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Planetary Instrument Concepts for the Advancement of Solar System Observations

Vital Components of a Planetary Pickup Ion Composition Spectrometer

Completed Technology Project

Project Introduction

Newly ionized atoms from planetary sources that are picked up by the solar wind and carried into the heliosphere contain information on the plasma and dust compositions of their origin. These pickup ions (PUIs) are collected by plasma mass spectrometers and analyzed for their density, composition, and distribution in both energy and velocity. In addition to measurements of planetary PUIs, in situ measurements of interstellar gas have been made possible by spectrometers capable of differentiating between heavy ions of solar and interstellar origin. While fascinating research has been done on these often singly charged ions, the instruments that have detected many of them were designed for the energy range and charge states of the solar wind and energized particle populations. To fully characterize these ion species, measurements are made using a triple-coincidence technique (a 'start' and a 'stop' detection to enable a successful time-of-flight measurement, and a total energy measurement). Pickup ions are typically only measured in double coincidence (a successful time-of-flight measurement), as their energy is usually below the energy threshold of solid-state energy detectors. This substantially degrades the signal to noise of PUI measurements and affects the mass resolution. For example, besides 3He/4He, isotopic abundance ratios typically cannot be measured in situ, and the mass resolution is insufficient to unambiguously separate the isotopes of elements at mass-per-charge > 20 amu/e. An instrument optimized for the complete energy and time-of-flight characterization of pickup ions will unlock a wealth of data on these hitherto unobserved or unresolved PUI species. The goal of this proposed work is to enable the next generation of pickup ion and isotopic mass composition spectrometers. Three objectives will be accomplished in this work: 1) Develop a large-gap Time-of-Flight–Energy sensor with more collecting power than heritage instruments; 2) Develop a 100-kV power supply for ion acceleration, not only to accelerate PUIs to energies above the energy threshold of solid-state detectors, but also to produce accurate enough time-of-flight measurements that isotopic composition can be determined; and 3) Develop improved electronics for integrated timing and position measurements with minimal electronics noise. Together, these technologies will lead to a new generation of space composition instruments, optimized for measurements of planetary pickup ions, yet applicable to pickup ions in the solar wind as well. These objectives will be accomplished by building on the previous work of the proposing team and by improving on heritage hardware designs. The proposed work is significant in that it will lead to the development of the vital components of a pickup ion composition instrument capable of the isotopic resolution of heavy ions in the solar wind, as well as energy measurements of planetary and cometary PUIs. The measurement of heavy ion isotopes is fundamental to understanding the evolution of matter in the solar system and the universe, as many of the isotopic ratios have steadily changed since the nucleosynthesis of the Big Bang. The technologies developed here will enable these measurements due to their broad applicability to different types of sensors and instruments. For time-of-flight spectrometers, these components will enable low-energy PUIs and isotopes to accelerate within the instrument to energies several times higher than current flight instrument capabilities, dramatically increasing the mass resolution. They also will enable isotopic measurements such as D/H using measurement techniques described in the literature.

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