Instrument Overview
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      The Solar-Wind Instrument on MS-T5 employs a
      Faraday cup to measure the bulk velocity, density
      and temperature of solar-wind ions.  This
      conventional cup, mounted flush with the outer skin
      of the spacecraft, consists of four grids and\ an
      ion collector.  The front grid G(1), is connected
      to the satellite skin.  A square-wave voltage is
      applied to the modulator grid G(2), alternating at
      400 Hz between 0 and V volt.  This produces a
      pulsating current by repelling and transmitting
      those incoming particles whose energy is between 0
      and a maximum voltage V of 1.5 kV, which can be
      selected from the ground.  The voltage will be
      fixed for most of the mission, although the
      instrument has several operating modes, such as
      automatic voltage change for every spin, automatic
      fine voltage sweep between two voltages, etc.  The
      third grid is grounded to block any leakage of
      modulating voltage from G(2) to the collector.  The
      fourth grid is negatively biased at - 100 V to
      suppress photo-electrons from the collector.  The
      sensor is not sensitive to the solar-wind stream
      beyond an angle of attack of +/- 60deg.  The table
      below summarizes the sensor's characteristics.
      Both DC and AC collector currents are detected.
      The former (ion current and secondary-electron
      current), which vary during each satellite spin,
      are used to check the angular characteristic of the
      Faraday cup itself and to monitor the
      photo-electron contribution to the collector
      current.  The modulated ion current measured
      between 2x10**-12 A and 10**-8 A is sampled at 128
      points between - 90deg and +90deg with respect to
      the solar-wind direction, i.e.  every
      180deg/128=1.4deg the collector Current is sampled
      as a function of the spin angle.  The bulk velocity
      and ion density are calculated from the point where
      the slope of the collector-current versus spin
      angle function becomes a maximum.
 
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Table:  Characteristics of MS-T5 Solar-Wind Instrument
 
Transmission of the grids   G(1) G(2), G(4) : 90%
                            G(3)          : 77%
                            Overall      : 70%
 
Distance between grids      4 mm
Collector diameter         90 mm
Maximum voltage to be
applied to G(2)             1.5 kV +/- 40 V
Collector impedance        10 M Ohms (with 20 pF)
                           collector-ground
                           stray capacity
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