4:15 PM - 4:30 PM
[PCG31-17] The design of the suprathermal ion mass spectrometer (STIMS)
Keywords:ion mass spectrometer, suprathermal ion, non-magnetized planet, planetary ionosphere
The STIMS consists of (a.) an energy analyzer and (b.) a mass analyzer. A field of view of the STIMS is about 4 pi sr per a half spin of spin-stabilized spacecraft. A target energy range is from 0.1 to 300 eV, which corresponds to suprathermal energies, and a mass range is from 1 to 50 amu. An energy resolution, ∆E/E, is less than 5%, and a mass resolution, M/∆M, is over 10.
(a.) An energy analysis of the STIMS is carried out in a top-hat type electrostatic analyzer, which deflects incident ions by 90 degrees and leads them to an entrance of the mass analyzer. Only ions that fly along a center radius of spherical electrodes are able to get to the mass analyzer.
(b.) The mass analyzer of the STIMS is mainly made up of a pre-acceleration section and a magnet section. The magnet section, which has a cylindrically symmetric structure, is divided into sixteen regions by permanent sector magnets. Firstly, in the pre-acceleration section, ions which got through the energy analyzer are accelerated or decelerated by an acceleration voltage E eV, whose magnitudes depend on mass number of the ions M amu. Secondly, the accelerated/decelerated ions experience Lorentz force in the magnet section, and reach a micro-channel plate (MCP), with semicircular trajectories. By sweeping magnitudes of the acceleration voltage E, only ions that conserve square root of product of their masses and kinetic energies, i.e., √ME, are able to reach the MCP.
As for observations of molecular ions, magnet type mass spectrometers have a great advantage that it is possible to detect the molecular ions without dissociations. However, this type of spectrometer has several disadvantages; (1) it is difficult to discriminate between noise signals and ion signals at the MCP because this type of spectrometer does not take signal coincidences; (2) this type of mass spectrometer tends to be heavier than other types of spectrometers due to installing magnets; (3) magnetic field of the magnets might cause undesirable effect on other observation instruments on spacecraft.
In this presentation, we will introduce design concepts and specifications of the STIMS.