Fusion Reactor Size

(Created prior to 1993)

Question:  What are the limits to the size of a fusion reactor?  I have read 
Sci-Fi stories that include the hero carrying a portable generator and heard 
rumors about a (discontinued) experimental fusion aircraft.  I would guess 
that you must have a certain mass to keep the reaction going, but what is the 
minimum?
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The basic process of a fusion reactor has no real minimum mass.  
But the complex equipment required to keep it going and make itefficient is 
enormous.  (With present technology) The basic process just needs a few atoms 
of tritium or deuterium.  It is the equipment that brings the atoms together 
at high velocity or heats them to high temperature that is complicated and 
bulky.  This is quite different from a fission reaction, where a minimum mass, 
called the critical mass, is needed.  I think the experimental nuclear 
airplane used a fission reactor.  A  typical critical mass for a fission 
reaction would be several kilograms.  This would be very portable, if it was 
not for the shielding needed to keep from frying everybody in the vicinity.  
Plus, you need cooling and heat exchangers to use the heat that is produced.  
Both fusion and fission reactors put out heat as their primary method of 
energy production.
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Actually, so far no fusion experiment has produced any more power 
than went into heating the stuff up in the first place.  That means that not 
only do we not have portable generators, we do not have ANY fusion reactors 
that actually produce power.  There is actually a proverbial "20 year" effect 
involved - reactors planned for 20 years in the future should finally be 
producing power, maybe even commercially.  Unfortunately, it hss sort of been 
that way for the past 30 years or so.

A. Smith
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