-> Apart from break-even there is another problem (well, of course there are 
-> several, but this one is obvious): The most developed fusion devices operate 
-> the Tokamak principle. This involves (basically) a very large transformer co 
-> that induces a magnetic field to help confining and a current to heat the 
-> plasma. Since a current is only induced by a changing magnetic field (and th 
-> a steadily increasing drive current) a Tokamak reactor has neccessarily to b 
-> operated in pulsed mode. The length of the pulses is the limiting factor for 
-> Asdex Upgrade right now, thus the plasma burns for only ~5s. For a power pla 
-> you'll have to find a way to produce electricity in a continuous manner. But 
-> you can probably solve this problem by making the pulse duration longer and  
-> building more than one device operating in different phases (like the cylind 
-> in a car's engine). 
-> The other principle on which a fusion reactor can be built is the so-called 
-> Stellarator. This would produce energy continuously by design, but it requir 
-> a more sophisticated system of (supposedly super-conducting) coils and rathe 
-> complex magnetic fields. Thus research on stellarators is behind research on 
-> tokamaks up to now, because tokamaks are easier to build. 
  
There is a third method - laser-driven pellet implosion. Basically, a 
small solid pellet of fusion "fuel" (i.e. containing deuterium and/or 
tritium) is dropped into a vacuum chamber. At the precise moment when 
it passes through a target position, it is struck from many directions 
simultaneously by intense pulses of light from lasers. These cause 
material to evaporate from the pellet's surface, producing an inward 
reaction force, compressing the pellet and raising its temperature to a 
combination where fusion occurs. The process sounds horribly difficult, 
but in fact fusion has been achieved this way. Break-even is another 
matter... 
  
Of course, this method is also inherently pulsed. 
  
                        dow 
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