Instrument Description

The instrument combines the selection of incoming ions according to their energy per charge by electrostatic deflection in a toroidal analyzer with post-acceleration by up to 25 keV/e and subsequent time-of-flight analysis. A top view and a cross-sectional view of the sensor showing the basic principles of operations are presented in Figure 1. The instrument is mounted with its axis of symmetry perpendicular to the spacecraft spin axis. The front end of the sensor is protruding out of the spacecraft surface so that its 360 o aperture has a free field-of-view.

The electrostatic analyzer (ESA) is of a toroidal top-hat type with a uniform response over 360 o of polar angle. As illustrated in Fig. 1, the analyzer consists of an inner toroid, to which a variable negative potential is applied, an outer toroid with a cut-out at the top, and a top-cap lifted above the outer toroid. Both, the outer toroid and the top-cap are normally held at ground potential, thereby exposing no high voltage to the outside world. A beam of parallel ion trajectories entering the aperture is focused to a certain location at the exit plane of the analyzer. This location determines the incident polar angle of the ions. With a cross-plate voltage of 2-5200 V (varied with logarithmically spaced steps), the energy range is 15-40000 eV/e for CODIF and ESIC. The corresponding numbers are 0.6 - 1300 V with an energy range of 5 - 10000 eV/e for TEAMS. The analyzer has an intrinsic energy resolution of E/E ~ 0.15. It is surrounded by a cylindrical collimator which serves to define the acceptance angles. The full polar angle of the analyzer is divided into 16 pixels of 22.5 o each. The full energy sweep will be performed 32 times per spin. Thus a two dimensional cut through the distribution function in polar angle with 11.25 o resolution in azimuthal angle is obtained every 1/32 of a spin period. For TEAMS an additional mode with 64 sweeps per spin (5.6 o resolution) has been implemented. The geometric factor over one individual pixel is:
. E/E . ~ 2.16 . 10 -3 cm 2 sr, where A denotes the aperture area and the azimuthal width of the analyzer. This value takes into account the transmissivity of all grids and support structures in the sensor. For TEAMS both halves are identical, thus covering the full sphere in half a spin period. The entrance apertures of CODIF and ESIC provide two different geometrical factors in order to maximize the dynamic range of the instruments, with the geometric factor of one pixel in the second half attenuated to 2.3 . 10 -5 cm 2 sr by an array of pin holes.

On CLUSTER and Equator-S where regions with very low temperature plasma will be encountered, the low-energy portion of the ion distribution ( 0 - 20 eV) is sampled by an additional retarding potential analyzer (RPA) at the entrance of the electrostatic analyzer with a geometrical factor of 0.04 cm 2 sr for the full 360 o acceptance angle and an aperture area of 0.04 cm 2 for beam distributions. In the RPA mode of the instrument the ions are collected through a separate aperture for the RPA, while the normal aperture is electrically blocked. The ions are pre-accelerated into the ESA to 150 eV/e by setting the outer electrodes to - 100 V.

Behind the analyzer the ions are accelerated by a post-acceleration voltage of 20 to 25 kV, such that the ions have at least an energy of 20 keV/e before entering the TOF-section. The energy per charge E/Q as selected by the electrostatic analyzer plus the energy e . U ACC gained by post-acceleration and the measured time-of-flight through the length d of the time-of-flight (TOF) unit are combined into the mass per charge M/Q of the ion according to:

M/Q = 2(E/Q + e . U ACC )/(d/ ) 2 . .

The quantity a represents the effect of energy loss in the thin carbon foil (~ 3 µ g/cm 2 ) at the entry of the TOF section and depends on particle species and incident energy. The flight path of the ions is defined by the 3 cm distance between the carbon foil at the entrance and the surface of the stop microchannel plate (MCP). The start signal is provided by secondary electrons, which are emitted from the carbon foil during the passage of the ions. The electrons are accelerated and deflected onto the start portion of the MCP by the appropriate potential configuration. The secondary electrons also provide the position information for the angular sectoring. To allow good angular resolution, a toroidal geometry has been chosen for the analyzer which pushes its focal point close to the carbon foil plane. The carbon foil is made up of 22.5 o sectors, separated by narrow metal strips. The electron optics are designed to strongly focus secondary electrons originating at a foil onto the corresponding MCP start sector, using the fringe fields of the radial foil support structures and radial fin separators between the sectors. The MCP assemblies are ring shaped quadrants with radii of 3 by 9 cm which serve both the Start and Stop signals. The signal output of the MCPs consists of a set of segmented plates (22.5 o each) behind the start MCP and (90 o each) behind the stop MCPs as well as thin wire grids with 85% transmission at a distance of 10 mm in front of the signal plates, all being at ground potential. The timing information is taken from the grids, while the position signals are taken separately from the segmented plates.

The sensor electronics of the instrument comprise two time-to-amplitude converters (TACs) to measure the time-of-flight of the ions between the start C-foil and the stop MCPs separately for two halves of the TOF section, 16 position discriminators at the Start and 4 position discriminators at the Stop section of the MCPs, as well as event selection logic. A schematic block diagram is shown in Fig 2. Each individual ion is pulse height analyzed according to its time-of-flight, incidence in azimuthal (given by the spacecraft spin) and polar angle (given by the start position), and the actual deflection voltage. The resulting digital outputs are encoded in a look-up table and the resulting energy-angle event is accumulated into incrementing memories with variable binning (according to the selected instrument mode) in energy and angle, separately for 4 different masses, or with long accumulation for up to 64 masses.

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