Development of an Ion Energy Mass Spectrometer (PACE-IMA) on Board the SELENE Satellite

The SELENE mission is planned for the investigation of the nature and origin of the Moon by a variety of measurements. The satellite will be launched in 2005, and will be put into the 100km altitude lunar orbit which covers the whole spherical surface of the Moon. One of the science instruments PACE-IMA (Plasma energy Angle and Composition Experiment - Ion Mass Analyzer) on board the SELENE satellite will measure the 3-D distribution function of the mass-discriminated ions around the Moon. The observation objectives of the IMA are the investigation of the desorption and transport mechanism of the lunar atmosphere, the composition of the lunar legolith, the examination of the interaction between the solar wind and the lunar surface, and so on. The IMA consists of an Energy Analyzer (EA) and a Mass Analyzer (MA). The IMA-EA has angular scanning deflectors at the entrance, spherical deflectors, and sensitivity control electrodes inside. The IMA-MA employs the Linear Electric Field Time-Of-Flight (LEF TOF) method in order to achieve the considerably high mass resolution because there are various and heavy ions around the Moon. Since this specific electric field (LEF) increases linearly with respect to the distance along the center axis, the equation of motion for charged particles in LEF is an equation of a simple harmonic oscillator. The bounce time of the particles from entering to exiting the LEF region is one half of the oscillation period which is independent of the dispersion of energy and angle caused by the passage of the incident ions through a carbon foil. Therefore, high mass resolution can be expected. We fabricated a proto-model of the IMA and experimentally calibrated its characteristics. The energy resolution, geometric factor, and angular coverage of the IMA-EA and the TOF profiles of the IMA-MA for various ion species will be compared with the calculated results. In addition, we will report the results of the numerical calculation of the Moon-originated ion flux which is expected to be observed by IMA.