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.
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.
Also http://en.wikipedia.org/wiki/JT-60 "During deuterium (D–D fuel) plasma experiments in 1998 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"
I would like to argue that prototypes have been what's called for. We don't even have a clear idea of what a commercial fusion reactor would look like. The cooling and tritium production issues, the structural materials, and the lifetimes required of the super cooling and superconducting systems, are massive problems that haven't really been solved yet.
It's possible that replacing D-D with D-T would have yielded break-even for a few seconds, (I've heard that claim before too) but I'm very skeptical; increasing the internal heat generation is sure to change the plasma conditions: for example, there would be a lot of adiabatic expansion, which, even restrained against the tokamak fields, is liable to have a variety of unstable modes.
I feel that with a modular design that includes remote handling capability you could test things out a lot faster on a full scale device. As to internal heating issues, I don't think there is much problem extrapolating from Q0.7 at JET to Q1.2 at JT-60.
Large Plasma devices are "the sexy" but IMO it's just another engineering problem at this point. And you don't design large scale power plants to sit at the outer edge of their capabilities 24x7. So yes, the highest energy pulses are unable after a few seconds, but we have built steady state fusion reactors they just operate further from their limits.
PS: Finding T is a problem, but "just build it bigger" and suddenly D-D (or 75%D - 25%T etc) becomes reasonable.
Edit: I am suggesting building building something with 4x ITER's budget which should give you ~8x the plasma volume and a lower surface area to volume ratio etc. We can also pump of the field strength etc. But you are limited to how much heat the wall can take so the plasma conditions don't need to be as efficient.
I do agree that the problem of controlling Q > 1 plasma becomes easier as you increase the volume. However, I don't think that's the biggest problem. As you yourself point out, you're limited in how much heat the wall can take. This is fundamentally the limiting factor in fusion power plant designs, unless a much more efficient direct conversion scheme is feasible. Why not instead take the tungsten panels you'd use and point a bunch of mirrors at it, achieving the same heat flux? Mirrors are just not that expensive.
PS: Do you work in fusion, by the way?
PPS: We should chat more. Send me a message at dfong at lightsailenergy.com
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.
EDIT: Taking JET's design (http://en.wikipedia.org/wiki/Joint_European_Torus) and building a 10GW reactor would actually be easier than trying to build a 1GW reactor with the same basic design.
Also http://en.wikipedia.org/wiki/JT-60 "During deuterium (D–D fuel) plasma experiments in 1998 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"