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The spherical segmented Langmuir probe in a flowing thermal plasma: numerical model of the current collection.

机译:流动热等离子体中的球形分段Langmuir探针:电流收集的数值模型。

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摘要

The segmented Langmuir probe (SLP) has been recently proposed by one of the authors (Lebreton, 2002) as an instrument to derive the bulk velocity of terrestrial or planetary plasmas, in addition to the electron density and temperature that are routinely measured by Langmuir probes. It is part of the scientific payload on the DEMETER micro-satellite developed by CNES. The basic concept of this probe is to measure the current distribution over the surface using independent collectors under the form of small spherical caps and to use the angular anisotropy of these currents to obtain the plasma bulk velocity in the probe reference frame. In order to determine the SLP capabilities, we have developed a numerical PIC (Particles In Cell) model which provides a tool to compute the distribution of the current collected by a spherical probe. Our model is based on the simultaneous determination of the charge densities in the probe sheath and on the probe surface, from which the potential distribution in the sheath region can be obtained. This method is well adapted to the SLP problem and has some advantages since it provides a natural control of the charge neutrality inside the simulation box, allows independent mesh sizes in the sheath and on the probe surface, and can be applied to complex surfaces. We present in this paper initial results obtained for plasma conditions corresponding to a Debye length equal to the probe radius. These plasma conditions are observed along the Demeter orbit. The model results are found to be in very good agreement with those published by Laframboise (1966) for a spherical probe in a thermal non-flowing plasma. This demonstrates the adequacy of the computation method and of the adjustable numerical parameters (size of the numerical box and mesh, time step, number of macro-particles, etc.) for the considered plasma-probe configuration. We also present the results obtained in the case of plasma flowing with mesothermal conditions reproducing the case of measurements onboard a low altitude spacecraft. Finally, we briefly discuss the capabilities of the SLP to deduce the plasma bulk velocity under similar conditions and present the first onboard measurements.
机译:分段作者Langmuir探针(SLP)最近由一位作者提出(Lebreton,2002),作为一种工具,除了可以通过Langmuir探针常规测量的电子密度和温度以外,还可以导出地球或行星等离子体的整体速度。 。它是CNES开发的DEMETER微卫星上科学载荷的一部分。该探头的基本概念是使用小球形帽形式的独立收集器测量表面上的电流分布,并使用这些电流的角度各向异性来获得探头参考系中的等离子体体积速度。为了确定SLP功能,我们开发了一个数字PIC(单元内颗粒)模型,该模型提供了一种工具来计算球形探针收集的电流分布。我们的模型基于同时确定探针护套和探针表面的电荷密度,由此可以获取护套区域中的电势分布。该方法非常适合SLP问题,并具有一些优点,因为它可以自然地控制模拟盒内部的电荷中性,允许护套和探针表面的网格大小独立,并且可以应用于复杂的表面。我们在本文中介绍了在等离子条件下获得的初始结果,这些条件对应于Debye长度等于探针半径。这些等离子体条件是沿Demeter轨道观察到的。发现模型结果与Laframboise(1966)发表的关于在非流动热等离子体中的球形探针的结果非常吻合。这证明了所考虑的等离子探针配置的计算方法和可调数值参数(数值盒和网格的大小,时间步长,宏观粒子的数量等)是否足够。我们还介绍了在等温条件下等离子流的情况下获得的结果,该结果重现了低空航天器上的测量情况。最后,我们简要讨论了SLP在相似条件下推导血浆体积速度的功能,并提出了首次机载测量结果。

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