The Neutral Ion Mass Spectrometer (NIM)
The NIM instrument is a highly sensitive mass spectrometer for neutral atmospheric gas and ionospheric ions specifically developed for the tenuous gases of planetary exospheres. It is a time-of-flight mass spectrometer using an ion mirror for performance optimisation. NIM consists of two major subunits, namely, the ion-optical system and the electronics.
It will be used to measure the chemical composition of the regular atmosphere produced by sublimation, energetic particle bombardment, and photon interaction with the surface of the icy Jovian moons. Its measurements include volatile species, contributions from non-ice material on the surface, and the measurement of the isotope composition of major species. In addition, NIM will also measure the ion composition of the ionospheres by direct ion measurement.
NIM has an open and a closed source. The latter extends the field-of-view (FoV) and ensures the capability of NIM to measure during the fast flybys, during the approaching and departing leg of the flybys, and during Ganymede elliptic orbits.
The NIM mass spectrometer consists of three main parts
- The ion-source
- The mass analyser (TOF) and
- The detector
Overview function of NIM mass spectrometer
- A sample inlet, dependent on the used ionisation technique (neutral mode or thermal mode) and the sample itself, feeds the ion-source, with neutral atoms and molecules. Ion-optical elements, such as electrostatic electrodes are used in the NIM instrument, to collimate and guide the ion beam through the ion-source towards drift region. The main task of the ion-source is then to ionise the neutrals and provide a collimated ion bunch for subsequent mass analysis.
- Subsequently, the mass analyser is responsible for the separation of the ions according their individual mass-per-charge ratio. In time-of-flight (TOF) mass spectrometry, separation between ions of different masses is achieved along the field-free drift path. This implies that ions with different masses have different velocities according to their mass. In fact, ions with a lower mass are faster than more massive ones, thus they have a shorter traveling time along the drift path and arrive earlier at the detector.
- Finally, the detector records the signal, which is proportional to the amount of ions hitting the detector, as function of time that is converted into a mass spectrum.
All these parts have to be operated in vacuum to provide a collision-free path for the ions, all the way from the ion-source to the detector.
The JUICE Mission requirements to NIM
The mechanical design of NIM is based on the design of the RTOF instrument of the ROSINA experiment and the NGMS instrument, to be flown on Luna-Resurs Mission of Roskosmos. However, NIM is designed and developed specifically for JUICE’s mission requirements. This includes measurements at high velocity during the flybys and orbits of Jupiter’s moons. A wide Field of View (FoV) is also required for NIM, since the spacecraft will by oriented differently during the diverse JUICE mission phases. In addition, NIM is able to conduct direct ion measurements of thermal plasma.
Besides the scientific requirements, also payload requirements are of importance. With the overall length of 360 mm, the NIM instrument is much smaller and lighter than its predecessors.
In addition, the environmental conditions during the mission have to be considered. The most critical issue in the Jupiter system is the harsh radiation environment. Jupiter has a very strong magnetic field, which leads to the most intense radiation belt in the solar system.