OBJECT                         = INSTRUMENT
INSTRUMENT_HOST_ID             = VEGA1
INSTRUMENT_ID                  = PUMA

OBJECT                         = INSTRUMENT_INFORMATION
INSTRUMENT_NAME                = "Dust Impact Mass Analyzer"
INSTRUMENT_TYPE                = "MASS SPECTROMETER"
INSTRUMENT_DESC                = "

\v
  Instrument Overview
  ===================
\v

  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<E(0)) and to eliminate those with energies that deviate too much from E(0).                                            
The purpose of this mirror system is to reduce the dispersion of the flight times for                                             
particles having the same charge-to-mass ratios. The reflector potential is switched                                               
between 1000 V and 1100 V every 30 s, in order to compensate for the higher                                                       
energies of ions created by the impacts of larger particles.                                                                      
  The time-of-flight is measured with reference to three signals: the light flash
recorded by the photomultiplier (3), the pulses detected by the target (M), and the 
accelerating grid (1). The signals induced in these sensors, in the first lens (9), in the                                          
ejecta trap (8) and in the detector (7), which are illustrated qualitatively in Figure 8c,                                        
are used to characterize the mass of the particles.                                                                               
  Additional information about PUMA is given in the table below.

Table. Dust Mass Spectrometer characteristics   

Target material                           Silver                           
Total drift length                        1 m  
Time of flight                            4 micro s (H+) - 40 micro s (Ag+)  
Ion detector                              Secondary electron multiplier
Dust mass range                           3x10**-16  -5x10**-10 g    
Atomic mass range                         1-110 amu                  
Mass resolution, m/delta m, at m = 107    200                        
Chemical composition accuracy per spectrum 10%                        
Maximum data acquisition rate             12 spectra/s               
Expected number of events                 10**3-10**4 impacts during flyby
"
END_OBJECT                        = INSTRUMENT_INFORMATION                      

OBJECT                            = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID                  = "ESA-SP1066"
END_OBJECT                        = INSTRUMENT_REFERENCE_INFO

OBJECT                            = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID                  = "NATURE321"
END_OBJECT                        = INSTRUMENT_REFERENCE_INFO

END_OBJECT      = INSTRUMENT                   
END