Instrument Overview
      ===================
      The ion mass spectrometer consists of two major subsystems.  One
      section, the High Energy Range Spectrometer (HERS) was designed to
      measure the ion abundances and the 3-dimensional velocity distribution
      outside the contact surface, where the ions are hot and there is
      considerable turbulence.  The other section, the High Intensity
      Spectrometer (HIS) was designed to measure the abundances of the
      low-energy ions inside the contact surface, where the ions were
      expected to have higher abundances and lower temperatures.  This
      section contained separate Mass and Angle Analyzers.  Both sections
      of the instrument were powered and controlled from a common power
      supply and control unit.
 
      HERS:
 
      HERS utilized a cylindrical, electrostatic mirror to redirect the ions
      from the direction of incidence to the entrance slit of the instrument
      and also to demagnify the range of accepted directions from 60 degrees
      in true elevation (angle with respect to the spacecraft spin vector) to
      30 degrees at the entrance slit of the spectrometer.  The 60-degree
      field of view in azimuth extends from 15 degrees to 75 degrees relative
      to the spin axis of the spacecraft and thus does not include the ram
      direction.  The field of view in azimuth (scanned by the rotation of
      the spacecraft) is 2 degrees. Cylindrical accelerating/decelerating
      grids are located immediately in front of the entrance slit which is
      3.60 mm by 1.26 mm.  Behind the slit is a 120-degree sector magnet
      followed by a second slit identical to the entrance slit.  The magnet
      is of samarium-cobalt with a pole gap of 5 mm and a field strength of
      0.335 Tesla (uniform to 0.5%).  The combination of the slits and the
      sector magnet provides a filter which selects a constant value of the
      normal component of momentum per unit charge of 7560/(M/Q) eV/e.  The
      output slit from the sector magnet is followed by an electrostatic
      deflector (ESD) which deflects ions out of the optical plane of the
      magnet towards the detectors.  Since the ions have already been
      selected for momentum per charge by the sector magnet, the sorting
      by energy done by the electrostatic deflector is equivalent to sorting
      by mass.  The ESD field is non-uniform 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 nearly straight and position along this
      line maps out the angle that ion trajectories make at the entrance
      slit. The ESD field is generated by applying voltages, via a resistive
      divider between two adjustable voltages, to a series of vane-like
      electrodes arranged appropriately inside the ESD.  The vane-shaped
      electrodes are also arranged so as to trap ions outside the range of
      M/Q which is being deflected onto the detector.
 
      The main detector for HERS is a micro-channel plate (MCP) with 50-mm
      diameter and curved channels.  The range of energies incident on the
      MCP is varied by varying the two voltages applied to the resistive
      divider.  As examples, voltages of +4667 and -667 put light ions
      (M = 2 - 4) on the MCP.  Voltages of 770 v and -110 v put medium ions
      (M = 12 - 26) on the MCP and voltages of 576 v and -82 v put heavy
      ions (m = 16 - 35) on the MCP.  A set of 4 discrete Channel Electron
      Multipliers (CEMs), each with a 5 x 12.8 mm entrance funnel, is used
      to detect protons.  They are placed such that the protons can be
      deflected with a weaker field than would be required to image the
      protons onto the MCP.  These also allow much higher counting rates
      than are allowed by the MCP.
 
      The output of the MCP is analyzed two-dimensionally with two orthogonal
      arrays of strip anodes.  An array of 8 strip anodes deposited on the
      back surface of the MCP senses the elevation angle of the ion events in
      the MCP while an array of 40 strip anodes deposited on a high-purity
      alumina plate located 0.1 mm from the output face of the MCP, senses
      the mass of the incident ion events.  The mass-anode plate is held at
      +50 volts relative to the output of the MCP.  Thus an ion entering the
      front of the MCP produces a positive pulse on one of the 8 angle anodes
      and a negative pulse on one of the 40 mass anodes.
 
      An event is recorded by the electronics whenever a pulse is produced
      at the mass anode which exceeds the threshold, which is selected from a
      set of 8 logarithmically spaced values.  An event is classed as good if
      there are coincident (within 1 microsecond) pulses at the angle and
      mass electrodes.  Events are recorded by the anode numbers on which the
      event was detected.  Multiple events within a single time window are
      flagged as multiple.  Since the incoming ions are filtered first for
      momentum by the sector magnet, the range of energies of ions of a given
      mass transiting the instrument is small (roughly +/-3% of the actual
      energy).  The instrument is scanned in energy using the accelerator/
      decelerator grids in front of the sector magnet.  The remainder of the
      instrument optics are floated at the accelerator potential.  The
      accelerating potential is swept at 8 Hz through a 4.35 kV triangular
      waveform, relative to a selectable central voltage chosen for the range
      of masses being measured.  This gives 32 complete energy scans per
      4-second rotation of the spacecraft.  Since the sweep is phase-locked
      to the spacecraft spin, two sweep phases (separated by 5.6 degrees of
      spacecraft rotation) are used alternately to avoid always measuring
      the same energy at the same rotational phase.  The mirror voltage is
      swept synchronously with the accelerator voltage.
 
      HIS:
 
      This sensor is intended to measure the ions in the inner coma, where
      they were expected to be at high densities, low temperatures, and low
      bulk velocities with respect to the nucleus.  The mass-range of this
      sensor was limited to 12 to 57 amu per charge and the velocities were
      to be measured in only a small range around the spacecraft encounter
      velocity of 69 km/s, both with respect to speed and with respect to
      direction of incidence.
 
      HIS contains both a mass analyzer (MA) and an Angle Analyzer (AA).  The
      MA uses both electrostatic and magnetic separation to give good mass
      resolution in the vicinity of M/Q = 16 - 20.  The intrinsic field of
      view is 2 degrees by 15 degrees, including the direction of the
      spacecraft spin axis.  Due to the rotation of the spacecraft, the FOV
      sweeps out a cone of half-angle 12 degrees with overlap near the center
      of the cone.  The AA is an electrostatic quadrispherical analyzer with
      5 miniature CEM detectors at the exit giving a resolution of 5 to 7.5
      degrees within a FOV of 2 by 25 degrees.  The cone swept out by this
      FOV has a half-angle of 22 degrees. The 2 by 25-degree fan contains
      five elevation-angle ranges and the azimuths swept out by the spin of
      the spacecraft are sorted into 16 bins. For five major ionic species,
      the velocity distribution around the ram direction can be inferred,
      thus providing the data necessary to interpret the data from the MA.
 
      The mass analyzer is an excellent analog to a prism spectrograph.
      After passing through an entrance slit, the divergent beam of ions is
      collimated by a quadrispherical analyzer while filtering out ions of
      all values of energy per charge other than the one selected by the
      voltages applied to the analyzer.  A permanent magnet (0.19T) disperses
      the beam according to the momentum per charge of the particles.  The
      entrance slit is then reimaged onto a detector by a second
      quadrispherical analyzer, providing a momentum-per-charge spectrum in
      the final focal plane.  The instrument is scanned in energy-per-charge
      by biasing the entire instrument beyond the first quadrisphere by a
      voltage Ub = U1 - Uc where U1 is the central energy per charge
      transmitted by the first quadrisphere and Uc is the required energy per
      charge for an ion of a given M/Q to hit the desired detector in the
      focal plane.  Ub is in the range -1400 to +1050 V.
 
      Discrete, dedicated detectors are mounted in the focal plane at the
      correct positions for ion species at 16-21 amu and also at the
      positions for the virtual ions at 17.5, 18.5, and 19.5 amu to monitor
      interference between channels.  In order to provide sufficient physical
      space to fit discrete detectors, the focal plane is imaged onto the
      surface of a prism-shaped, activated, lead-glass block.  There are 9
      rectangular holes, roughly 0.6 by 2.0 mm, the bottoms of which are
      connected to the rear of the block by straight 0.4mm-diameter channels.
      The channels are drilled at various angles so that they are well
      separated where they emerge from the block.  The funnels of specially
      fabricated CEMs are attached to the block at each hole using conductive
      epoxy.  The front and back surfaces of the block are gold-plated to
      provide conductivity and a voltage of 700 V between the two surfaces
      allows the channels in the block to act as preamplifiers for the
      discrete CEMs.  This assembly is known as der Igel' after the German
      word for the hedgehog which its shape suggests.  The high voltage to
      the CEMs can be set to any of four levels between 2.5 and 3.4 kV.
 
      The angle analyzer (AA) bridges the gap between HERS and HIS, providing
      a wider field of view than the MA and providing resolution in elevation
      so that, in the event that the ions in the inner coma have a
      significant temperature or a significant bulk velocity with respect to
      the nucleus, the data from the MA can be usefully interpreted.  The
      quadrispherical AA plates are similar to those of the first
      quadrisphere of the MA and are connected to the same voltage supply.
      The collimated beam is incident on aluminum dynodes and the secondary
      electrons are collected by a set of 5 CEMs with high voltages
      selectable in the range 2.5 to 3.1 kV.  The amplifiers on these CEMs
      can handle count rates to 2x10**6 counts per second.
 
      Science Objectives
      ==================
      The primary scientific objectives of the IMS team were:
        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.
 
      Calibration Description
      =======================
      The entire IMS was calibrated in the ion-beam calibration system at the
      University of Bern, using H+, H2+, He+, CH3+, CH4+, Ne+, N2+, Ar+, and
      CO2+. Dynamic calibration was performed under the control of the IMS
      control unit (IMS-3), with fast, linear sweeps of deflection and
      acceleration potentials with the usual encoding of data.  A static
      calibration was also performed, under control of a special ground-test
      system, which allowed stepping the potentials at discrete values.
 
      The dynamic calibration, which yielded low duty cycles because of the
      wide range of energies swept compared to the energy of the incoming
      ions, was used primarily to assign the various bins of the sweep
      voltage to values of Q/M.
 
      The static calibration was used to investigate the optics and to
      optimize voltages.  The static calibration was also used to measure the
      response as a function of direction and energy of the incoming ions.
      In the static calibration, the energy of the beam was linearly wobbled
      over the energy range of the detectors.  The wider of the two angular
      dimensions of the field of view was scanned by rotating the turntable
      on which IMS was mounted while the narrower dimension was measured by
      calibration runs at several discrete settings.
 
      Results of the calibration are described by Balsiger et al. (1986 in
      The Giotto Mission, Its Scientific Investigations, ESA SP-1077, p129)
      and by Balsiger et al. (1987 J. Phys. E: Sci. Instrum. 20, p753).
      Further details of the calibration facility are described by Ghielmetti
      et al. (1983 Rev. Sci. Instr. 54, 425).
 
      Operational Considerations
      ==========================
      IMS functioned as planned during the Giotto encounter with P/Halley.
      However, after the encounter, HERS was no longer functional and only
      HIS was operational for the encounter with P/Grigg-Skjellerup.  Because
      the axis of spacecraft spin was not coincident with the ram direction
      during the encounter with P/Grigg-Skjellerup, the range of angles which
      were sampled was not symmetric about the ram direction and this must be
      taken into account when interpreting those data.
 
        Instrument Mass		: 9.2 kg
        Instrument Manufacturer	: UNIVERSITY OF BERN
        Platform or Mounting Name   : Spacecraft Body
 
      Electronics
      ===========
      Electronics ID               : IMS-3
 
      This electronics box contains all the low-voltage electronics,
      including an isolating power converter.  It supplies all voltages
      needed to drive the various detector amplifiers, the logic, and the
      high-voltage power circuitry.  The high-voltage stepping in both
      detectors is accomplished with control voltages generated by PROMs
      which feed digital-to-analog converters. Two microprocessors, one
      each for HIS and HERS, receive and compress digitized data from the
      two sensor boxes and also control the modes of the instrument.  The
      microprocessor for HERS also controls all general IMS command and
      telemetry operations as well as all interactions of IMS with the
      spacecraft.
 
      Instrument Section/Operating Mode Descriptions
      ==============================================
 
        HERS
        ----
        This mode is intended for use during cruise and in the outer coma
        when the data from HERS are given priority over those from HIS for
        telemetry. During each 4-second spin of the spacecraft, 64 complete
        energy scans are made for a single range of masses (light, medium,
        or heavy), corresponding to 64 separate azimuthal bins, each 5.6
        degrees wide.  In successive sweeps, the energy range alternates
        between increasing and decreasing such that a given energy can be
        sampled at two different relative positions within an azimuthal bin.
        A complete cycle consists of 4 spins which are used in one of four
        different sub-modes, selected by varying the voltage applied to the
        resistive divider for the ESD: 1. protons only for 4 spins.  2.
        alternating spins of protons and light ions, 3. no protons - light,
        medium, heavy, and medium ions, and 4. all masses - protons, light,
        medium, heavy.  Sub-mode 2 is intended for cruise phase to measure
        the solar wind and sub-mode 4 is intended for encounter.  Sub-modes
        1 and 3 were intended for use if a detector deteriorated.
 
        During cruise phase, and whenever during the encounter phase HERS
        is given priority for telemetry, data are transmitted for each spin.
        Each ion event is recorded as a 24-bit word containing the anode
        numbers for mass and elevation angle, the energy and azimuth bin
        numbers, and the direction of the sweep.  For protons, instead of
        recording anode numbers, the total counts in each CEM are recorded
        for each azimuth and energy bin.  At high count rates, data
        compression is used to fit the data into the available telemetry
        rate.  In the inner coma, when HIS has priority, each of the mass
        ranges is held for two spin periods instead of one and the telemetry
        is adjusted accordingly.
 
        FOV Shape Name                 : RECTANGULAR
        Horizontal FOV                 : 2.
        Vertical FOV                   : 60.
 
 
        HIS
        ---
        In the inner coma, when HIS is given priority for telemetry, the
        complete cycle is 4 spins of the spacecraft.  The basic program
        consists of a 64-step energy scan, repeated 16 times per spin period.
        The sets of voltages for deflection, acceleration/deceleration, and
        for the quadrispherical lenses, are stored in a PROM in IMS-3.  Per
        spin, the total set of 14 CEMs yields 14366 individual count rates.
        These are reduced by compression to an array of 1004 8-bit words.
        Compression is achieved by i. summing related count rates, ii.
        omitting insignificant channels, and iii. quasi-logarithmic
        compression of the remaining count rates.  A full cycle can last 1,
        2 or 4 spins depending on IMS mode (HIS or HERS priority) and
        depending on the actual telemetry rate.
 
        For the AA, only those E/Q channels containing the most abundant ions
        (M/Q = 17, 18, 19, 28, 44) are transmitted separately for each of the
        16 azimuthal bins in a spin.  The other 59 E/Q channels are
        integrated over a full spin and then only CEM 1 (which looks in the
        direction of the spin axis) is recorded directly in the telemetry
        while the other 4 CEMs are themselves summed before transmission.
 
        The MA has two distinct modes of operation since one can choose the
        appropriate acceleration/deceleration voltage and the corresponding
        voltage on the quadrisphere to direct any given mass to any given
        position on the Igel.  In the N-program, any given mass is always
        directed to the same dedicated detector over the appropriate range
        of E/Q.  This mode thus allows one to measure velocity distributions
        with a single detector and minimize calibration uncertainties.  For
        example, in the water-group ions, mass 18 will always be directed at
        CEM 4.  The layout of the holes on the faceplate of the Igel was
        designed to optimize the detection of species in the water group.
        When other mass ranges are directed at the Igel, some masses are not
        centered on the detectors.  In the H-program, individual channels in
        the Igel are dedicated to specific velocities, centered at the
        nominal encounter velocity of 69 km/s.  In this program, all species
        at velocity 69 km/s always appear at CEM 6 (and other velocities at
        other CEMs).  This program is better suited to determination of
        relative abundances.  The choice between N- and H-programs is made
        via telemetry, choosing either all N, all H, or alternating N- and
        H-programs with a 3:1 or 1:1 ratio over groups of 4 spins.  There is
        also the option to wobble the voltage on the external deflector to
        move the field of view by 2 degrees for mass 12 (0.5 degrees for mass
        57) away from the spacecraft.
 
        FOV Shape Name                 : RECTANGULAR
        Horizontal FOV                 : 2.
        Vertical FOV                   : 15.