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This experiment achieved a stable plasma for 26 seconds.

What do we learn from it in general(why it was done)? and specifically about the possibility of achieving positive energy flow ($Q>1 $) by fusion?

user2679290
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1 Answers1

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specifically about the possibility of achieving positive energy flow (Q>1) by fusion

It tells us very little about breakeven, simply because it isn't meant to.

One can, of course, extrapolate out from given results to those that might be typical of a production machine. However, every time we've tried that in the past we failed. Invariably, when you build the machine that the last device said would work, it don't work.

The clearest example of this was the series of tokamaks that were built in the mid-1970s. Notable among these was PLT, which repeatedly generated the temperatures required for breakeven (and then some). All that was needed was to make it bigger and introduce tritium.

So the race was on. Three machines emerged, TFTR, JET and JT-60. All three set the explicit goal of hitting breakeven - JT-60 was originally the "Breakeven Test Reactor" IIRC. All three failed to get close. JET finally did a decade later. Of course if you read post-failure histories from those labs, they all state breakeven was never the goal, although with the miracle of Google Books one can trivially verify it very much was.

And then, of course, there's LLNL's history of ICF. Shiva was going to do breakeven. Oops. Nova, totally going to do it. Oops. Ok, this next one will absolutely do it, so we're going to put it right in the name, the National IGNITION Facility... Oops.

So now the designers tend to be much more circumspect. X-7 is roughly of the same generation as the PLT in terms of power and confinement and experimental goals. So there would be at least one more machine between it and something like ITER, call it the JET-a-like. And that machine may well just up and fail.

So in the end, X-7 doesn't tell us a whole lot about breakeven. It does tell us a whole lot about how to build X-8, or Y-7 or whatever they're up to now. Beyond that, all bets are off.

Maury Markowitz
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  • The results previously achieved are similar in terms of stability time? – user2679290 Feb 06 '20 at 10:43
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    Yes, I think it's roughly equal to PLT. X-7's record temp so far is 40 million, PLT hit 50 and then went some way beyond that. Confinement time is not really directly comparable because PLT worked in pulses, but it hit 80 msec compared to around 200 for X-7. So I think it's fair to say they are similar in terms of performance. – Maury Markowitz Feb 06 '20 at 14:15