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
Ion escape processes are critical issues to solve atmospheric evolution of non-magnetized planets, e.g., Venus and Mars. Many studies about the ion escape have been conducted by both observational and theoretical methods. There is, however, a problem that qualities of in-situ observations have not been sufficient to identify the detailed suprathermal plasma dynamics, especially about molecular ions, around the non-magnetized planetary ionospheres. A suprathermal ion mass spectrometer (STIMS) has been designed for future in-situ observations of three dimensional velocity distributions for suprathermal ions around the planetary atmospheres.
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.
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.