Development of an Ultra-small Mass Spectrometer for Future Lunar and Planetary Exploration

Mass spectrometers have been widely employed as payloads for planetary exploration missions. The instruments measure the atomic and molecular compositions in planetary samples and are widely applicable to quantitative/qualitative analysis. Unlike remote sensing, mass spectrometers need to be in close to the target bodies for in-situ analysis, thus developing small mass spectrometers is an important issue in expanding the scope of analytical targets. There are additional possibilities for small mass spectrometers, namely piggyback or secondary payloads for missions. The small mass spectrometers may accompany sample return missions and provide complementary observations (e.g., multi-point; time continuity).In this study, we are developing an ultra-small neutral mass spectrometer of which ion optics fits into a volume of 1U (10×10×10 cm³). The mass spectrometer being developed is based on Electron Impact (EI) ionization method and time-of-flight (TOF) mass analyzing technique. The EI ion source is newly developed to achieve low power consumption (~0.5 W), high electron emission (>1.0 mA), and high ionization efficiency (~10⁴ nA/Pa for N2 gas) with small size (~7×3×3 cm³). For the mass analysis part, ion trap electrodes are employed to increase TOF while reducing volume. The ion trap electrodes allow ions to pass around the same optics at specific frequency for each m/z, resulting in a small optics size and long TOF. So far, we developed a test model with an overall optics volume of ~7×7×8 cm³ and a weight of ~0.3 kg. As for performance with a single-lap, the mass resolution was m/Δm ~ 50, the sensitivity was ~1×10^-6 (counts/s)/(particle/cc), and the detection limit (i.e., S/N=1) was ~1×10⁴ particle/cc, respectively. In addition, we have completed a design with an improved mass resolution to obtain mass spectra for more than 2 laps, using a numerical simulation. According to the results of the simulation, the improved mass resolution (m/Δm > 1,000) will be achieved with > 100 laps.It should be noted that the new model does not require high voltage pulses; the instrument is expected to be small enough even if the flight-qualified electrical system is included. The timing of ion trap can be arbitrarily determined, making it possible to provide multiple observation modes that take into account the fundamental trade-off between resolution and sensitivity in TOF instruments. Additionally, the new ion source with low power consumption and high emission current is one of the achievements of this work and can be applied to other mass spectrometers as a stand-alone system; the ion source is planned to be used for JAXA/ISRO LUPEX mission.