It’s hard to overstate how much the world would be changed by success in net energy generation by nuclear fusion. A mini-Sun, on Earth, on demand. But every time you hear about nuclear fusion, it seems the timeline for it just got extended again. It’s always 20 or 30 years away, isn't it?
Well, here’s a refreshing change, to say the very least: The Journal of Plasma Physics has just released a special issue stating, via 7 papers, involving 12 different research teams, that due to recent advances in theory, design, and materials, the first working demonstration of net energy production by nuclear fusion, with no need to add external heat once it gets going, has been moved up by a decade. It’s now projected for 2025.
Joe Biden will be presidin’ as this thing fires up right here in the U.S. Sheesh, it’s near enough that you’ll still own some of the same pairs of underwear. (OK, er… I will, anyway.) The gauntlet has been thrown down!
"Virtually all of us got into this research because we're trying to solve a really serious global problem," said study author Martin Greenwald, a plasma physicist at MIT and one of the lead scientists developing the new reactor. "We want to have an impact on society. We need a solution for global warming — otherwise, civilization is in trouble. This looks like it might help fix that."
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Overall, Greenwald says, the work that has gone into the analysis presented in this package of papers “helps to validate our confidence that we will achieve the mission. We haven’t run into anything where we say, ‘oh, this is predicting that we won’t get to where we want.” In short, he says, “one of the conclusions is that things are still looking on-track. We believe it’s going to work.”
It’s a good sign that the research teams behind this said back in 2015 that they might be able to achieve it within a decade, by 2025 — and here we are in 2020, and they haven’t changed the timeline. The reactor will be called SPARC, and construction begins in June.
Up to now we’d been told that France’s gigantic ITER reactor was our first legitimate shot at this, and that the first real demonstration there couldn’t happen until at least 2035.
So … what happened all of a sudden?
The single most important thing is that recently, materials that superconduct at reasonable temperatures have become more manufacturable and available. That means higher magnetic fields can be generated under reasonable conditions, which in turn means fusion reactors can be much smaller, cheaper, and easier to run and maintain.
To keep fusion going here on Earth, where we don’t have the crushing gravity of the interior of a star, you’ve got to heat up a plasma (gaseous charged particles) to millions of degrees and keep it tightly contained with a strong magnetic field so it doesn’t just expand out aimlessly. Those nuclei need to keep colliding, and hard, to keep the chain reaction going, so they have to stay close together and move around fast:
I covered the basics and challenges of the nuclear fusion reactor, or tokamak, awhile ago — and in a fun way, I hope — so I won’t go back over all that again here.
But the main thing you need to do is induce a strong magnetic field within the reactor. That forces the plasma to zoom around the reactor — an electric current — and it also keeps the plasma contained, instead of hitting the walls and losing all its heat and fizzling out.
You induce that magnetic field by running strong electrical currents in wires wrapped around the tokamak, and a superconductor helps you do that by conducting that current with essentially no energy loss. The more you can juice up the magnetic field, the smaller your reactor can be and yet achieve the same energy gain. ITER’s reactor will be about 40 feet across, while SPARC will only need to be about 12 feet:
The key to the whole thing is these high-temperature superconductors (HTS), so let’s compare what ITER is using to what SPARC will use and see what the big difference is.
The best superconductor that was out there when ITER was designed was niobium-tin (Nb3Sn). Here’s a giant spool of that, ready to be incorporated into an ITER electromagnet.
ITER proudly states that they will use enough of this wire to circle the Earth at the equator twice, but I‘m not so sure that’s a good thing. SPARC will use rare-earth barium copper oxide (REBCO), which is a bit brittle, more of a ceramic material than a metal, so it’s pressed into tape:
REBCO has two huge advantages over niobium-tin:
1) it superconducts at the temperature of widely available liquid nitrogen (77 Kelvins, or about −321.1°F), while niobium-tin requires a much colder temperature (at most 18.3 Kelvins, or −426.7°F). You’d need liquid helium to accomplish that, and it costs 15 times as much as liquid nitrogen. Plus you’re running a much bigger reactor, so you’d need a lot more of it.
2) You can pound more than twice as much current density through REBCO as you can through niobium-tin before the superconductivity breaks down, and that means you can generate stronger magnetic fields within the reactor.
So what is the upshot of all of this? Both ITER and SPARC are projected to achieve a “Q” value greater than 10. That means you get 10 times as much energy out as you put in.
The “overview” paper in the collection of seven from Journal of Plasma Physics concludes:
SPARC is a compact, Q > 2 tokamak and is the next step on the path to timely and economical fusion energy. It is well into the conceptual design phase and is on track to begin construction in 2021. As shown using the conservative physics methodology outlined in this paper, SPARC has considerable margin to achieve its goal of Q > 2, and should be able to achieve a fusion gain of Q≈11 with nominal assumptions. The nominal Q≈11 discharge is well within the burning plasma regime and likely provides an opportunity to study various aspects of burning plasma physics. This high performance is possible in a compact device due to the high magnetic fields enabled by HTS magnets. The high field of SPARC allows for high current and high density, placing SPARC in a unique operating regime where typical density and β limits do not constrain high-performance operation.
Burning plasma!
That is a self-sustaining mini-Sun, made by us little people. Maybe we will save ourselves yet….