From: daellis94@gmail.com   
      
   So, I've been exposed more to the idea of 'Q' when talking about nuclear   
   fusion reactions during my time on this forum, and it has gotten me wondering   
   something.     
      
   Talking about a reactor that can achieve a Q of 10 or 100 or 500 is fine.    
   It's not hard to imagine why a Q of 500 is much more impressive than a Q of   
   10, and how it might be much harder to accomplish.     
      
   Are there known theoretical maximum values for Q for the fusion reactions   
   usually considered by folks like us?  That is, for deuterium-tritium,   
   deuterium-deuterium, helium-3-deuterium, boron-hydrogen, etc.     
      
   As far as I can tell, any fusion reaction, be it a deuterium nucleus slamming   
   into a tritium nucleus, will be giving off a certain amount of energy.  We   
   know this.  It will form a helium nucleus and eject a neutron, and we know the   
   energy each product    
   will represent.  I would imagine, as well, that, for any given pair of fusion   
   reactants, the nuclei must slam together with at least a given amount of   
   energy, otherwise the reaction will not take place, and the nuclei will simply   
   bounce off of one    
   another.     
      
   Surely, the energy released by the reaction divided by the minimum energy   
   required would give the maximum value for Q that it is physically possible to   
   achieve for any given fusion reaction.     
      
   Am I right in thinking this?  Does anyone know what such Q values are for the   
   reactions that are normally of most interest to us for science-fiction   
   purposes?  Would the theoretical maximum Q of He3/H2 fusion, for example,   
   leave a lot of room for    
   technological growth, say, with a theoretical maximum of Q=10 000 or something   
   like that, or is it much lower, maybe in the less impressive realm of a few   
   hundred?   
      
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