Instrument Overview
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      The dust mass spectrometer measures the chemical
      composition, the size and the spatial density of
      solid particles using a time-of-flight technique,
      with particular emphasis on the determination of
      the Li, C and B isotopic ratios.  The operating
      Principle of PUMA, illustrated in Figure 8a, is
      similar to that of the PIA instrument flown on
      Giotto.  The dust particles enter through a baffle
      and impact on a silver target (M) at a speed of 78
      km/s.  The particles and a certain amount of the
      target material are vaporized and partly ionized.
      The two Vega spacecraft have different targets; one
      type is mounted in a cartridge as in the PIA, the
      second has a corrugated surface such that a larger
      amount of projectile ions enter the analyzer.  The
      target is at + 1020 V; the ions are accelerated by
      a grid (1), which is held at a potential of -2000
      V, and enter the field-free drift tube at zero
      potential (4).  These charged particles are sent by
      the electrostatic reflector (5) into the second
      drift tube (6) and on towards the detector (7).
      The ions trajectories are focused by the lenses
      (9), (10) and (11).  A set of three electrodes
      (12), consisting of an inner grid at +1000 V
      between two grids at zero potential, prevents ions
      with energies less than 1 keV from reaching the
      detector.  The geometry of the reflector is
      designed in such a way as to bunch ions of the same
      species (particles with energies E>E(0) travel a
      larger distance than those with energies E