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Development of an ion energy mass spectrometer for lunar plasma observation
Shoichiro YOKOTA


The plasma environment around the Moon has several unique features that are strongly affected by the interaction between the solar wind and the Moon. Since the Moon has neither global dipole magnetic field nor substantial atmosphere, it has been considered that the Moon just obstructs and absorbs the solar wind plasma. The interplanetary magnetic field is barely perturbed from its upstream orientation because it diffuses into the very weak conducting layers at a rapid rate. Hence, neither a bow shock nor a magnetosphere is formed in front of the Moon. It has, however, recently attracted the interest that the Moon and possibly some of the planetary satellites and the asteroids maintain some atmospheres while these atmospheres are thin and sometimes transient. The rarefied atmospheres are continuously produced primarily by solar photons and the solar wind. Just after the atmospheric particles are photoionized, some of them are picked-up by the solar wind and lost to the space. In addition, the sputtering by the solar wind, one of the source mechanisms, presumably produces the secondary ions reflecting the composition of the lunar surface. In-situ measurements of such ions will provide much information on the lunar surface and atmosphere.

The ions sputtered by the solar wind from the lunar surface include various ion species such as Na, Mg, Al, Fe, etc. However, ordinary space plasma spectrometers aiming at discrimination of the Earth's magnetospheric ions cannot distinguish such heavy ions. Time-of-flight technique using carbon foil whose secondary electrons generate start signals offers large throughput and small resources. The instrumental mass resolution of the conventional field-free time-of-flight systems, however, is limited because the dispersion in the flight time depends critically on the energy degradation and angular scattering of the particles caused by the passage through the thin carbon foil. Thereby, time-of-flight device of the spectrometer applies a peculiar electric field, called `linear electric field', which increases linearly to the penetration length of incident ions for higher mass resolution than that of traditional time-of-flight techniques. In this electric field, ions bounce in simple harmonic motion, where energy and flight path no longer affect the flight time and thus the mass resolution. We have succeeded in designing the complicated instrument, and have confirmed the validity of the calculated characteristics by the laboratory experiment.

For a future lunar orbiter mission named `SELENE', we have developed an ion energy mass spectrometer that measures three-dimensional distribution functions of mass-discriminated ions with a high sampling rate by using a top-hat electrostatic energy analyzer and time-of-flight mass analysis. On the other hand, collaboration with optical experiments preferring three-axis stabilized spacecraft in planetary missions poses a problem for complete coverage of all possible plasma arrival directions for three-dimensional energy analyses. Hence, we have added angular scanning deflectors to the cylindrically symmetric ion analyzer for hemispherical (2 $\pi$ str) field-of-view. Ion analyzers need suitable sensitivities for surrounding space plasma fluxes, whose intensity is much diverse depending especially on plasma regions such as the solar wind, the planetary magnetospheres and so on. The ion fluxes emitted from the Moon and picked-up by the solar wind are very weak compared to the solar wind. In order to possess a wide range of sensitivity, the analyzer is equipped with sensitivity control electrodes.

A position-sensitive time-of-flight MCP detector has also been developed for the installation on the ion energy mass spectrometer. By using a resistive anode and a conductive grid, the MCP detector is capable of obtaining position and timing signals simultaneously. This compact MCP detector we designed on the basis of the test experiments functionally deals with the position signals with frequency of MHz and timing signals with frequency of GHz for time-of-flight measurement.

Although there are few clear in-situ measurements of the lunar ions near the Moon, several ground-based observations and laboratory experiments suggest that a substantial amount of ions generated from the lunar atmosphere or surface exist around the Moon. In order to estimate the flux of the lunar ions, we have carried out numerical calculations by using the production rate obtained from previous studies. The results of our estimation indicate that our newly-developed ion energy mass spectrometer on board a lunar orbiter will certainly measure the lunar ions with a sufficient spatial resolution with regard to the lunar surface study.

Cross section of IMA and trajectories of incident ions and secondary electrons
fig.1Cross section of IMA and trajectories of incident ions and secondary electrons

 

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