***** File GIOIMS.TXT
NOTE: This file was created by scanning the original hardcopy article
and only the Figure captions are included.
The Giotto Ion Mass Spectrometer
H. Balsiger, K. Altwegg, F. Buhler, J. Fischer,
J. Geiss, A. Meier & U. Rettenmund
Physikalisches Institut, Universiry of Bern, Bern, Switzerland
H. Rosenbauer, R. Schwenn, J. Benson,
P. Hemmerich, K. Sager & G. Kulzer
Max-Planck-Institut fur Aeronomie, Lindau, West Germany
M. Neugebauer, B.E. Goldstein & R. Goldstein
Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, USA
E.G. Shelley, T. Sanders & D. Simpson
Lockheed Palo Alto Research LAboratory, Palo
Alto, California, USA
A.J. Lazarus
Department of Physics and Center for Space Research, MIT,
Cambridge, Massachusetts, USA
D.T. Young
Los Alamos National Loboratory, Los Alamos, New Mexico, USA
Abstract
A wide range of ion species and velocity distributions are expected to be found as
the Giotto spacecraft traverses the coma of Halley's Comet. The outer coma is
characterized by the interaction between solar wind and cometary plasmas, the inner
coma by the outflow of cometary neutrals and their ionization products. The resultant
demands on instrument dynamic range preclude use of a single sensor for
measurements of ion composition. The Giotto Ion Mass Spectrometer (IMS) therefore
consists of two sensors: one optimized for the outer and the other for the inner coma,
with each obtaining complementary information in the region for which it is not
optimized. Both sensors feature mass imaging characteristics, thereby permitting
simultaneous measurements of several ion species by means of multi-detector arrays.
Resultant mass-per-charge resolution is >= 20. In addition to mass per charge, the
energy per charge, the elevation and azimuth of incident ions are measured. Calibra-
tion and in-flight solar-wind data show that the IMS will meet its scientific goals for
the Halley encounter.
1. Introduction
The interaction of a comet with the solar wind is fundamentally very complex and
dynamic. Cometary material first makes its presence felt in the solar wind in the form
of neutral gas photo-ionized and accelerated by the solar wind far upstream from the
comet nucleus at distances > 10**6 km. Although their density is initially very low,
these 'pick-up' ions (e.g. H+, C+, O+, OH+, CO+, H20+) mass load the solar wind
and begin slowing it. Nearer the comet a bow shock may form, but this is not certain.
The solar-wind slowing process may also create a broad region of plasma turbulence
as picked-up ions are thermalized and incorporated in the flow of solar wind around
the comet. Implantation of cometary ions in the solar wind has the further consequence
that the solar wind interaction with cometary material occurs far outside the contact
surface that is expected at ~ 10**4 km from the nucleus (Ip, 1980). Once the contact
surface is crossed the spacecraft is immersed in cold, low-velocity (~ 1 km/s)
outflowing cometary ions. The region inside the contact surface, often called the
'inner coma', contains relatively high ion densities, and perhaps magnetic fields,
which stand off the solar-wind ram pressure. Within the inner coma, at the latest,
neutral gas and dust particles impinging on Giotto will create a plasma cloud around
the spacecraft, giving rise to electrical charging and creating spurious ions (and elec-
trons) detectable onboard by the Ion Mass Spectrometer (IMS). In this region some
interference with the ion measurements is expected.
This thumbnail sketch of the flyby through Halley's coma illustrates the two primary
science objectives of the Giotto IMS:
1. To measure accurately the relative abundances of both solar and cometary ions
in the cometary coma, and
2. To determine ion velocity distributions as a function of position within the coma.
The design principles of one part of the IMS (the HERS, see below) have been describ-
ed by Neugebauer et al. (1982). The complete IMS as adapted to the Giotto mission
was discussed by Balsiger et al. (1981). The present paper documents the final instru-
ment design as well as the successful implementation, calibration, and initial flight
testing of the IMS sensors.
2. Approach
The ion composition of the outer coma is dependent upon details of the interaction
of solar wind and cometary plasmas. In order to determine composition, we must
obtain good measurements of the three-dimensional velocity distribution of individual
ion species. From the latter we determine species flow velocities (speed and direc-
tion), temperatures, and number densities. Ion composition and velocity distributions
are expected to be strong functions of radial distance from the nucleus (Fig. 1).
Moreover, recent results from the electron plasma analyzer onboard the International
Cometary Explorer (ICE) during its intercept of Comet Giacobini-Zinner indicate
sharp variations in bulk plasma flow speed, density, and electron temperature (Bame
et al., 1985).
Figure 1. Model of ion distribution along
comet-Sun line in the coma of Halley. In this
model the bow shock is at 4 x 10**5 km (Ip, 1980)
Based on the above discussion, an ion mass spectrometer suitable for high-speed
flyby of Halley's comet near 1 AU must cover a wide range in particle phase space.
At the time the Giotto instruments were designed, little was known about the cometary
plasma environment. Today, following the successful ICE flyby of Comet Giacobini-
Zinner, the impression remains that wide instrument response in angle, energy and
ion mass will be needed for the Halley flyby. No single present-day ion instrument
is able to cover this wide range of plasma properties while still providing the necessary
mass resolution. We have therefore designed two different sensors specialized for
measurements in these regions. The sensor for the outer coma is called 'HERS' (High-
Energy Range Spectrometer) and the one for the inner coma 'HIS' (High Intensity
Spectrometer). Their characteristics are summarized in Table 1.
Although energy and angular distributions are important parameters (and hence will
be measured) the emphasis of IMS lies on an accurate determination of ion composi-
tion. Since not only chemical and elemental, but also isotopic, abundances are of high
cosmogonic interest, good mass separation is required. Furthermore, mass analysis
must be performed in such a way that prior knowledge of the ion velocity distribution
is not assumed. Hence both components of the IMS are true mass analyzers using
variable electric fields and static magnetic fields for determining energy per charge
(E/Q) and mass per charge (M/Q), respectively. (In this paper M is ion mass in amu.
Q is the ion charge state, and E is energy in eV). Both sensors have M/Q imaging
capabilities that increase sensitivity and, because several ion species are detected
simultaneously, increase the time resolution as well.
Despite these similarities, the two sensors are different in many details (see next sec-
tion), which leads to quite different properties. The range of energy and angle covered
by HERS is large, although as one sacrifice in the tradeoff study the cometary direc-
tion could not be included in the field of view (Fig. 2). On the other hand, HIS has
a more limited field of view, but concentrates on the cold cometary ions incident from
the forward direction (Fig. 2). Both will be operated throughout the encounter
measurement period. In the outer coma, two-thirds of the available data rate will be
dedicated to HERS, one third to HIS. These ratios are reversed in the inner coma using
a time-tagged command from the ground.
Because of the hazard of gas and dust impact that might lead to physical damage
or a large background ion flux, neither sensor will view directly into the Giotto-
Halley relative velocity vector (the ram direction). Protection of the sensors is achiev-
ed by mounting them within the spacecraft body and projecting their fields of view
into the desired directions using electrostatic deflection devices.
--------------------------------------------------------------------------------
Table 1. Summary of IMS chamcteristics
Parameter HERS HIS
--------------------------------------------------------------------------------
E/Q range 10 eV/e to 2.0 - 4.5 keV/e 300 to 1400 eV/e
depending on M/Q
M/Q range 1 to 35 amu/e in 4 groups 12 to 57 amu/e
M/Q resolution (M/delta M) >= 20 at 20 amu/e >= 20 at 20 amu/e
Elevation angle range +15deg to +75deg -3deg to +12deg (MA)
-3deg to +22deg (AA)
Elevation angle resolution 7.5deg (15deg for H+) 5deg to 7.5deg
Azimuth angle range 360deg (spin-scanned) 30deg (spin-scanned)
Azimuth angle resolution 5.6deg 22.5deg
Spectral time resolution 16 s 4 s
Density range 10**-3 to 10**2 cm**-3 10**-2 to 10**4 cm**-3
--------------------------------------------------------------------------------
Weight 9.2 kg
Average power 8.5 W
Telemetry, total 3253 bit/s
--------------------------------------------------------------------------------
Telemetry
Outside contact surface 2169 bit/s 1084 bit/s
Inside contact surface 1084 bit/s 2169 bit/s
Figure 2. IMS angular coverage and fields of
view for the HERS and HIS sensors. The more
limited field of view of the HIS MA is shown
shaded. The -z direction refers to the Giotto
spin axis, which is directed towards the comet
nucleus
3. Instrument Description
In order to simplify hardware interfaces, HERS and HIS were built as separate
units, called IMS-1 and IMS-2, respectively. The interface to the spacecraft is provid-
ed by IMS-3, which includes low-voltage power converters and the common data-
processing unit, whereas the high-voltage converters, preamplifiers, signal condition-
ing and test electronics are located directly in the IMS-1 and IMS-2 units. Figure 3
shows the principles of the IMS electronics, and Figure 4 shows the three IMS boxes
laid out as on the spacecraft platform.
3.1 Sensor for the outer coma (HERS, IMS-1)
3.1.1 Objectives
The prime objective of the High-Energy Range Spectrometer is to measure the ion
abundances and three-dimensional velociry distributions outside the cometary contact
surface. The HERS sensor is designed to provide the mass resolution necessary to
resolve the important cometary ion species over a wide range of temperatures and flow
velocities within a field of view lying between 15deg and 75deg from the comet ram direc-
tion (including the solar wind direction, Fig. 2).
3.1.2 Main sensor components
The basic concept of the HERS sensor has been described previously by Neugebauer
et al. (1981) and Balsiger et al. (1980). Referring to the schematic drawing of HERS
in Figure 5, the main components of the sensor consist of (i) a curved electrostatic
mirror, (ii) cylindrical accelerating/decelerating grids, (iii) a magnetic analyzer, (iv)
an electrostatic deflector and (v) ion detectors. The electrostatic mirror compresses a
60deg external field of view to 30deg internal to the sensor with nearly uniform angular
magnification over the full range. The mirror consists of a pair of nonconcentric,
cylindrical, gold-plated high-transmission grids, the front one of which is maintained
at ground potential, while the rear one is held at a potential positive enough to reflect
the ions of interest in that portion of the measurement cycle (the cycling programme
is described below.)
A photograph of the HERS instrument with the outer cover removed is shown in
Figure 6.
3.1.3 The magnet
Two 3.60 mm high by 1.26 mm wide slits located symmetrically on either end of
the 120deg sector magnet provide a constant normal-component-of-momentum filter
over +/- 15deg angular acceptance in the plane of the magnet (Fig. 5). The samarium-
cobalt magnet has a pole gap of 5 mm and a magnetic field strength of 0.335 T
Figure 3. Block diagram of the IMS
Figure 4. The IMS boxes laid out as on the spacecraft platform, with (from right to left) IMS-1 (HERS), IMS-3 and IMS-2 (HIS).
Figure 5. Cutaway view of the HERS sensor showing principles of operation
Figure 6. The HERS sensor with cover removed (for description see Fig. 5)
(uniform to 0.5% over the usable portion of the gap). The constant momentum per
charge corresponds to an ion energy per charge of 7560/(M/Q) eV/e. The internal
angular acceptance of the instrument out of the magnet plane (spacecraft azimuth) is
an average of 2deg FWHM. This is determined by the slit height and separation and by
the amount of vertical focussing in the magnetic fringing field. The external angular
acceptance depends, of course, on the acceleration/deceleration voltage.
3.1.4 The electrostatic deflector (ESD)
The electrostatic deflector serves as an energy-per-charge analyzer, and since the
normal momentum has been fixed by the magnet, it becomes a mass-per-charge
analyzer. The electric field in the analyzer deflects ions out of the optical plane
of the magnet towards the detectors. The ESD field is nonuniform in such a way that
all ions of a given M/Q are focussed in the detector plane. The focal line for ions of
different elevation is very nearly straight, and the position along this line maps out
the angle that ion trajectories make at the entrance to the magnet slit S1 (Fig. 5). The
higher the M/Q of the ion, the nearer to the ESD entrance slit S2 this line lies.
In Figure 6 the ESD is seen to the rear of the magnet. The structure on the topmost
face supports some of the signal-processing electronics for the detectors. The
microchannel plate (MCP) detector is hidden by this structure, but the four channel
electron multipliers (CEMs) are visible at the rear top corner.
The nonuniform, approximately two-dimensional electric field is generated by
applying voltages to a set of vane-like electrodes arranged appropriately in the
analyzer. Figure 7 shows a cross section of the ESD, including the results of computer
simulation of ion trajectories. The ESD entrance slit S2 is at the lower right corner
with the vanes shown in projection. Voltages are applied to the vanes by taps on a
resistive divider between two supply voltages. For the case shown, +4667 and -667
volts are used to bring 'light' ions of M/Q = 2 to 4 onto the main detector area. The
main detector, an MCP, allows two-dimensional imaging, and will be discussed in
more detail below. Protons are focussed by the same voltages onto a separate set of
CEMs. By switching to other voltage combinations, other mass ranges can be detected
on the MCP. Since the ion energy entering the analyzer is inversely proportional to
Figure 7. Cross-section of the HERS
electrostatic deflector. Envelopes of ion
trajectories are shown for the light ion range
(M/Q=2 to 4 amu/e). Vanes behind slit S2 act
as a particle trap (note trajectories for
M/Q=12 amu/e)
M/Q, higher mass ranges are detected with lower voltages on the analyzer. (The cor-
responding voltages are +770 V and -110 V for 'medium' ions defined as
M/Q=12-26, and +576 V and -82 V for 'heavy' ions with M/Q=16-35.)
For the case shown in Figure 7, all ions with M/Q>4 are deflected onto the region
in front of the MCP. In order to minimize the effects of scattering, the vanes in this
region are arranged to act as a particle trap (see M/Q= 12 in Fig. 7). Similarly, when
heavy ions are being detected, the light ions pass nearly straight through the ESD and
are trapped by the vane arrangement at the end opposite the entrance slit. The vanes
in this far region are electrically split, with potentials arranged such that the electric
field near the vanes retards the escape of low-energy secondary ions.
3. 1.5 Detectors
Only protons are detected on the CEMs at the appropriate voltage combination men-
tioned above. These separate detectors for the protons are used to avoid unreasonably
high voltages necessary to deflect them onto the MCP, and because anticipated proton
count rates are more easily accommodated with a CEM. A line of four 5x 12.8 mm
funnel-type CEMs is used. Each covers an external elevation angle range of 15deg. Any
loss of detector gain during flight is measured by test routines (see below) and can
be corrected by selecting one of four high-voltage bias levels. We presently operate
the CEMs at -2450 V with discriminators in the output pulse circuits fixed at a
threshold of 0.1 pC.
All heavy ions (M/Q> 1) are detected on the 50 mm diameter (45 mm diameter
sensitive area) curved-channel MCP, in each of three mass ranges set by the ESD
voltages given above. As for the CEMs, the MCP high voltage is selected in flight
from one of four levels. It is currently set at -1650 V.
In order to obtain simultaneous two-dimensional information (one dimension for
mass and one for elevation angle), the MCP is equipped with two orthogonal sets of
pickup anodes (Liptak et al., 1984). These produce electrical signals that allow a
unique determination of the location of an event on the MCP. Figure 8 shows a
photograph of the angle-sensing anodes deposited directly on the output surface of the
MCP. Each of these eight anodes covers an external elevation angle of 7.5deg. The
Figure 8. The angle-sensing anodes deposited
on the back of the HERS MCP detector
second set or mass-sensing anodes, consists of 40 gold-plated strips deposited on the
face of a high-purity alumina plate. A photograph of the mass anode plate is shown
in Figure 9. This plate is located parallel to and 0. l mm removed from the output face
of the MCP. It is held 50 V positive relative to the MCP to attract the pulse of secon-
dary electrons emitted by the latter. Thus, when an ion strikes the input side of the
MCP, a positive pulse is produced on one of the eight angle anodes and a negative
pulse on one of the 40 mass anodes. One of eight logarithmically spaced threshold
levels is selectable separately for the discriminator of each type of pulse (i.e. mass or
angle). We presently use 0.024 pC for the mass threshold and 0.012 pC for the angle.
For an ion to be registered by the encoding electronics, it is a prerequisite that it
produces a charge pulse above the mass discriminator threshold. A coincidence
(within 1 micro s) between both mass and angle pulses is required to produce a 'good' event
initiated by the arrival of an ion at the MCP. In such a case the event is defined by
its location (angle anode and mass anode numbers) and a flag to indicate that a good
event occurred. If the angle pulse is missing, or if more than one of either or both
pulses occur within the allotted time window, the event is not discarded, but rather
a 3 bit flag is used to indicate the pulse combination produced. Analysis of flagged
versus unflagged events provides a quantitative assessment of the overall efficiency
of the MCP system.
3.1.6 Acceleration/deceleration system
Since the magnet is a momentum/charge filter, only a very narrow range of energies
+/- 3% on average) for each M/Q is passed by the system. In order for the magnet to
transmit a wider energy range, the acceleration/deceleration system accelerates or
decelerates ions to a momentum range that the magnet will accept. This voltage is
applied between the middle and innermost grids shown in Figure 5. (The outermost
grid is at + 10 V to exclude spurious impact-produced low-energy ions from the
system.) All parts of the optics downstream of the inner grid float at the accelerating
potential. This floating 'platform' can be seen in the centre of Figure 6, supporting the
electrostatic deflector, magnet, and the innermost accelerating grid assembly. Several
of the required high voltages must be generated at this floating level. Power is supplied
through an isolation transformer, while data and telecommands are transmitted by
means of opto-couplers.
Figure 9. The mass-sensing anodes for the
HERS MCP detector. Forty gold-plated strips
are deposited on a high-purity alumina plate
which is mounted behind the MCP (see Fig. 7)
The accelerating potential is swept in a triangular waveiorm, relative to a selectable
central voltage at a frequency of 8 Hz. This gives 32 complete cycles per 4 s spin
period. The amplitude (peak-to-peak) is fixed at 4.35 kV, while the central value
depends on which of the four mass ranges is being measured. The resultant voltage
sweep ranges are -7932 V to -3580 V for protons, -3967 V to +385 V for light
ions, -662 V to +3690 V for medium ions, and -500 V to +3852 V for heavy ions.
Because the sweep is phase-locked to the spin period, two sweep phases, 5.6deg apart,
are used in alternating measurement cycles. This avoids always measuring a given
energy at the same set of azimuth angles. One waveform ramp is divided into 64 bins,
spaced quasi-logarithmically to give roughly constant energy resolution (Delta E/E) over
the full range. The energy bin number is telemetered as part of the address of each
ion event detected.
The mirror voltage is swept synchronously with the accelerating voltage in such a
way that its value approximately tracks the energy of the ions being measured.
3.1.7 HERS measurement modes
During each 4 s spin period, 64 complete energy scans are performed in one of the
four mass ranges mentioned above. This yields one energy sweep for each of the 64
azimuth bins of 5.6deg for all ions within the selected mass range. (In two consecutive
azimuth bins the sweep voltage runs up and down, respectively.) The basic HERS
mode, used when HERS has priority in telemetry allocation, consists of four spin
periods. (The HIS mode, used when HIS has priority, has eight spin periods.) These
modes, selectable by telecommand, determine the mix of mass ranges in one instru-
ment cycle. Modes include: l. protons only, 2. alternating protons and light ions only,
3. no protons (light, medium, heavy, medium), and 4. all masses (protons, medium,
heavy, light). Mode 2 typically would be selected for measuring solar wind during
cruise phase, while mode 4 is intended for encounter. Modes 1 and 3 have been includ-
ed in case a detector deteriorates.
During cruise and passage through the outer coma, when HERS has priority, data
are telemetered for each spin. In the inner coma, when HIS has priority, each mass
range is held for two spins and telemetry is adjusted accordingly. Each ion event
causes a 24 bit word to be formed containing the MCP elevation angle and mass anode
numbers, the pulse coincidence flags, the energy and azimuth bin numbers during
which the event occurred, and whether the voltage was sweeping up or down. Events
are then placed chronologically into the telemetry queue. In the case of protons, a 24
bit word is also used, but instead of MCP location the total number of counts per
energy bin for each CEM (along with the energy and azimuth information) enters the
telemetry stream. A result of zero counts per bin is not transmitted. At very high count
rates a data-compression scheme goes into effect to fit the measurements into the
available telemetry rate.
3.2 Sensor for the inner coma (HIS, IMS-2)
3.2.1 Objectives
The High Intensity Spectrometer (HIS) is designed to complement HERS in the in-
ner coma where we expect cometary ions at high densities, low temperatures, and with
low bulk speeds relative to the nucleus. The composition of these ions will be deter-
mined between 12 and 57 amu per charge. Their velocity distributions will be
measured in a limited range around the Giotto-Halley relative velocity of 69 km/s,
both with respect to absolute speed and the angles of incidence. These data will be
used to obtain the composition of the volatile fraction of the nucleus and ultimately
the chemistry and dynamics of the inner coma.
3.2.2 Main sensor characteristics
The HIS contains two separate analyzers (Figs. 10 and 11), the Mass Analyzer
(MA) and the Angle Analyzer (AA). The MA uses a combination of electrostatic and
magnetic deflection systems designed to give good separation between adjacent masses
near H2O+ (M/Q= 16-20). Its intrinsic field of view is 2deg X 15deg, including the
direction of the spin axis (Fig. 2). Due to Giotto's spin, the resulting field of view is
conical, with a half angle of 12deg and slight overlap at the centre.
Figure 10. Schematic of the HIS sensor
Figure 11. The HIS sensor with cover removed
(for description refer to Fig. 10)
The AA is an electrostatic quadrispherical analyzer. Five miniature CEM detectors
at its exit allow for a resolution of 5deg to 7.5deg within the fan-like field of view of
2deg X 25deg total width. Here the resulting conical field of view has a half angle of 22deg
(Fig. 2). This viewing fan contains five elevation-angle ranges, which are split up
electronically into 16 azimuthal segments each. For five major ion species, the angular
velocity distribution around the ram direction can be inferred (see 3.2.5), which is
important for interpreting the MA's results.
Both HIS sensors are mounted in such a way that their viewing directions are nearly
identical. Their measurement programmes are stepped in parallel and in part they use
common supply voltages. An external electrostatic plane-plate deflector bends the ion
trajectories out of the dust particle path and into the shadow behind the spacecraft dust
shield, where the main part of the IMS is located.
3.2.3 The Mass Analyser (MA)
The basic principle of this mass spectrometer can best be understood with reference
to the classical optical analogy sketched in Figure 12. The entrance slit S1 is located
in the focal plane of the first convex lens L1 and a parallel beam of light leaves L1.
A prism provides dispersion, while the lens L2 focusses the image of S1 onto the
Figure 12. Optical principle of the HIS sensor:
(a) classical optical analogue,
(b) ion optics of the HIS
image plane P, forming a spectrum of the source. In the MA the role of the lens L1
is played by the quadri-spherical analyzer L1: it transforms an incoming divergent par-
ticle beam into a parallel beam. Of course, only those particles with the appropriate
energy per charge given by the applied voltage (0 to -200 V on the inner plate) are
transmitted. The permanent magnet (0.19 T) acts like a prism, with dispersion depen-
ding on the particles' momentum per charge. Lens L2 (+/- 66 to +/- 333 V) images S1
onto the plane P, providing a momentum-per-charge spectrum there. At a certain posi-
tion on P we will find particles with both identical energy per charge and momentum
per charge, and thus with identical mass per charge. The spectrum formed in plane
P is therefore interpreted as an M/Q spectrum. Computer modelling was used to
optimize the optical system described above, details of which will be published
separately.
If a given detector on the image plane is to be dedicated to a specific M/Q value,
the principle of the HIS as explained so far would not allow any scanning of the E/Q
range of incident ions. Ions are constrained to arrive at the detector with a fixed E/Q
value given by the magnet field strength and its length, and by the position of the
detector. In order to scan the E/Q distribution of incoming ions, the entire section of
the instrument beyond L1 must be biased (Fig. 3) such that
U(1) - U(bias) = U(const.)
where U(1), denotes the central energy per charge of ions transmitted by the first elec-
trostatic analyzer L1, U(bias) (-1400 to +1050 V) is the voltage applied to float the
rest of the analyzer, and U(const.) is the energy per charge the ion of appropriate M/Q
must have in order to hit the desired detector.
One of the main boundary conditions for designing the MA sensor was the goal of
optimum mass separation in the water ion group. Therefore, separate dedicated detec-
tors for ion species 16-21 were placed in the image plane. At the positions of the
virtual masses 17.5, 18.5, and 19.5, additional detectors were installed in order to
monitor possible interferences hetween neighbouring masses.
For some of the ion species, e.g. H2O+, count rates in excess of 1 x 10**6 s**-1 are ex-
pected on detector areas as small as 2 mm**2. This prevents the use of MCPs and
rather suggests the use of CEMs. The distance between images of adjacent mass
numbers in the water-ion group is only 1.2 mm, however, and intermediate detectors
are needed for masses 17.5, etc. These small spacings therefore required development
of a special CEM-based miniature detector system.
Central to this miniature detector system is a prism-shaped activated lead-glass
block (made of material similar to MCPs and CEMs) with gold-plated surfaces where
high conductivity is required (Fig. 10). The block's front surface, which lies in the
image plane, has nine rectangular holes roughly 0.6x2.0 mm, the bottoms of which
are connected by a 0.4 mm diameter straight channel to the back side of the prism.
The channels are drilled at different angles such that their exits are spread over a larger
area than the entrances in the image plane. Funnels of specially fabricated CEMs are
directly attached to the block's exits using conductive epoxy. Both the front and back
surfaces of the glass prism are gold-plated. A voltage of 700 V applied between then,
lets the activated channels act as preamplifiers for the ions hitting holes in the image
plane. Because the completed detector array is reminiscent of a hedgehog, it is refer-
red to as an 'Igel', which is the German name for this animal. For the CEMs four
different HV levels between 2.5 kV and 3.4 kV can be selected by telecommand.
3.2.4 The Angle Analyser (AA)
The main purpose of the AA is to extend the MA's angular field of view and to allow
for some resolution in elevation. This capability of the AA is important for interpreta-
tion of MA data in the case of high ion temperatures or of some ion bulk motion
relative to the nucleus. Thus, to some extent, the AA bridges the gap in angle and
energy coverage between the HIS and the HERS.
The AA is equipped with a detector system consisting of five CEMs (operated at
one of four selectable HV levels between 2.5 kV and 3.1 kV) and appropriate
amplifiers capable of handling count rates in excess of 2x10**6 s**-1. The CEMs are not
hit directly by the transmitted ions but rather by secondary electrons produced on
aluminium dynodes. The quadrispherical AA plates have the same R/deltaR values as the
lens L1 in the MA and thus are connected to the same voltage supply.
3.2.5 HIS measurement modes
The basic HIS program consists of a 64-step energy scan repeated 16 times per spin
period. During each scan, voltages for deflection, acceleration/deceleration and on the
lenses Ll and L2 are swept in a rather complicated way depending on the measurement
mode. The actual step values of the sweeps are stored in a PROM in IMS-3. Per spin,
the 14 CEMs yield a total of l4 366 individual count rates. The 14 366x16-bit words
are reduced by onboard compression to an array of 1004 8-bit words. Compression
is achieved by: (i) summation of related count rates, (ii) omission of insignificant chan-
nels, and (iii) quasi-logarithmic compression of the remaining count rates. After the
end of a cycle (1,2, or 4 spins, depending on telemetry rate and IMS mode), the data
are processed further, then compressed and finally telemetered during that same spin.
The actual time delay between a measurement and its transmission at nominal
encounter conditions will not exceed 4 s. This fact may become important in the inner-
most coma where the end of the mission can be expected at any time.
The data scheme of the AA is rather simple: only for those E/Q channels in which
the most abundant ions (M/Q= 18, 19, 20, 28, 44) are expected are the counts of all
five CEMs behind the AA transmitted separately for each of 16 azimuthal bins per
spin. For all other 59 E/Q channels, counts are integrated over a full spin for CEM
1 only (this is the CEM that looks in the forward direction). The remaining four CEMs
are summed together over a full spin period and a single number sent back.
In order to understand the MA's data scheme, one must be aware of a basic property
of this instrument: by choosing the proper post-acceleration or deceleration and mak-
ing corresponding adjustments in the L2 voltage, each mass species can be directed
to any desired position on the Igel, and hence to any CEM. This property can be used
to set up two distinct modes: the so-called 'N- and H-programmes'.
In the N-programme each species has its own dedicated detector within the
appropriate E/Q regime. Any velocity value of a given species causes the particles to
hit the same detector. To make optimum use of the focussing properties at the Igel's
centre and to keep the Igel's size and number of detectors within reasonable limits,
the whole range of masses is split up into seven groups. For example, in the 'water
group', mass 18 will always be registered by CEM 4, the peak of its nominal velocity
distribution occurring at E/Q channel 14 corresponding to 69 km/s. For this mass
channel as well as for mass channels M/Q= 14-17, 19-21,26-29, 32, and 44, all
16 azimuthal channels during one spin are transmitted individually, while in the other
cases they are just summed over. Both flanks of the nominal mass-18 velocity distribu-
tion, which is focussed on a central CEM, are covered quite well. Other ions, e.g.
mass 16 or 32 (focussed on CEMs I and 9, respectively), yield only one flank of their
distributions. This scheme assigns best coverage to the expected most-abundant ion
species. Less abundant species will not yield enough counts to measure their angular
and velocity distributions.
The detector pattern on the Igel has been designed such that it fits best the
requirements for resolving the water group. This pattern does not necessarily fit other
groups equally well. In the latter case, some ion species will not be focussed at the
centres of the detectors, but rather at their edges or even on the rims in between them.
Therefore, at heavier masses (above 46), the group scheme was dropped and replaced
by the same scheme as in the H-programme described below.
In the so-called H-programme individual detectors are not dedicated to fixed M/Q
values but rather to fixed velocities centred on the nominal encounter velocity of
69 km/s. In other words, the peak of the velocity distribution of any species will be
seen at CEM 6 as E/Q is scanned, provided only that the plasma is truly at rest in the
comet frame of reference. In this programme the same ion species will appear on dif-
ferent detectors, according to its velocity distribution.
Each programme has its special merits: the H-programme is better suited to measur-
ing relative abundances of different species because each is measured with the same
detector (CEM 6). The N-programme allows better determination of the velocity
distributions of several ion species because they are measured for each selected ion
with a single CEM. Use of a single CEM for either mass (H-programme) or velocity
(N-programme) distributions takes account of any variations in individual CEM
sensitivities.
Via telecommand we can select either N or H programmes only, or N and H alter-
nating every spin with a 3:1 or 1:1 ratio. If the latter mode is used, an additional
'wobble' voltage is applied to the outer plate of the external deflector, which in all
other modes is grounded. The voltage follows a sawtooth pattern (0 to -15 V) with
a frequency of 1 kHz and effectively expands the field of view of both the MA and
the AA in the direction away from the spacecraft (by 2deg for mass 12 or 0.5deg for mass
57). This feature becomes useful in case the whole spacecraft charges up negatively.
3.3 Common power and data-processing unit, IMS-3
The common electronics box for the HERS and the HIS links the IMS to the Giotto
telemetry and telecommand subsystems and to the power bus. IMS-3 contains all the
low-voltage electronics, including an isolating power converter. It supplies all
voltages necessary to drive the various detector amplifiers, the logics and the high-
voltage power circuitry.
Two microprocessors, together with their peripheral logic elements, receive and
compress digitized data from the sensor boxes. One microprocessor, aided by two
64 kbit memories, is assigned to mode control and data compression for HERS. The
other, together with another 64 kbit RAM, governs the HIS instrument, its modes and
data handling, as well as all general IMS command, telemetry and housekeeping inter-
faces with the spacecraft. Digital-to-analog converters with PROMs at their inputs
provide the necessary control voltages for all high-voltage stepping in both
instruments.
In the event of microprocessor failure, all software controlled logic can be bypassed
such that hardwired logic feeds the contents of the various counters directly onto the
telemetry bus. However, in this emergency mode the sampling rate would be highly
reduced. Finally, in-flight test modes enable us to verify the proper functioning of all
electronics, including monitoring of high voltages and detector thresholds as well as
the contents of the RAMs and PROMs.
4. Instrument Calibration
The entire IMS was calibrated in the ion-beam calibration system at the University
of Bern. This facility has been used to calibrate mass spectrometers on Geos and
several other spacecraft (Ghielmetti et at., 1983). The ion species used for IMS
calibration were H+, H2+, He+, CH3+, CH4+, Ne+, N2+, Ar+ and CO2+.
The flight and flight-spare units of the HERS and HIS sensors were tested using two
methods. The so-called 'dynamic calibration' was performed under the control of the
IMS-3 unit, with fast linear sweeps of deflection and acceleration potentials and the
usual data encoding. Dynamic runs yielded low detector count rates because the
acceleration voltage sweeps through a wide range compared with the sensor energy
window and hence the duty cycle is very low. Such runs are, however, needed for
the allocation of voltage sweep bins to ion energies.
In addition to the dynamic calibration, a 'static calibration' was performed in which
IMS-3 was replaced by ground-support electronics built specifically for these tests.
With the static method, only DC levels of the instrument potentials could be command-
ed, but this allowed us to step each voltage through any arbitrary range of values. In
this way, the optics of the sensor were investigated, and voltages were optimized (e.g.,
the HERS ESD and acceleration ranges). Moreover, response as a function of the
energy and direction of the incoming ions could be measured in a very short time for
each selected level of the acceleration/deceleration potential.
During static calibrations, the beam energy was linearly wobbled over the energy
windows of the two sensors. The wider of the two orthogonal angle windows (Fig. 2)
was scanned linearly by rotating the turntable on which the sensor was mounted. The
narrower of the angle windows was stepped through in a sequence of runs.
Figure 13 shows the angular response of the HERS for H2+ ions with a mean
energy of 2420 eV (i.e. with 1330 V acceleration), integrated over the energy win-
dow. The sensor response function was found to depend on the direction of ion in-
cidence in a well-behaved manner. The modulation of the response function in
Figure 13 is produced by potentials from the individual acceleration grid wires. High
deceleration voltages tend to cause stronger modulation.
The angular response of the HIS mass analyzer for 20Ne+ ions is shown on
Figure 14. The ion energy was wobbled with an amplitude of 13 eV (corresponding
to a rather cold ion beam in the case of the rammed cometary ions for which HIS is
intended). The average energy corresponded to the relative velocity during encounter.
This run was performed in order to simulate the response of CEM 4 (nominally allot-
ted to 18 amu/e) for water ions. Counts in CEM 5 adjacent to CEM 4 can be seen
at the extreme epsilon angles.
Figure 15 demonstrates the mass resolution of the HERS for CH3+ and CH4+ ions.
The figure shows the distribution of CH3+ and CH4+ counts over mass anodes 1 to
40 for one angle anode. With the ion beam filling the energy and angle windows, the
mass peak width is approximately two anodes (FWHM). This width is nearly constant
for all ions in all mass ranges and our estimate of the HERS mass resolution is
M/deltaM=25. Note that the contribution of one peak to the centre of the neighbouring
peak is less than one percent.
The mass resolution of the HIS is illustrated in Figure l6. This display of several
dynamic runs shows counts accumulated in each mass/energy bin when unidirectional
beams of CH2+, CH3+, CH4+, 20Ne+, and 22Ne+ arrived at the HIS from the ram
direction with the encounter velocity. Each peak appears in its predicted bin. Counts
are normalized to the same peak value, whereas no counts were obtained in any of
the blank areas. The latter symbolize mass/energy bins transmitted, whereas shaded
areas indicate bins not transmitted (see Section 3.1.5 for the data-compression
scheme). Mass resolution is clearly sufficient to identify, for relatively cold ion
Figure 13. Angular respnose of the HERS
MCP for H2+ ions. The beam energy of
2420 eV is wobbled to flll the energy window.
This run simulates the response to 4He**2+ ions
of 4840 eV. Contours in the lower panel show
20, 40, 60, and 80% of maximum response.
Phi (azimuth) and lambda (elevation) angles
correspond to the narrow- and wide-view angle
shown in Figure 2 for the HERS. The upper
panel gives the MCP response integrated over
eight angle anodes and phi. The integrated
response over both angles (geometric factor) is
~ 9 X 10**-3 cm**2 ster eV/e for this acceleration
voltage
Figure 14. Angular response of the HIS mass-
anlyser for 20Ne+ ions. The beam energy was
440 eV with a wobble of 13 eV (peak-to-peak).
Contours are plotted at logarithmic intervals
over two decades. Alpha and epsilon
correspond to the narrow- and wide-angle of
the shaded field of view of the HIS in
Figure 2. The origin (alpha, epsilon = 0) is the
cometary ram direction
Figure 15. HERS mass separation: composite
spectrum for CH3+ and CH4+ measured in
sequence. The integrated response of all 4O
mass anodes is shown for ion beams filling the
energy and angle windows. Thin and dashed
lines show the response from the two
individual runs
Figure 16. HIS mass separation: composite
spectrum of five ion species with beam
energies of 24.5 eV/amu (i.e. encounter
energies). Energy wobble was 13 eV (peak-to
peak, equivalent to one energy step). Counts
are normalized to the number counted in the
nominal E/Q bin of each ion species. No data
are transmitted for the shaded areas
Figure 17. Colour spectrogram of solar-wind
data collected on 24 October 1985 summed
over 128 spins in the light ion range (i.e. data
were averaged over a total period of 34 min).
The frame on the left of the figure is a plot of
energy channel number (vertical axis, with ion
energy increasing downward) against mass
channel number (horizontal axis), with the
count rate indicated by the colour scale shown
to the right. The mass scale corresponds to the
light ion range (M/Q=2 to 4 from left to
right). The black curved lines are lines of
constant velocity over the mass range and
correspond (from top to bottom) to 70,200,
400, and 600 km/s. These same velocities are
separately marked for the protons (the left-
most band labelled P) with crosses. The right
panel shows the ion angular distribution
excluding protons. Note the proton (red and
black) and 4He2+ (blue and green) peaks at
about equal velocities of 540 km/s
beams, ions in the water range separated by 1 amu/e.
These static and dynamic runs define the IMS response functions for the ion species
and acceleration/deceleration levels actually tested. For other species and for beam
energies and directions not measured directly, we are developing simplified physical
models to predict the response functions for both instruments.
5. In-Flight Performance
The first in-flight operation of the IMS occurred on 7 September 1985 and showed
nominal performance. At this time the solar aspect angle at the spacecraft precluded
solar-wind measurement by the HERS, so that only the general health of the IMS could
be assessed. Operation on 9 October 1985 showed the solar wind beginning to enter
the HERS field of view (with nominal solar aspect angle 105deg). Both protons and
He++ were observed by the HERS. Owing to its more restricted energy and mass
ranges, the HIS sensor does not respond to the solar wind. By 24 October the solar
wind was well within the HERS field of view (solar aspect angle was 120deg), and we
present results of preliminary analysis of HERS data from this day. The HIS sensor
electronics were fully exercised during several periods and performed well. Detector
background is low and calibrations showed both detector thresholds and analyzer plate
voltages to be nominal.
Figure 17 is a colour spectrogram of data collected during 128 spins (512 s). The
flux peak (blue and green) at the lower left shows the presence of ions with M/Q = 2,
which we identify as solar wind 4He2+. The maximum count rate averaged over the
128 spins was about 32 s**-1 at an energy corresponding to a velocity of 540 km/s for
M/Q=2. The width of this peak corresponds to five mass channels, as was the case
for the laboratory calibration spectrum shown in Figure 15. The vertical extent is a
measure of the energy distribution, i.e. temperature, of the ions. Note that the high-
velocity tail of the distribution is apparently cut off at the high-energy limit of the
HERS.
The instrument is programmed in such a way that integration times for high energies
(above E/Q channel 32) are longer than for low energies. In the colour plot this pro-
duces a systematic energy dependence in the appearance of the background count rate.
The vertical band on the left labelled 'P' shows proton count rates averaged over
the 128 spin period. Protons (collected by the CEMs) are not on the same mass and
velocity scales as the other ions (collected by the MCP), but are shown together for
convenience. The averaged maximum proton count rate is > 1.6 X 10**4 s**-1; the peak
instantaneous count rate was, in fact, about 2.5 x 10**5 s**-1. The peak of the distribu-
tion occurs at an energy corresponding to a proton velocity of about 500 km/s.
The frame to the right in Figure 17 shows the ion count rate in angle space, with
32 azimuth bins on the vertical axis and 8 elevation angle bins (each 7.5deg wide) on
the horizontal axis. It can be seen that the ions are contained within three azimuth bins
and are fully within the elevation angle field of view.
On 31 October, the solar-wind velocity had decreased to ~400 km/s, and thus ions
up to M/Q ~3 arrived in the HERS energy range. Figure 18 is a mass spectrum
measured on this day, showing the total raw counts accumulated in the angle range
containing elevation channel 1 and azimuth channels 15 to 17. The spectrum has not
been corrected for background (corresponding to an average of about 3 counts per
mass channel), nor was the response function of the instrument taken into account.
The theoretical positions of the most abundant solar-wind ions in this M/Q range
are indicated. The spectrum fully agrees with previously obtained solar-wind M/Q
spectra, even in detail (Kunz et al., 1983). The valley between He2+ and O7+ cor-
responds to the resolution expected from HERS calibration data. Whereas O6+ is
clearly isolated, the O7+ peak shows the well-known shoulder that is composed
mainly of C5+ and Ne8+.
Figure 18. Solar-wind mass spectrum from 31
October 1985 taken with the MCP detector
(light ion range). Counts are integrated over
500 spins (i.e. the total sampling period was
130 min) for one elevation and three azimuthal
channels. Raw data, uncorrected for
background, are given
Figure 18 demonstrates that the HERS can meet its scientific goals, namely those
of measuring solar wind and hot cometary ion composition with good mass resolution.
It is important to note that this resolution is maintained for high ion temperatures,
similar to those expected in the interaction region of the comet. Although no scientific
data has yet been returned from the HIS, engineering and calibration data lead us to
expect that the HIS scientific goals will be met as well.
Acknowledgements
IMS represents the work of many people over a period of more than four years. For
fear of leaving anyone out, we do not mention any individuals, but rather thank here
all the technicians at the several institutes who have contributed to the IMS. Because
of Giotto's extremely tight schedule, the effort was sometimes at the limit of what can
be expected even from a dedicated collaborator.
We thank the Giotto Project Team, in particular D. Dale, J. Credland, C. Berner
and H. Bachmann, as well as the Project Scientist, R. Reinhard, for their patience and
continuous support in solving some difficult problems.
We also wish to thank our co-investigators, H. Bridge, W.T. Huntress, W.-H. Ip,
R.G. Johnson, R.D. Sharp and E. Ungstrup for stimulating discussions and contribu-
tions during the definition phase of the instrument. In particular, we thank A.
Ghielmetti for his invaluable advice on ion optics and calibration, and W. Lennartsson
for design of the electrostatic mirror. One of us (DTY) gratefully acknowledges the
help of M.J. McCurdy and S. Trujillo for careful preparation of the manuscript.
We also acknowledge the support of our funding agencies, the US National
Aeronautics and Space Administration (Contracts NASW-3572, NASW-3729 and
NAS7-9 18); the German Bundesministerium fur Forschung und Technologie (Con-
tract 0l 0F 0323) and the Max-Planck Society; the Swiss National Science Foundation
(Grants 2.034.81 and 2.007.83) and the State of Bern; and the US Department of
Energy.
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