On Thursday, July 26, 2018 at 12:26:02 PM UTC-5, David Ellis wrote:   
   > 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?   
      
   The Q factor is a measure of how much energy that you are getting out of a   
   reactor versus the amount that you are putting into the reactor to keep it   
   going.    
      
   If you have a self-sustained fusion reaction, then the energy from one set of   
   reactions can supply the energy for the next. So if the first generation of   
   reactions produces 200 times the power that you need to catalyze a reaction   
   then it can allow 200    
   times as many reactions to happen, which then release 40,000 times the initial   
   power input. The trick is the ability to keep the produced power inside of the   
   plasma long enough to allow for additional reactions to be catalyzed.    
      
   Energy leaves a plasma based on its surface area, but power produced would   
   seem to be based on its volume. So if the radius increased by a factor of 10   
   then the surface area increased by a factor of 100, but the volume by a factor   
   of 1000. This means    
   that energy will stay in the plasma longer and the temperature of the plasma   
   will rise, which will further increase the reaction rate. Confinement time   
   will also increase. These are reasons why the power output of tokamaks   
   actually scale with the    
   increase of the radius to the 4th power, not the 3rd. The power density and   
   the Q factor are therefore increased by simply building a bigger reactor.   
      
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