Europa PIMS prototype faraday cup development

Abstract

The Europa Clipper mission (Concept: [1]; Updates: [2]) is equipped with a sophisticated suite of 9 instruments to study Europa's interior and ocean, geology, chemistry, and habitability from a Jupiter-orbiting spacecraft. This mission to Europa is tasked with characterizing Europa's subsurface ocean and assessing Europa's habitability. The magnetic sounding investigation exploits currents induced in Europa's interior by the moons exposure to variable magnetic fields in the Jovian system to infer properties of Europa's subsurface ocean such as its depth, thickness, and conductivity. This technique was applied to Galileo observations and demonstrated that Europa indeed has a subsurface ocean. Complicating this investigation is the fact that Europa is embedded in a complex Jovian magnetospheric plasma, which rotates with the tilted planetary field and interacts dynamically with Europa's ionosphere affecting the magnetic induction signal. Plasma from Io's temporally varying torus diffuses outward and mixes with the charged particles in Europas own torus producing highly variable plasma conditions at Europa. Thus, characterization of the ocean by magnetic sounding requires an accurate specification of how the plasma properties, specifically the mass density and flow velocity of the plasma change as it flows across Europa. The PIMS (Plasma Instrument for Magnetic Sounding) measures the plasma surrounding Europa to characterize the magnetic fields generated by plasma currents. PIMS uses Faraday Cups to measure the plasma mass density and flow velocity. The PIMS instrument builds upon the Voyager PLS Faraday Cup instrument [3]. PIMS works in conjunction with the Interior Characterization of Europa using Magnetometry (ICEMAG) investigation to magnetically sound Europa's subsurface ocean. The measured plasma properties along with detailed physics-based modeling of the moon-magnetosphere interaction establish the contributions to the total magnetic field of plasma currents. Taking advantage of the differing spatial forms of fields from plasma and induction currents, an inversion scheme is able to markedly increase the accuracy with which the inductive field signature can be characterized from PIMS measurements, which in turn determines the ice shell thickness, ocean depth, and conductivity. The PIMS team has recently completed the design, fabrication, test, and characterization of the breadboard Faraday Cup sensor. This sensor was designed to meet the performance requirements specific to the Europa mission including signal level, energy range, angular resolution, sample rates, and field of view. Additionally, the PIMS sensor was specifically designed to address the challenges inherent to the Jovian environment, including spacecraft charging, deep dielectric discharge, and total ionizing dose. Finally, the PIMS Faraday Cup was designed to meet spacecraft accommodation requirements such as thermal, structural, microphonics, magnetics, electromagnetic interference, and grounding. The purpose of this paper is to describe the PIMS instrument design process and results. The paper will discuss analyses of the Europa environment and the resulting effect on the Faraday Cup design. The required materials selection, electrical, mechanical and thermal interfaces, high voltage considerations, and connector selections will all be discussed in detail. Finally, we present laboratory characterization measurements from the prototype unit demonstrating performance and the impact of these selections.