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.
 
 
 
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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)
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