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:
A
.
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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