Instrument Overview
    ===================
 
      This instrument detects the electric charges
      contained in the plasma cloud generated by a solid
      particle impacting on a gold target.  The principle
      of this technique is similar to that adopted for
      the impact plasma detectors of SP-2.  The magnitude
      of the positive and negative charges is
      proportional to the mass of the particle Q=Am,
      where A= 10**3C/s for an impact velocity of 78
      km/s.  The SP-1 system includes two similar
      detectors.  Each unit is made up of a base plate
      (1) covered by a gold target (2) at zero potential,
      perpendicular to the dust flow, and an array of
      strip collectors (4).  The collectors are parallel
      to the dust flow; they are mounted on an insulator
      (3) and their edge is protected from impacts by a
      shield (5) connected electrically to the structure.
      Adjacent collectors are biased at potentials of 30
      V, with opposite polarities.  The two sets of
      electrodes in each detector detect a positive and a
      negative current pulse, which are analysed and
      recorded by the electronics unit.  The entrance to
      one detector is covered by a plastic foil, to
      obtain additional information on density and/or
      calibration factor.  The characteristic parameters
      of SP-1 are given in Table 8.
 
 
 
 
 Table 8. Dust Particle Impact Plasma Detector
 characteristics
 
 Target material               Gold (0.1 mm)
 Toil sensor area              160 cm**2, one sensor covered  with 0.6
                               and 2 micro m thick plastic foil
                               on Vegas-1 and 2, resp.
 Impact charge range          3x10**-14  -10**-8 C
 Estimated mass range         3x10**-17  -10**-11 g
 Integration time             2 s (high data rate)
                              2.5 min (low data rate)