Characterization of neutral gas densities in the plume and the discharge chamber of an ion thruster using two-photon absorption laser induced fluorescence spectroscopy

双光子吸收激光诱导荧光光谱是精确测定低温等离子体中原子和分子基态密度的一种很好的方法。对于一些物种，如H、N、O、Kr或Xe，两个激光光子的同时吸收允许从基态激发，同时绕过真空紫外光中的激光波长。然而，与已发表的各种H、N和O原子的等离子体诊断研究相比，这种方法很少被用于研究稀有气体等离子体放电中的原子数密度，例如在Kr或Xe中。在离子源的操作方面，稀有气体通常用于进行与地面应用中的离子束溅射沉积或表面修饰有关的过程，此外，作为电子航天器推进装置的推进剂，特别是栅极离子和霍尔效应推进器的推进剂。在这里，羽流以及放电区的实验表征对于更好地理解重要的物理现象是必不可少的，例如在推进器出口附近出现电荷交换等离子体分量。此外，需要这些研究的结果来优化电推进装置。在推进器等离子体中，中性气体物种的产生主要是由于推进剂在放电室中的不完全电离，作为中和剂流动的一部分，以及作为地面测试设施中的背景原子。特别是，快离子和慢热中性气体原子之间的电荷交换碰撞可能导致离子束的电离原子回流，这可能导致离子碰撞对航天器或推进器部件充电或侵蚀等关键问题。因此，由于中性气体不能用标准的离子束诊断工具如Faraday cups或ExB、RPA和Langmuir Probe探测到，因此，针对中性原子数密度的定量分析的等离子体诊断研究是特别感兴趣的。在拟议的项目中，将使用双光子吸收激光诱导荧光技术进行测量，以确定离子推进器羽流中中性氙气的局部基态密度。根据测量的数密度分布，估计电荷交换过程中产生的中性粒子的比例。此外，在合适的推进器工作条件下，应在多普勒频移吸收波长处直接测量快中性粒子的分数。此外，将在放电中进行实验研究，将适当的光学通道安装到腔体上，以便检查对验证数值放电模型至关重要的中性气体密度分布。除了氙气的实验外，离子源也是如此。将对以氪为推进剂的推进器进行研究。

Two-photon absorption laser induced fluorescence spectroscopy represents an excellent method to accurately determine atomic and molecular ground state densities in low temperature plasma. For several species such as H, N, O, Kr, or Xe, the simultaneous absorption of two laser photons allows excitation from the ground state, while circumventing laser wavelengths in the VUV. However, compared to various published plasmadiagnostic studies on atomic H, N and O species, this method has been applied rather seldom for investigations of atom number densities in rare gas plasma discharges such as in Kr or Xe.With respect to the operation of ion sources, rare gases are commonly used to conduct processes related to ion beam sputter deposition or surface modifications within terrestrial applications, and, furthermore, as propellants of electric spacecraft propulsion devices, in particular of both gridded ion- and Hall-effect thrusters. Here, the experimental characterization of the plume, as well as of the discharge region, is indispensable to achieve a better understanding of important physical phenomena such as the emergence of a charge exchange plasma component near the thruster exit. In addition, results of these investigations are needed to optimize electric propulsion devices.In the thruster plasma, neutral gas species mainly occur due to incomplete ionization of the propellant in the discharge chamber, as part of the neutralizer flow and as background atoms in ground test facilities. In particular, charge exchange collisions between fast ions and slow thermal neutral gas atoms may cause a backflow of ionized atoms of the ion beam, which can result in critical issues such as electric charging or erosion of spacecraft or thruster components by ion impingement. Therefore, plasmadiagnostic investigations aiming at the quantitative analysis of number densities of neutral atoms are of particular interest, since neutral gas species can not be detected with standard ion beam diagnostic tools such as Faraday cups or ExB, RPA and Langmuir probes.Within the proposed project, measurements using the two-photon absorption laser induced fluorescence technique will be carried out to determine local ground state densities of neutral xenon in the plume of an ion thruster. From the measured number density distributions, neutral particle fractions emerged from charge exchange processes shall be estimated. In addition, the fraction of fast neutrals shall be measured directly at the Doppler-shifted absorption wavelength for appropriate thruster operating conditions. Moreover, experimental investigations in the discharge will be conducted mounting appropriate optical access to the chamber housing, allowing for the examination of neutral gas density distributions that are critical to the validation of numerical discharge models. Additional to the experiments in xenon, ion source resp. thruster operating with krypton as propellant will be investigated.