Investigation of Coatings for Langmuir Probes: Effect of Surface Oxidation on Photoemission Characteristics

Abstract

Surface oxidation of Langmuir probes is an important issue for probe measurements in space environments. Followed by our previous work about the oxidation effect on the collection of ambient plasma electrons and ions, here we present its effect on photoemission from the probe surface of various materials. Photoemission is either a contamination for traditional Langmuir probes or a necessity for electric field probes in low-density plasma. Our results show that all materials after oxidation have a varying degree of reduction in photoemission. The photoemission of copper, gold, and niobium drops most significantly followed by DAG213 (a resin-based graphite coating), TiN (titanium nitride), and rhenium. Iridium, DAG213, and AquaDAG (graphite coating) have the largest photoemission after oxidation, making them appropriate coating candidates for electric field probes. Both DAG materials show a large photoemission enhancement after the oxidation products are cleaned off the surface. A long exposure test shows that the photoemission from iridium slowly degrades. Due to the high surface conductivity of oxidized iridium shown in previous work, it is suggested that iridium can be oxidized before flight to minimize the photoemission when being used as a coating for Langmuir probes. Overall, iridium is found to be a coating material appropriate for both electric field probes and Langmuir probes.

In‐Situ Measurements of Electron Temperature and Density in Mars' Dayside Ionosphere

“…The publication of first-result T e and n e in Mars' ionosphere from the MAVEN's LPW instrument had sizable uncertainties and possible overestimates of both T e and n e (Ergun et al, 2015). To better constrain models and understand the photo-chemical process in Mars' ionosphere, a detailed recalibration effort was implemented using self-consistent 3D simulations, laboratory tests, and detailed examination of several years of data (Ergun et al, 2021;Samaniego et al, 2018Samaniego et al, , 2019. This effort has greatly improved the accuracy of T e and n e in Mars' lower ionosphere.…”

Calculation of the Atomic Oxygen Fluence on the Van Allen Probes

“…It is known that atomic oxygen exposure changes DAG‐213 coated surfaces (Samaniego et al., 2019), so it is illuminating to calculate this impact in the context of this model. We can approximate the amount of degradation to a small patch of both the top and bottom surfaces of the DAG‐213 coated spheres using the top and bottom atomic oxygen fluence values from Figure 17.…”

“…To develop a better understanding of these effects, the Space Environment and Experiments Branch of NASA has preformed many studies investigating in situ spacecraft atomic oxygen erosion, as well the observable effects of atomic oxygen in ground‐based experiments. While the focus of many of these studies has been on the erosion depths of various materials used on spacecrafts (Banks et al., 2004; Bourassa & Gillis, 1992), observed issues with spacecraft instruments after perigee lowering in LEO (Pedersen, 1995), or changes to the photoemission properties of certain materials when exposed to laboratory created oxygen‐rich environments (Samaniego et al., 2018, 2019), there are fewer published works on the process of modeling atomic oxygen exposure before or after the mission occurs. This paper will outline this process in depth to provide a platform for the atomic oxygen exposure analysis of past, current, and future missions.…”

“…The six Langmuir probes on the instrument consist of Aluminum spheres coated in DAG‐213, an epoxy resin and graphite coating used to ensure there are minimal work function variations over the surface of the spheres (J. R. Wygant et al., 2013). Exposing DAG‐213 to high‐density oxygen plasma has been shown to directly inhibit the material's photoemission capabilities; the oxidized surfaces produce photoelectrons at a reduced rate (Samaniego et al., 2019). Modeling the oxygen exposure to the spacecraft will help determine whether the instrument's measurement quality could have been affected by oxidation or erosion of the sensors over the course of the mission.…”

Kinetic Modeling of Langmuir Probes in Space and Application to the MAVEN Langmuir Probe and Waves Instrument

“…However, high-ram velocity AO impacts appear to have caused the LPW sensors to deviate from ideal response after a few months in orbit at Mars. Recent studies using O + beams have verified and characterized the non-ideal response of several types surfaces coatings (Samaniego et al, 2018(Samaniego et al, , 2019 The ideal current of a spherical sensor (black) and a cylindrical sensor (blue) with the same area. If eV >> T e , the asymptotic current of a sphere is linear whereas the current of a cylinder is proportional V 1/2 .…”