LIEFSI – Laboratory Investigation of Electric Field Sensor Instabilities

The quantitative study of space plasma electrodynamics requires the accurate measurement of in situ electric fields. In order to optimize the accuracy of the double probe method for both DC and AC field measurements a variety of sensor and antenna geometries and biasing schemes have been used on past and present missions. These mechanical designs and operational techniques serve to reduce offsets and attenuation due to the nearby spacecraft structure or the E field antenna structure itself, but have unexpectedly been the source of either anomalous response ' e.g. unexpected voltage gain ' or outright oscillations, rendering the measurements ambiguous at best and unusable at worst. Examples of this in the recent past include the behavior of the Polar-EFI, Demeter-ICE, CNOFS-VEFI, and possibly Van Allen Probes EFW-AXBs, hindering the analysis of the fluctuations responsible for the bulk of geo-effective wave-particle interactions, and degrading the measurement of quasi-DC E-fields with intense background noise and non-linear offsets to sensor potentials. While theoretical analyses exist for the relevant antenna-plasma interactions, in situ observations provide only circumstantial evidence for their validity. Recent successful measurements of the relevant sensor-plasma interactions in the laboratory setting point the way to a conclusive test of that theoretical analysis, a way forward to avoid the pitfalls found in earlier designs, and a process to prove out new, more ambitious designs for AC E-field antennas before launch. LIEFSI ' the Laboratory Investigation of E Field Sensor Instabilities ' addresses this golden opportunity to significantly improve our practical knowledge of electric field sensor design, and to support the investigation of fundamental plasma processes that contribute to all aspects of space physics. Such an effort, when successful, shall directly support progress on all three of the NASA Heliospheric Research Plan (HRP) and Decadal Survey (DS) goals of Exploration, Advancement, and Development by enabling the design, fabrication, and operation of more robust and capable electric sensors throughout heliospace. The results of the LIEFSI effort do not support a particular instrument or design effort, but shall significantly increase the chance of success of all subsequent efforts by the community. Its results shall provide a clearer understanding of the mechanisms by which these sensor instabilities occur, their parametric dependence on plasma conditions and sensor dimensions and geometry, allowing for more certain design of E-field sensors on future missions and enhanced science return from such measurements. These two goals lead naturally to the following two science questions: 1. What geometric and dimensional parameters control the existence and properties of electric field sensor instabilities? 2. What plasma parameters control the existence and properties of electric field sensor instabilities? 3. How do these instabilities affect the quality of the electric field estimates? With these questions in mind, we propose the following experimental plan, described in detail below: 1. Determine, using existing theories of E-field sensor instabilities and plasma impedance probe design and NRL Space Plasma ' Space Chamber (SPSC) laboratory measurements the relationship between the vacuum and in situ admittance matrices for the scaled models of the Polar-EFI and RBSP-EFW sensor assemblies for a set of plasma parameters relevant to ionospheric, plasmaspheric, magnetospheric, and/or heliospheric conditions. 2. Demonstrate using scaled versions of the relevant sensor bias and feedback systems that the measured admittances do in fact lead to instabilities quantitatively or qualitatively similar to those observed on-orbit in terms of frequency and parametric dependence.

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