As I recall the actual limitations where two fold, it's a research device so they did not want the extra radiation produced from an actual DT reaction so they stuck with DD.  And it was cheaper to build it as a pulsed device and let it cool down, vs having an active cooling system.<p>From what I can tell the problem over the last 25 years with fusion is people keep saying what is the least amount of money we can spend and still make something useful. But, actually building a fusion reactor costs ~20 billion so we have been stuck prototypes when we need to shoot for the moon.<p>EDIT: Taking JET's design (<a href="http://en.wikipedia.org/wiki/Joint_European_Torus" rel="nofollow">http://en.wikipedia.org/wiki/Joint_European_Torus</a>)  and building a 10GW reactor would actually be easier than trying to build a 1GW reactor with the same basic design.<p>Also <a href="http://en.wikipedia.org/wiki/JT-60" rel="nofollow">http://en.wikipedia.org/wiki/JT-60</a>  "During deuterium (D–D fuel) plasma experiments <i>in 1998</i> plasma conditions were achieved which would, if the D–D fuel were replaced with a 1:1 mix of deuterium and tritium (D–T fuel), have exceeded break-even"