Development of an electron impact ion source with high ionization efficiency for future planetary missions

Ion sources using electron impact ionization (EI) methods have been widely accepted in mass spectrometry for planetary exploration missions because of their simplicity. Previous space-borne mass spectrometers were primarily designed with the EI method using rhenium tungsten alloy filaments, enabling up to 100–200 ​μA emission in typical cases. The emission level is desired to be enhanced because the sensitivity of mass spectrometers is a critical requirement for the future in situ mass spectrometry related to the measurement of trace components in planetary samples. In this study, we developed a new high-emission EI ion source using a Y2O3-coated iridium filament, which has a lower work function than rhenium tungsten alloy. The size of the ion source was 30 ​mm ​× ​26 ​mm ​× ​70 ​mm, and its weight was ∼70 ​g. We confirmed that when consuming ∼3.0 ​W power, the ion source records 1–2 ​mA electrons, which is 10 times greater than the conventional models’ electron emission level. We verified the linearity of ionization efficiency and the electron current in the range of 0.1–1 ​mA, which indicates our new model increased the ionization efficiency. We conducted performance tests on the prototype with the 3.0 ​W heating condition, confirming a high ionization efficiency (∼104 ​nA/Pa). In addition, we conducted endurance tests of the ion source and demonstrated the persistence of the ionization efficiency for 30 ​min ​× ​100 cycles.