Simulation-based interpretation of spacecraft particle sensor measurements

粒子传感器，如朗缪尔探测器或粒子成像仪，在许多卫星和实验室等离子体实验中用于推断等离子体参数，如密度和温度。在其最简单的形式，朗缪尔探头包括一个电极，可以偏置到可变电压，从它可以测量电流。收集的电流作为偏置电压的函数，即所谓的探针特性，然后可以在理论或计算模型的基础上进行解释，以产生局部等离子体参数。近一个世纪以来，人们在这个问题上做了很多工作。许多理论模型，以及最近的一些计算模型已经被开发出来，用来描述朗缪尔探测器在等离子体中的响应。然而，在实践中，探针测量几乎普遍被解释为能够在操作模式下产生实时答案的简化分析模型。不幸的是，这些解释是出了名的不确定，误差条可能是100%。这些巨大的不确定性的原因来自理想化模型的使用，其中只考虑了相关物理过程的一小部分。例如，一些模型只考虑静止的非磁化等离子体中的探针，另一些模型考虑了等离子体流动而忽略了环境磁场，还有一些模型考虑了磁场，但忽略了等离子体流动或其他影响，如光电子或二次电子发射。基本上，在所有情况下，等离子体都被假定为空间均匀的，而附近卫星或实验物体的存在及其几何形状被忽略。解决这一困境的一个很有希望的方法是在详细的计算机模拟的基础上解释粒子传感器的测量结果，这些模拟能够解释等离子体-物质相互作用的多物理场特征，同时也说明了测量的几何形状。不幸的是，由于进行此类模拟所需的时间和计算资源，这在实时操作模式下是不可能的。在这项拟议的研究中，一个需要探索和发展的实际解决方案是为每个要支持的实验或太空任务建立一个库或数据库，从中可以使用适当的回归技术推断等离子体参数。与目前的实践相比，新方法将显著提高推断等离子体参数的准确性。拟议的研究将集中于为选定的航天器和实验建立解决方案库，开发和评估不同的回归技术。这项研究的预期结果是传感器测量解释范式的改变，这将依赖于详细的动力学模拟结果，而不是理想化的分析模型。这一建议是根据多年来在卫星环境模拟方面的经验提出的。

Particle sensors such as Langmuir probes or particle imagers, are used in many satellites and laboratory plasma experiments to infer plasma parameters such as the density and temperature. In its simplest form a Langmuir probe consists of an electrode which can be biased to variable voltages, and from which the electric current can be measured. The collected current as a function of bias voltage, the so-called probe characteristic, can then be interpreted on the basis of theoretical or computational models, to yield local plasma parameters. Much work has been done on this topic over nearly a century. Many theoretical, and more recently, several computational models have been developed to describe the response of Langmuir probes in a plasma. In practice however, probe measurements are almost universally interpreted in terms of simplified analytic models capable of producing real time answers in operation mode. Unfortunately these interpretations are notoriously uncertain, with error bars that can be of order 100%. The reason for these large uncertainties comes from the use of idealised models in which only a fraction of the relevant physical processes are taken into account. For example, some models only consider a probe in a stationary unmagnetised plasma, others account for plasma flow but neglect ambient magnetic fields, yet others account for a magnetic field, but ignore plasma flow or other effects such as photoelectron or secondary electron emission. In essentially all cases, plasma is assumed to be spatially uniform, and the presence of nearby satellite or experimental objects and their geometry is ignored. A promising solution to this predicament is to interpret particle sensor measurements on the basis of detailed computer simulations capable of accounting for the multiphysics which characterises plasma-material interaction, while also accounting for the geometry in which measurements are made. This is unfortunately not possible in real time operation mode owing to the time and computational resources required to do such simulations. One practical solution to be explored and developed in this proposed research, is to construct a library or data base for each experiment or space mission to be supported, from which plasma parameters could be inferred using a suitable regression technique. Compared to current practice, the new approach would lead to significant improvements in the accuracy of inferred plasma parameters. The proposed research would concentrate on producing solution libraries for selected spacecraft and experiments, developing, and assessing different regression techniques. An expected outcome of this research is a change of paradigm in the interpretation of sensor measurements, which would then rely on detailed kinetic simulation results rather than on idealised analytic models. This proposal is based on many years of experience in satellite-environment modelling.