Post Launch Performance of a Hot Cathode for Electron Ionization of a Space Borne Time-of-Flight Mass Spectrometer (NIM on board JUICE)

On 14 April 2023, the JUICE spacecraft was launched to the Jovian system to study the emergence of potentially habitable worlds around gas giants. The Neutral-Ion Mass Spectrometer (NIM), developed by the University of Bern, will characterise the atmospheres of the Galilean moons and analyse subsurface material ejected by Europa’s plumes. NIM uses a power-efficient hot cathode filament which creates an electron beam to ionize atoms and molecules for mass spectrometric analysis.

For this mission, we employ customized yttrium oxide (Y₂O₃) cathodes produced by Kimball Physics, based on the ES-525 design. For example the filament legs are lengthened to minimize heat loss through conduction. Additionally, a thicker coating is applied to enhance longevity. Given the criticality of correct cathode operation, two cold-redundant cathodes are installed in the NIM instrument.

This study compares the performance of the space-qualified cathodes in the Proto Flight Model (PFM) instrument, post-launch (in orbit) commissioning, with both expected performance metrics and laboratory-tested cathodes in the Flight Spare (FS) instrument.
During commissioning, the PFM cathodes underwent conditioning lasting several hours. While both cathodes were successfully conditioned, cathode 2 exhibited performance comparable to FS cathodes, whereas cathode 1 deviated from the pre-flight performance. This deviation was further investigated through additional investigations and tests. Preliminary findings suggest that launch-induced vibrations caused slight bending of the cathode legs, resulting in asymmetry between the emitting disk and the surrounding repeller electrode.

The cathodes operate within a nominal emission range of 100 to 300 μA. Without active beam shaping, power consumption varies between 1.2 and 1.6 W (up to 1.8 W for the deviating PFM cathode 1) with a current draw of 860 to 980 mA (up to 1030 mA). Optimal beam shaping increases the current requirement by approximately 20 mA. Despite limited available data, we successfully fit our measurements to the Richardson-Dushman equation, describing the relation between operation parameters and emission current, enabling a comparison with theoretical emission expectations.

The heating current drawn by the cathode is expected to increase over the long term (up to a lifetime of 10,000 operational hours) due to degradation of the coating. In contrast, short-term behaviour (up to 100 hours) reveals a "learning" effect: cathodes exhibit improved performance when used under specific active beam-shaping configurations, an effect disrupted after exposure to air.

During the post-launch commissioning of the cathodes in orbit, the LV subsystem was commissioned as well. The commissioning, including the cathodes, bake-out heater, low-voltage electrodes, as well as the necessary electronics was successful. Long-term monitoring of the cathodes' performance in both laboratory and space environments continues.