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

Abstract

Abstract. 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 microsatellite 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 nonflowing 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 deCorrespondence to: E. Séran (seran@cetp.ipsl.fr) duce the plasma bulk velocity under similar conditions and present the first onboard measurements.

The fixed-bias Langmuir probe on the Communication/Navigation Outage Forecast System satellite: Calibration and validation

“…This is due in part to the large thermal velocity of light ions, but it is mostly due to the effect of the negatively biased probe on the motions of the ions. Light ions that are not in the direct path of the probe are more easily accelerated toward the probe and collected on all surfaces, as demonstrated by the simulations of Sèran et al 21 Heavier ions such as O + are accelerated toward the sphere as well, but the trajectories are not changed enough for these ions to be collected on the anti-ram surface. This is illustrated in Figures 8 and 9 of Sèran et al Additionally, it is possible under the right combination of conditions that light ions may be collected in greater abundance on the anti-ram surface of a spherical probe than on the ram surface due to this acceleration effect (see Figure 11 of the same paper).…”

Accelerated Steady-State Electrostatic Particle-in-Cell Simulation of Langmuir Probes

“…For example, collisions can significantly affect ion current to Langmuir probes (e.g., Schulz & Brown 1955;Sternovsky & Robertson 2002;Sternovsky et al 2003;Sudit & Woods 1994;Zakrzewski & Kopiczynski 1974), and a number of studies have used PIC codes with Monte Carlo collision capabilities to study this, using 1D cylindrical or spherical models (with 3D velocities) and a Boltzmann electron distribution (Taccogna et al 2004;Tejero-del-Caz et al 2016;Voloshin et al 2015) or fully kinetic electrons (Cenian et al 2005;Iza & Lee 2006;Soberón 2006;Trunec et al 2015;Zikán et al 2019), including surface effects such as secondary electron emission (Cenian et al 2014). PIC has been critical in simulating Langmuir probes with asymmetric 3D geometries (Chiaretta 2011;Hilgers et al 2008;Hruby & Hrach 2010;Imtiaz et al 2013;Podolník et al 2018;Séran et al 2005), and has been used for probe and sheath analysis in the presence of magnetic fields (Bergmann 2002;Podolník et al 2018). The effect of nonuniform plasma and flowing plasma conditions are other examples ripe for exploration with PIC (Imtiaz et al 2013;Knappmiller & Robertson 2007;Olson et al 2010;Séran et al 2005).…”

Langmuir probe

“…The basic concept of this probe is to mea-sure 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. To ascertain the capabilities of this instrument, Séran et al (2005) developed a numerical particles in cell (PIC) model to compute the distribution of the current collected by a spherical probe. According to their model calculation, it is clear that the ion velocity measurement accuracy is not as good as that provided by the ion analyzer technique, at least at the present stage of the instrument development.…”