Development of a Low-energy Ion Mass Spectrometer for Inner Magnetospheric Research

It has been known that the Earth's inner magnetosphere is the region where energetic particles of a wide range of energies from a few eV to about 10 MeV coexist and it changes dynamically with geomagnetic storm, the massive energy release phenomena. However, processes of energetic particle acceleration, transports, and losses are not fully understood. In order to understand energetic particle dynamics in the inner magnetosphere, it is highly desired to observe particles with energies from a few eV up to a few hundreds of keV without any unobserved energy gaps and to separate dominant ions-H+, He+, O+, and so on. In spite of such importance, most observations of the low energy ions get significant amount of background noise due to large fluxes of high-energy particles in the inner magnetosphere. Our purpose is to develop a low-energy ion instrument which can reduce the effects of high-energy particles to obtain accurate data. A combination of an electrostatic analyzer and a time-of-flight (TOF) mass spectrometer provides energy-per-charge (E/q), velocity (V), and then, mass-per-charge (M/q). We designed an electrostatic analyzer which measures ions from 10 eV/q up to 25 keV/q with energy resolution of 15%, angular resolution of 22.5 degrees, and g-factor of 10-2cm2 sr keV/keV. In order to reduce the background noise due to high-energy particles, following points are considered: 1) a double-coincidence technique with TOF method, 2) minimization of anode areal size of a detector (MCP), and 3) thickening the electrode/chassis as a shield. TOF requires two signals, generated by an incident particle itself and secondary electrons emitted from an ultra-thin foil at a passage of the incident particle. Trajectories of the secondary electrons can easily be deflected by a static electric field inside the sensor, since their energies at the generation are rather small (typically less than 10eV). In our design, axis-asymmetric positioning of electrodes (i.e., non-zero electric field in azimuthal direction inside the sensor) is a key element to get the focusing area on the MCP surface for the secondary electron trajectories. Using this axis- asymmetric effect, the secondary electrons are collected in a small area (0.35cm2/22.5deg) in contrast to those of incident particles (3.6cm2/22.5deg) without losing angular resolutions. Combining the above points, the noise count rate due to high energy particles can be reduced to two orders of magnitude lower than expected count rate of target (not noise) particles. We propose this instrument for low energy ion measurements in the inner magnetosphere.