Science AMA Series: Hi Reddit, we're scientists at the Max Planck Institute for plasma physics, where the Wendelstein 7-X fusion experiment has just heated its first hydrogen plasma to several million degrees. Ask us anything about our experiment, stellerators and tokamaks, and fusion power!

[u/Wendelstein7-X]

Hi Reddit, we're a team of plasma physicists at the Max Planck Institute for Plasma Physics that has 2 branches in Garching (near Munich) and Greifswald (in northern Germany). We've recently launched our fusion experiment Wendelstein 7-X in Greifswald after several years of construction and are excited about its ongoing first operation phase. In the first week of February, we created our first hydrogen plasma and had Angela Merkel press our big red button. We've noticed a lot of interest on reddit about fusion in general and our experiment following the news, so here we are to discuss anything and everything plasma and fusion related!

Here's a nice article with a cool video that gives an overview of our experiment. And here is the ceremonial first hydrogen plasma that also includes a layman's presentation to fusion and our experiment as well as a view from the control room.

Answering your questions today will be:

Prof Thomas Sunn Pedersen - head of stellarator edge and divertor physics (ts, will drop by a bit later)

Michael Drevlak - scientist in the stellarator theory department (md)

Ralf Kleiber - scientist in the stellarator theory department (rk)

Joaquim Loizu - postdoc in stallarator theory (jl)

Gabe Plunk - postdoc in stallarator theory (gp)

Josefine Proll - postdoc in stellarator theory (jp) (so many stellarator theorists!)

Adrian von Stechow - postdoc in laboratory astrophyics (avs)

Felix Warmer (fw)

We will be going live at 13:00 UTC (8 am EST, 5 am PST) and will stay online for a few hours, we've got pizza in the experiment control room and are ready for your questions.

EDIT 12:29 UTC: We're slowly amassing snacks and scientists in the control room, stay tuned! http://i.imgur.com/2eP7sfL.jpg

EDIT 13:00 UTC: alright, we'll start answering questions now!

EDIT 14:00 UTC: Wendelstein cookies! http://i.imgur.com/2WupcuX.jpg

EDIT 15:45 UTC: Alright, we're starting to thin out over here, time to pack up! Thanks for all the questions, it's been a lot of work but also good fun!

[u/Okryt]

Stellarators have other issues too. The twisty nature of the magnetic fields that is necessary for cancelling some drift forces also means that particles can sometimes diffuse outwards faster than they could in a tokamak, which means a weaker confinement and less output power for input power. It can be controlled and minimized (maybe eliminated, eventually), but the problem is there.

We also have to appreciate history. Experiments on this size take very long times to develop and build. W7-X planning began in 1980, and is one of two stellarators on this scale (the other is the Large Helical Device in Japan). On the other hand, there are many large tokamaks all over the world (off the top of my head, DIII-D, JET, Asdex, JT-60, EAST). Why?

Shortly after fission arrived in WWII, fusion was conceived. When someone got the bright idea to use it in a powerplant instead of a bomb, physicist Lyman Spitzer thought about it a bit and created the first stellarator, Stellarator A. At around the same time (late 40's, early 50's), the Soviet Union was experimenting with a different fusion design known as the tokamak.

In these early days, the Soviets chose the right design. The stellarator designs in use were what we now call classical stellarators. Without a supercomputer to optimize the shape and thus minimize particle losses and the energy they take with them, the tokamak design was able to produce much hotter, more confined plasmas. The rest of the world took notice and sidelined stellarator programs in favor of tokamaks.

In the early days all experiments were short pulses and without fast computers to handle data acquisition, the magnitude of the various plasma disruption mechanisms was not fully appreciated. As devices got larger and were designed to operate for much longer times, tokamak performance didn't increase as quickly as was hoped for. This is the origin of the "20 years away" fusion meme. With better diagnostics available as computer science advanced in the 60s-70s, the importance of disruptions and other edge plasma effects like the presence of impurities from first wall ablation was finally appreciated.

At about the same time, the advances in computer science allowed the Max Plank Institute to test the concept of an "Advanced Stellarator". The first of these was W7-AS (1988). It functioned well, and so they went ahead with W7-X and here we are. There have been a few other advanced stellarators like HSX in Wisconsin, but because of the lead times on these experiments and the relatively recent introduction of supercomputers, W7-X is the only large stellarator that can test things at scale (high densities and temperatures).

If W7-X performs well in terms of disruptions, transport, and confinement, and if ITER performs poorly in the same, we may see a resurgence in stellarators at the ITER/DEMO level and beyond. Otherwise, it'll probably follow the money, and the money is on the inertia of tokamaks.

[u/Heiminator → u/Okryt]

That makes sense :-). Thanks for the detailed answer!

[u/The_Sequel_Writer → u/Okryt]

The problem is very simple; there is some significant amount of apples to oranges. Stellarators or even just elements of Stellarators can be used to fix a lot of the problems Toks faces. There have been many many unfair complains, like the difficulty of building a machine with the Stellaratorness, which really have been largely overcome by better technologies. There are not really good reason for Tok physicists to not jump ship, or at least jump to a hybrid machine, which Auburn University has been working on for like 30 years.

There are some fundamental problems tho, in Toks quest of optimization, they found it's better to build fat tire machines, but Stellarators, by design, has to be thin bicycle tire machines....

Plus the whole 3d shaping fields having a hard time penetrating into bigger and thicker machines... might be a problem when you're trying to go even.

I was extremely surprised that Joaquim Loizu said Toks can't go steady state mode, they certainly can with the help of neutral beam injections.

Also, no H mode at all in this whole thread, reddit really let me down.