Welcome to today’s lesson in fusion reactors, where there are startlingly few explanations in layman’s terms! Today’s topic: divertors.
A divertor is basically an exhaust system for a fusion reactor, venting both waste heat (which can melt the machine) and waste helium (transmuted from hydrogen, which dampens the reaction). All tokamaks these days are diverted, but there’s still so much left to learn!
It’s very hard to find good diagrams that aren’t too technical, so here’s that promo picture of SPARC again. I’ve circled the divertors in red:
SPARC will have two divertors, one on top and one on the bottom. That’s called a “double-null” configuration. Many tokamaks just have a lower divertor, which is called “single-null.” Each divertor has two legs, one inner and one outer. You can see that SPARC’s divertors have super long outer legs and little stubby inner legs — I’ll talk about why in a later post!
Promo pics are all well and good, but let’s look at some real photographs! I’ve been focusing too much on American tokamaks on this tumblr, so let’s move a bit farther afield. Here are some examples from overseas:
This is ASDEX-Upgrade, a tokamak in Germany. ASDEX-U has been one of the most important experiments for divertor science! It just has a lower divertor, so it always burns single-null plasma. On the right, you can see a photograph where the plasma glows brightly as it strikes the tips of the divertor legs.
This is EAST, a tokamak in China. EAST has both upper and lower divertors, so it can run in either single- or double-null mode. It currently holds the record for longest-burning plasma, where it runs for minutes at a time! On the right you can see a timelapse of a double-null plasma, where the divertors on both the top and bottom glow.
One of the biggest challenges with divertors is heat. There are always two spots where the plasma directly strikes, and these points get hot. Very hot. Like, spacecraft reentry hot, or hotter. Building a divertor that doesn’t melt is hard. Off the top of my head, I think DIII-D in San Diego uses a graphite divertor, WEST in France is an all-tungsten machine, and JET in the UK has used tungsten and beryllium.
There’s a lot more to talk about: the “detached” and “swept” divertors at Alcator C-Mod at MIT, pros and cons of single-null and double-null, the whole deal with the long-legged SPARC divertors, and so on. I’ll cover some of that later! But for now, I leave you with one of the most baffling questions in divertor research:
Who decided to spell it “divertor” instead of “diverter?” That’s just cruel to science journalists, and doubly cruel to their editors.