After the application of time-of-flight (TOF) mass-spectrographs in space had been successfully pioneered with the AMPTE mission in 1984 (Gloeckler et al., 1985; McEntire et al., 1985; Möbius et al., 1985) TOF instruments have become a standard tool in space plasma physics. TOF instruments have been flown in various different configurations on missions, such as VIKING, GIOTTO, VEGA, PHOBOS and Ulysses. They are also aboard the GGS spacecraft. The wealth of the ion composition results from these missions has clearly demonstrated the excellent capabilities of this type of instrumentation in both the energy range of the bulk plasma as well as in the energy gap between about 30 keV and several hundred keV/nucleon, which existed between the regimes of magnetic mass spectrographs and cosmic ray telescopes.
More recently the further evolution of this type of instrument has branched into two directions to optimize their use for different applications. Along one line the mass resolution of this class of instruments has been substantially enhanced by the introduction of isochronous TOF spectrographs (Hamilton et al., 1990; Möbius et al., 1990). Such spectrographs are on Wind, SOHO (Hovestadt et al. 1989) and Cassini (McComas et al., 1990). Whereas the emphasis in this line of instruments is on high mass resolution in order to study the detailed composition of space plasmas including minor species, isotopic ratios and the identification of molecular ions, the other parallel effort went towards a fast plasma analyzer with simultaneous mass resolution. In order to simultaneously determine the 3-dimensional distribution function and the mass-composition of ions, the concept of the fast 3-D plasma instrument on AMPTE/ IRM (Paschmann et al., 1985) has been combined with the TOF technique and post-acceleration for the ESA/NASA CLUSTER mission (Rème et al., 1993). Similar instruments will be flown on FAST (Carlson 1992) and on Equator-S. The major achievement of these instruments will be to allow the accumulation of full velocity distribution functions for 4 major species (e.g., H + , He 2+ , He + and O + ) within 1 spacecraft spin on Cluster and Equator-S or within 1/2 spin period on FAST. The instantaneous coverage of a two-dimensional cut of the distribution function is achieved within one energy sweep whose typical duration is 60 - 250 msec. This capability will significantly enhance our knowledge of the dynamics in multi-ion plasmas. In this paper we will describe the basic features, some critical design elements and results from test and calibration of these instruments. We will also emphasize the modular concept of this approach that allowed
- an efficient split of the workload among different institutions and
- the adaptation of the instrument for several missions, while leaving the bulk of the instrument identical, thus saving a substantial design effort.