Instrument Overview
    ===================
 
      The instrument includes six detectors: two ion
      analysers, one electron analyser, two collectors
      which measure the integral plasma ion flux, and one
      sensor which monitors the integral electron current
      produced from its surface by photon and particle
      bom- bardment.  The ion detectors are designed for
      the study of the energy distribution of the solar
      and cometary particles, and not for the
      investigation of their three- dimensional velocity
      distribution.  Each sensor consists of a
      quadripolar electrical lens (2), a hemispherical
      plate analyser (3) and a channeltron (4).  The
      energy spectrum is swept once per second by
      stepping the bias voltage U(A) applied between the
      plates.  The detector oriented towards the Sun has
      a conical of view (1) with a half angle of 15deg,
      wide enough to detect the solar wind both upstream
      and downstream of the cometary bow shock.  The ion
      detector oriented along the spacecraft-comet
      velocity vector is intended for operation in the
      innermost part of the coma, where the cometary ions
      are expected to have thermal velocities that are
      negligible with respect to their relative drift
      velocity of 78 km/s.  This system can therefore be
      used as a mass spectrometer in the range 1-110 amu
      with a resolution m/delta m = 20; the minimum
      measurable ion density is 10**-3/cm**3.  The
      electron analyser has an aperture of 0.03 cm**2, an
      angular resolu- tion of +/-2.5deg, and an energy
      resolution of +/-5%; its geometrical factor is
      10**-5 E (keV) (cm**2.sr.keV).  Its aperture is
      oriented perpendicular to the Sun and relative-
      velocity directions in order to eliminate
      interferences generated by photon and particle
      impacts.  The energy spectrum is scanned by
      applying a series of logarithmically distributed
      voltage steps U(A) to a cylindrical deflection unit
      (1).  The entrance of the instrument is controlled
      by a collimator (2) and a grid (3); the detector is
      a channeltron (4).  The integral detector which
      measures the solar-wind ions consists of a
      collimator, a number of grids to limit the flux of
      photoelectrons, and a collector.  A potential of
      +3500 V is periodically applied to one of the grids
      in order to stop the ion flux and evaluate the
      interference background.  The aperture has an area
      of 5 cm**2 and the dynamic range of the
      measurements extends from +/- 10**-11 A to +/-
      3x10**-9 A.  The integral detector, which is
      aligned with the relative-velocity vector, has
      similar features, but the electrodes are configured
      such that the incoming dust par- ticles and neutral
      molecules can only impact on the first diaphragm
      and on the collec- tor.  By biasing the electrodes
      differently, in an alternative configuration, it is
      possible to discriminate between the cometary ions
      and the charged particles produced by various
      impact ionisation processes.  The impact plasma
      monitor is a gold electrode, biased at a fixed
      potential of -17 V, which measures the integral
      flux of electrons emitted by the impact of neutral
      gas, dust particles and photons, in the range
      10**-10-3x10**-5 A.  The characteristics of the
      various detectors are summarised in the table
      below.
 
 
 
---------------------------------------------------------------------
Table. Plasma Energy Analyser characteristics
 
1. Channeltron detector
 
Particles            Solar ions        Cometary ions  Electrons
---------------------------------------------------------------------
Pointing direction   Sun               Relative        Perpendicular
                                       velocity        to Sun and relative
                                                       velocity
Energy range         50 eV-3.5 keV     15 eV-3.5 keV   3 eV-10 keV
                                                       3 eV-30 eV
Points per spectrum  60                120             30
Time resolution      1 s               1 s             1 s
Aterture             15deg half cone   15deg half cone 5deg X 10deg
---------------------------------------------------------------------
 
2. Electrode detector
 
Particles            Solar ions        Cometary ions  Emitted Electrons
-----------------------------------------------------------------------
Pointing direction   Sun               Relative        Relative
                                       velocity        velocity
Energy range         Integral          Integral        Integral
Points per spectrum  1                 1               1
Time resolution      0.125 s           0.125 s         1 s
Aterture             45deg half cone   8deg half cone  90deg half cone
 
 
Instrument summary
 
The PLASMAG-1 instument package included six different sensors:
A0 plasma impact detector for measuring neutral particle flux,
   (manufactured by R. Grard, ESA/ESTEC, Noordwijk, The Netherlands)
A1 Faraday cup for measuring integral ion flux from solar direction,
A2 Faraday cup for measuring integral ion flux and neutral particles
   from ram direction,
A3 spherical electrostatic analyzer for measuring ions from ram
   direction in the energy range of 15 eV - 3.6 keV in 120
   logarithmically spaced channels,
A4 spherical electrostatic analyzer for measuring ions from solar
   direction in the energy range of 60 eV - 30 keV in 60
   logarithmically spaced channels,
A5 cylindrical electrostatic analyzer for measuring ions perpendicular
   to the ecliptic plane in the energy range of 2 eV - 10 keV in
   30 logarithmically spaced channels.
 
 
      Scientific objectives
 
      The electrostatic ion analyzers A3 and A4 were
      designed to study the change of the energy
      distribution of the solar wind and cometary ions as
      a function of the distance from the nucleus.  The
      detector oriented in the ram direction could
      determine the mass distribution of cometary ions
      close to the nucleus with a resolution of
      m/dm=20-25.  The ram Faraday cup A2 had four modes
      of operation.  Two of these provided information on
      the neutral particle flow from the comet.
 
      For further informations on instrument overview and
      scientific objectives (also first results) see the
      following papers:
 
      1) Gringauz et al., The Vega probe instrument for
      measuring charged particles with energies less than
      25 keV, in Cometary exploration Vol.  III., ed.
      by T.I. Gombosi, pp. 333-350, Central Res.Inst.
      Physics, Budapest, 1982.
 
      2) Gringauz et al., First in situ plasma and
      neutral gas measurements at Comet Halley,
      Nature 321, 282-285, 1986.
 
      3) Apathy et al., Plazmag-1 experiment, in
      Exploration of Halley's Comet, ESA SP-250,
      Vol. I., ed. by B. Battrick, E.J. Rolfe, and
      R. Reinhard, pp. 65-70, Paris, 1986.
 
 Operational considerations
 
 The sensitivity of the electron analyzer A5 was very low.
 Detectors A3 and A5 of Vega-1 were fatally damaged during the cruise
 phase.Detector A3 of Vega-2 was fatally damaged 2.5 min before
 closest approach. The channeltrons of A3 were saturating at higher
 count rates.
 
 Calibration description
 
 The sensors were calibrated at the Max-Planck-Institut for Aeronomy
 in Katlenburg/Lindau, Germany. The viewing angle and energy resolution
 of the electrostatic analyzers were determined in vacuum chamber
 using monoenergetic ion and electron beams, respectively.
 (For further details see reference #3 above.)
 The spectrometers used Soviet-made channeltrons as particle detectors.
 An in-flight calibration was performed every day to select the
 optimum voltage for the channeltrons.
 A post-launch calibration was performed in order to correct the saturating
 count rates of detector A3 of Vega-2 using the integral fluxes provided
 by the ram Faraday cup A2.
 
 
 Principal investigator:
     K.I. Gringauz, Space Res. Inst., Moscow,
 
 Co-principal investigator:
     T.I. Gombosi, Cent. Res. Inst. Physics, Budapest,
 
 Co-investigators:
     A.P. Remizov, M.I. Verigin, L.I. Denchikova,
     A.V. Dyachkov, I.N. Klimenko, G.A, Vladimirova,
     Space Res. Inst., Moscow
     I. Apathy, T. Szemerey, S. Szendro, A.J. Somogyi, K. Szego,
     M. Tatrallyay, A. Varga, Cent. Res. Inst. Physics, Budapest,
     E. Keppler, Max-Planck-Inst. Aeronomy, Lindau, Germany.
 
 
 
      Instrument section
 
      Field of view of the detectors:
      A1: 90x90 deg (centered on solar direction),
      A2: 26x26 deg (centered on ram direction),
      A3: 32x12 deg (centered on ram direction),
      A4: 36x32 deg (centered on solar direction),
      A5: 7x7 deg (perpendicular to ecliptic plane).
 
      Effective apperture:
      A1: 1.6 cm**2
      A2: 0.93  cm**2
      A3: 0.0193 cm**2
      A4: 0.00455 cm**2
      A5: 0.0036  cm**2
 
      Energy resolution:
      A3: 3.2 %
      A4: 3.85%
      A5: 7.5 %
 
      One complete spectrum was taken by A3, A4, and A5 in every
      second during the BRL (high telemery rate) seance.
      Sampling times for each energy channel:
      A3: 0.005 sec
      A4: 0.005 sec
      A5: 0.021 sec
 
      During the cruise phase data were recorded by an on-board
      magnetometer at lower telemetry rate.