Power-Efficient Electron Emitters for Electron Ionization in Spaceborne Mass Spectrometers

Robust cathodes are used as electron emitters to ionize neutral atoms and molecules by electron ionization in spaceborne mass spectrometers. These instruments require an electron emission current of about 50 to 1000 µA from the emitters. The emitters must be power-efficient, robust with respect to handling, and withstand launch into space. In addition, the lifetime driven by the mission design influences the selection of the cathode. The required lifetime is a result of the sum of the nominal scientific operation time in space, the time operated during both pre-launch and post-launch commissioning, potential mission extension if applicable, and margin. For example, a simple lunar landing mission such as the Neutral Gas Mass Spectrometer (NGMS) on board the Luna 27 mission requires a nominal total lifetime of the cathodes of about 4,000 hours. In contrast, the team specified 10,000 hours as a requirement for the Neutral and Ion Mass spectrometer (NIM) on board ESA’s large-class Jupiter’s Icy Moon Explorer (JUICE) mission. Here, we present a study to identify the flight cathodes for NGMS/Luna-Resurs, NIM/JUICE, and their successor instruments MANIaC and its Neutral Density Gauge (NDG) on board Comet Interceptor. The analysis includes a detailed survey of concepts presented as review with subsequent laboratory investigation of promising candidates. The realized concepts included prototypes with, on one hand, innovative concepts relying on field effect and secondary electron emission namely two types of Carbon Nano-tubes (CNTs) and an UV-LED triggered microchannel plates (MCPs), overcoming the microtips Spindt type cathodes. On the other hand, ten types of thermionic emitters are analyzed with materials including BaO, LaB6, WRe, WTh, and Y
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O
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. The tests conducted in this framework included laboratory testing with respect to the mentioned requirements especially focusing on both normal and accelerated life testing and advanced surface analysis (SEM with BSE, SE, and EDX). We established a simple metric to compare the results of these tests. In addition to the empirically derived parameters for such a metric, the current market situation, i.e., the availability of the cathodes, strongly influenced the selection of flight cathodes. For example, due to a shift in technology of consumer electronics for (cathode-ray tube) televisions, several best-of-class oxide cathodes suddenly became unavailable. In addition, potentially suitable cathodes used for calibration of the ROSINA instrument on board the ROSETTA mission became unavailable as well. Among the remaining candidates, we selected an yttrium-oxide coated thermionic disc cathode for NGMS consuming about 1.6 W satisfying its lifetime requirement. For NIM, the same cathode was further enhanced to achieve a nominal power consumption of about 1.1 W, whilst increasing their lifetime to comply with the mission requirements. Recently, we successfully commissioned one of the cathodes on board the flight instrument of NIM during the Near-Earth Commissioning Phase (NECP) shortly after launch, attesting the quality of the analysis presented in this paper. This success enables NIM to analyze the neutral exospheres of Jupiter’s icy moons Europa, Ganymede, and Callisto and potentially providing insights on their subsurface oceans.