Pseudospark discharge is a low-pressure high-voltage pulsed discharge with hollow electrodes, and the overall discharge experiences several distinctive sub-phases. The instability phenomena, characterised by the sudden oscillations or distortions of discharge voltage and current, restrict the applications of pseudospark discharge. This study presents an investigation into phase transitions and instabilities in pseudospark discharge by combining the evidences from multiple discharge waveforms, time-resolved images, electron beam current profiles, 2D kinetic simulation results, and the spectral line emission of cathode material. The process of a normal pseudospark discharge is firstly discussed, and the threshold conditions of the transition from the superdense glow discharge (SGD) phase to the high current phase with arc cathode spots are addressed. According to temporal profiles and voltage-current characteristics, two types and four occurrences of instabilities are distinguished: type I—in the pre-breakdown phase; type II-1—in the SGD phase; type II-2—in the transition of cathode spots propagating from cathode hole edge to cathode plane; type II-3—in the re-initiation of spots near current reversal. It is found that the instability of type I is caused by the stepwise penetration of virtual anode plasma, and type II is closely related to the cyclic nature and finite lifetime of cathode spots. It is induced by the fact that the decay of cathode spots is faster than the formation under certain conditions, and the existing spots are not able to provide the current required by the external circuit. Important features and the suppression methods of the instabilities are discussed based on the proposed mechanisms.

中文翻译：

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揭示赝火花放电中的相变和不稳定性

赝火花放电是一种空心电极低压高压脉冲放电，整个放电经历几个独特的子阶段。以放电电压和电流的突然振荡或畸变为特征的不稳定现象限制了赝火花放电的应用。本研究结合多个放电波形、时间分辨图像、电子束电流分布、二维动力学模拟结果和阴极材料的谱线发射的证据，对赝火花放电中的相变和不稳定性进行了研究。首先讨论了正常赝火花放电的过程，并讨论了从超密辉光放电（SGD）阶段过渡到带有电弧阴极点的高电流阶段的阈值条件。根据时间分布和电压-电流特性，可区分两种类型和四种不稳定性的发生： I 型——击穿前阶段；II-1型——SGD阶段；II-2型——阴极斑点从阴极孔边缘向阴极平面的过渡；II-3型——电流反转附近点的重新启动。研究发现，I型的不稳定性是由虚拟阳极等离子体的逐步穿透引起的，而II型的不稳定性则与阴极点的循环性质和有限寿命密切相关。这是由于在一定条件下阴极斑点的衰减速度比形成速度快，现有的斑点无法提供外部电路所需的电流而引起的。