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INSTRUMENT_NAME = "PLASMA ENERGY ANALYZER"
INSTRUMENT_TYPE = "ELECTROSTATIC ANALYZER"
INSTRUMENT_DESC = "
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."
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