Geometrical effects of microchannel plates: Grazing incidence operation of time-of-flight mass spectrometry and comparison to standard carbon foil.

Abstract

Direct detection of plasma composition in situ is integral to our understanding of ionospheric, magnetospheric, and heliospheric dynamics. Heavy, singly charged ions have been shown to contribute significantly to the mass and energy redistribution within the magnetosphere. However, most studies neglect the individual contributions of CNO group ions (instead attributing all effects to oxygen) due to the lack of mass separation of the spectrometer. In this paper, we investigate how the mass resolution of a traditional Time-of-Flight (ToF) mass spectrometer responds to different geometric applications of "grazing incidence" scattering. We apply this technique by utilizing standard Microchannel Plates (MCPs) to generate the "Start" signal, instead of the traditional carbon foil, reducing overall scattering via physically limiting plasma collisions to shallow angles of incidence. We considered only geometric differences, comparing LD 20 and LD 40 (length to diameter ratio) MCP effects on mass resolution observed by the ion composition and distribution function analyzer; traditional carbon foil spectra are contrasted against that produced by replacing one foil with these MCPs and subsequently illuminated with H+, H2 +, He+, N+, H2O+, N2 +, Ar+ ions over energies from ∼15 keV to 29 keV, similar to those seen in magnetospheric plasma. We found that among the MCP geometries, the increasing LD ratio corresponds to spectra that observe smaller FWHM and faster mean ToF, indicative of a significant drop in scattering. Additionally, the mass resolution is significantly better than when using the traditional carbon foil for masses above 4 amu. However, the geometry limits detection efficiencies due to the plasma's reduced cross section of collision with the surface that generates the "Start" signal.