper presents results on the onset of plasma instabilities in a 25 A-class cathode, investigated using a fast-scanning axial probe capable of probing the cathode orifice region. By means of FFT and CWT analysis, the mode frequencies have been identified and their spatial localization on the centerline with respect to the cathode orifice has been observed. In the absence of an applied magnetic field, ionization instabilities have been detected at low mass flow rates, with characteristic frequencies that increase as the flow rate decreases. In the presence of an axial magnetic field, rotational instabilities couple with turbolent ion acoustic instabilities, leading to the generation of more energetic ions, and also interact with ionization instabilities when the cathode operates in plume mode. The CWT plots demonstrate that rotational bursts are stronger just downstream of the keeper orifice, where sputtering occurs. The dynamics of these rotational instabilities, tracked with the scanning probe, supports the presence of kink-type instabilities in the plume, even at low discharge currents.

等离子体不稳定性（Plasma Instabilities）是决定空心阴极（hollow cathodes）寿命、进而影响电推力器（electric thrusters）工作寿命的核心因素。目前已识别出三类主要的不稳定性：电离不稳定性（ionization instabilities）、旋转不稳定性（rotational instabilities）以及湍流离子声不稳定性（IAT）。电离不稳定性通常在阴极以羽流模式（plume mode）运行时出现，并随放电电流（discharge currents）与质量流量比（mass flow rate ratios）的降低而减弱。旋转不稳定性在施加轴向外磁场时即可发生，即便阴极处于斑点模式运行（spot-mode operation）状态；而湍流离子声不稳定性的出现频率高于100kHz。多项研究表明，等离子体不稳定性与高能离子（high-energy ions）的产生相关，这类高能离子会引发维持极（keeper）表面的溅射侵蚀，进而限制空心阴极的使用寿命。本文针对25安级空心阴极的等离子体不稳定性起振特性展开研究，采用可对阴极孔区（cathode orifice region）进行探测的快速扫描轴向探针（fast-scanning axial probe）完成实验。通过快速傅里叶变换（FFT）与连续小波变换（CWT）分析，我们识别出了各不稳定模式的特征频率，并观测到其相对于阴极孔的中心线空间分布情况。在未施加外磁场的工况下，低质量流量条件下可检测到电离不稳定性，其特征频率随流量降低而升高。当施加轴向磁场时，旋转不稳定性会与湍流离子声不稳定性发生耦合，产生能量更高的离子；而当阴极以羽流模式运行时，旋转不稳定性还会与电离不稳定性产生相互作用。连续小波变换绘图结果显示，旋转不稳定性的爆发强度在维持极孔下游区域更强，该区域恰好是溅射（sputtering）发生的位置。通过扫描探针追踪得到的旋转不稳定性动力学特性，证实了羽流中存在扭结型不稳定性（kink-type instabilities），即便在低放电电流工况下亦是如此。