Azimuthal Ion Dynamics at the Inner Pole of an Axisymmetric Hall Thruster

The azimuthal dynamics of ions along the inner pole of a Hall thruster with a centrally mounted cathode and a magnetic shielding topography are experimentally investigated. A time-averaged laser-induced fluorescence diagnostic is implemented to characterize the azimuthal ion velocity distribution, and its moments are computed numerically to infer bulk rotation speed and ion temperature. It is found that the time-averaged ion swirl velocity grows to 2 km/s in the near-pole region, and the cathode ions exhibit ion temperatures in the azimuthal direction approaching 8 eV. Both of these quantities exceed the speeds and temperatures anticipated from classical acceleration and heating. Time-resolved laser-induced fluorescence is then employed to investigate the role of plasma fluctuations in driving the time-averaged ion properties. Semicoherent fluctuations at 90 kHz are observed in the ion velocity distribution and its associated moments. These oscillations are correlated with the gradient-driven anti-drift wave, which propagates azimuthally in the near-field cathode plume. Quasilinear theory is used to construct a 1D model for acceleration and heating of the ion population as a result of the anti-drift mode. This approach demonstrates qualitative agreement with the time-averaged ion velocity and temperature, suggesting that the anti-drift mode may be a dominant driver of azimuthal ion acceleration and heating in front of the cathode keeper and the inner half of the inner front pole cover. These results are discussed in terms of their relevance to the erosion of thruster surfaces in the near-field cathode plume.

针对搭载中心式阴极且采用磁屏蔽构型的霍尔推力器（Hall thruster）内极区域的离子方位动力学特性，开展了系统性实验研究。采用时间平均激光诱导荧光（laser-induced fluorescence, LIF）诊断手段表征离子方位速度分布，并通过数值计算其矩量以推导出离子整体旋转速度与温度。研究结果表明，在极区近场区域，时间平均后的离子旋流速度可达2 km/s，阴极区域离子在方位方向的温度接近8电子伏特（eV）。上述两项参数均超出了经典加速与加热机制下的预期数值。随后采用时间分辨激光诱导荧光诊断技术，探究等离子体波动对时间平均离子特性的驱动作用。在离子速度分布及其相关矩量中，观测到频率为90千赫兹（kHz）的半相干波动。该类振荡与梯度驱动反漂移波存在关联，此波在近场阴极羽流中沿方位方向传播。基于准线性理论（quasilinear theory），构建了反漂移模式引发的离子群体加速与加热的一维（1D）模型。该模型与时间平均后的离子速度与温度呈现出定性一致性，由此表明反漂移模式可能是阴极保持器前方以及内前极盖内半区域内离子方位加速与加热的主要驱动因素。最后结合近场阴极羽流区域推力器表面的侵蚀问题，对上述研究结果展开了讨论。