From: dietz@dls.net   
      
   iain-3@truecircuits.com wrote:   
      
   > 2. Large ion mass.  ?  I assume any electric field you set up is   
   >    going to deliver a fixed number of electron-volts to each ion on   
   >    the way out.  Heavy ions would appear to need more eV to reach   
   >    the same exhaust velocity as lighter ions.  Why is this good?   
      
   For a fixed grid spacing, there is a limit to the current/area   
   that a single stage electrostatic acceleration can achieve, due   
   to the 'space charge' of the ions in transit across the gap   
   between the grids (basically, the charge on the ions cannot   
   exceed the charge on the grids).   
      
   For a given exhaust velocity, the voltage must be higher   
   for the heavy ions.  This is a *good thing*, since the current   
   will also be higher.  And since the mass/charge is also going   
   up, we see that the thrust density of the engine is proportional   
   to the square of the ion mass/charge ratio (for fixed exhaust   
   velocity and grid spacing.)   
      
   	Paul   
      
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