Reddit AMA: Fusion Power

A very interesting discussion about our future. What follows is the (edited) first page of the AMA, please click on the source link for further information.

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! : science

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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!

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

EDIT 14:00 UTC: Wendelstein cookies!

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!


Just reading that article, it seems like there were lots of problems faced in the building of the Stellarator. What would you say was the hardest obstacle that you managed to overcome? And can you run through a(n extremely) simplified version of how you overcame it? Thank you.

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

From the theoretical point of view it was necessary to understand the problems which result from three-dimensionality of the stellarator (loss of continuous symmetry and the related conservation laws). Regarding the construction the main problems were the construction of the superconducting non-planar coils. Also putting a big machine (about 700t) together with a tolerance of about 1mm is very demanding (e.g. wielding parts together will, if not done carefully enough, lead to a non-tolerable welding distortion). So, the most simplified version how to overcome construction problems is: work extremely carefully and constantly check quality (which will take time) (rk)


When I hear about Stellarators specifically, there’s alot of talk about how those are particularly difficult to simulate the behavior of. Where exactly does all this added complexity come from compared to, say, a tokamak? Why does the twisting make that much of a difference?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

The main point is that the tokamak plasma is two-dimensional and the stellarator plasma is three-dimensional. This makes stellarators about one order of magnitude more difficult to simulate. (rk)

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

It is computationally more intensive to do calculations for a system that has no obvious symmetry. The complexity of the physics that we must understand is in most areas higher, but in some, lower than for the tokamak. To give an example where the complexity is lower by stellarators: The tokamak is a self-organised configuration – the plasma creates part of the confining magnetic field, but can also affect its own confining field much more. The stellarator has its confining magnetic field dictated from the coils and cannot perturb it strongly. (ts) In some sense, the stellarator is a stiff cage with some leaks in it, a tokamak is a wobbly cage with much less leaks. So the wobbliness of the tokamak makes it somewhat more complex to understand when it comes to large-scale stability.

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

If you imagine how a grid would need to look like on which you want to describe your particle motion, a stellarator needs a much finer grid to correctly show all the twists and wiggles in the magnetic field. (jp)


When do you think will fusion power become a reliable source of energy?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

According to the EFDA Roadmap it is planned that the demonstration reactor DEMO should produce first electricity 2050 (as usual: if everything works as expected). It will just be a prototype. After this one can start producing reactors on a large scale. So, the time when fusion power will become a reliable source of energy then depends how fast further reactors can be build. But roughly I would say, not before 2060. (rk)


Let’s hope for the best, this is the type of technology that will herald a new age of clean and unlimited energy 🙂


As far as I can calculate, fusion power might be 40% cheaper than fission power. Fuel cost close to zero, no waste to dispose, decommissioning cheaper. But that’s FAR from “a new age of clean and unlimited energy”. It’s quite possible that by the time we have fusion power, power from renewables will be so cheap that fusion power won’t even be economically viable.


The problem with renewable today is that the current power grid we have is much more viable with either nuclear energy or fossil fuels. A nuclear plant is thus much better able to integrate with he current grid than renewable like solar and wind. Not saying solar and wind are not a viable source. They are, but not something that can replace the entire power grid of a region for example.

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

You are right, my friend 😉

Wind and Solar are not base-load. They have a fluctuating nature. Thus, one needs large-scale energy storage and back-up systems (both not existing until now; there are not even technologies for large storage)

Thus, fusion power is in that sense benefical as it provides a base-load continous power to the grid! (fw)


Wow thanks for the reply! I have one question as it relates to fusion though, you or one of your peers posted out that the current timeline of 25 years is largely dependent on budget constraints. My question is how much money is required to really speed up the process? Can it be sped up or are you guys still in the stage of studying how fusion works?

Also, can the Wendelstien power plant be replicated in other countries or is Germany holding its tech close to the chest,

Finally earlier in the month you may have heard of gravitational waves being detected. I read that one of its practical uses, should you our detectability get better, is study the inner workings of stars, would that help the process along?

Also I wanted to thank you, you’re truly at the forefront of future energy power. 🙂

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

More money would enable us to build more experiments to pursue different ideas to fusion. Also it would be necessary to build a neutron radiation facility needed for developing fusion material.

Wendelstein in its current design is not a power plant. For a power plant you have to build it approximately four times larger. The design of Wendelstein is published and we are an international institute with lots of collaborations so there is no need for Germany to hold the technology to its chest.

The fusion process (Deuterium plus Tritium) itself is extremely well understood and basic nuclear physics so there is no further research necessary. Probing the inner working of stars is certainly interesting but does not help with fusion since the main problem is to confine and heat the plasma. (rk)


I see, the only real obstacle is further research and engineering on how to confine and hear plasma. Thank you for your replies. I hope your work bears real fruit soon. Please come back for more AMAs whenever time allows you.


Isn’t pumped-storage hydroelectricity, exempting its many issues with widescale deployment, a viable, and technologically feasible, answer to the issue of energy storage and back-up, at least temporarily?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

Of all fundamental forces known to physics, gravity is the weakest by far. That is the reason why gravitational storage systems never reach the capacities we would need. The entire german hydropower capacity, for example, amounts to ~40GWh. That is just about half an hour of supplying the german peak load.(md)


Yes, the current grid is limited, and not designed for a multi-small-intermittent-source environment. But I’ve read that it works fine with 40% or 50% renewable intermittent sources on it. Still need existing nuclear or gas to back it up.

But that will change. Grids will become smarter. We’ll have local solar-farms and wind-farms, and storage. And some household solar and storage. Tidal power and storage in coastal regions. Sure, we’re not yet ready to replace entire regional power grids with 100% renewables. But there’s no reason it can’t happen over, say, the next 50 or 75 years.


Perhaps it’ll be relevant for space travel? Not a lot of windmills up there 🙂


Yes, I think that’s an unstated motive behind some redditors support for nuclear (fission and fusion). Nothing else really works for serious space travel.


Hopefully this timeline will be accurate so I can see this happen before I die!


Well, you what they say, fusion is the power of the future – and always will be.

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

It may take a long time — even beyond your lifespan. But think of your children and grand-children. Fusion power is a legacy. And future generations will thank us for the efforts we made. 😉 (fw)


“A society grows great when old men plant trees whose shade they know they will never sit in”


Starting work on the great cathedrals must have felt something like this.


Alternatively, what are the main hurdles which stand in the way of fusion power, how significant are they, and how difficult are they likely to be to overcome?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

Despite the fantastic progress……..

1960’s: tokamak plasmas confined and heated to about 10 million degrees; 1990’s: plasmas heated to more than 100 million degrees with first release of 16MW of fusion power for 24MW of input power, for less than a second; 2020’s ITER is aiming at 500MW of fusion power for 50MW of input power, for several minutes;

……….there are some physics and engineering challenges to overcome:

(1) the problem of heat exhaust (particles and heat must be channeled to the edge of the machine, but materials can only withstand a certain amount of heat flux density)

(2) the problem of tritium breading (the easiest fusion reaction is Deuterium-Tritium but Tritium is not found in nature and must be generated inside the reactor)

(3) the problem of steady-state (one would like to operate a fusion power plant continuously; tokamaks cannot do that, although they can produce long pulses; stellarators can in theory operate steady-state)

(4) disruptions (this is a problem only present in tokamaks: sometimes the plasma becomes unstable and is quickly lost, potentially damaging the machine; while not dangerous, these should be prevented)

……..there are others but I think (1)-(4) are the most crucial. (jl)


I am asking as an absolute layman: Problem 3 and 4 only seem to exist in Tokamaks but not in Stellarators. Why are you still evaluating both design types if one seems to have clear advantages over the other?


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.

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

Because the tokamak so far has had significantly better confinement of the plasma energy. We aim to show that W7-X has been optimised enough that it will have tokamak-like confinement. (ts)


Because like in anything, trying different routes can bring to light solutions to problems you never even knew you had. Basically, never put all your eggs in one basket.


And to add to this, if the answer is “25 years”, that’s been the answer since the 60s.

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

This is an old joke every fusion scientist enjoys very much 🙂 But fusion is much more difficult to achive than people thought in the 60s. Also one must take into account that progress is a function of money. So, putting more money into fusion research would speed up things considerably. But this is a political question. Also fusion need big machines which take a long time (about 10 years) to construct and to operate. (rk)


Progress is a function of money.

I like this line a lot. It should be used more often, or at the very least, be printed on a t-shirt and sold for progress.


indeed. i would buy them, but only if the money goes 100% toward Wendelstein7-X


How many Progress Units would you charge for it?


19.99 milliprogresses +s&h


How much funding do you receive and how much funding would be ideal to speeding up that timeline?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

A lot more would be nice! Our national budget (Germany) is around 150 million euro (don’t quote me on that!), of which a large part (120 million euro) goes to IPP – this includes both our Garching and Greifswald branches, so 2 massive experiments. That may sound like a lot of money, but especially in Germany it’s very little compared to our renewable energies budget, for example.

It would be nice if we could internationally afford another big prototype like ITER. Putting all our eggs in one basket is difficult but necessary with the current global budget. If only we could have a stellarator reactor prototype! (avs)


As a German I think the amount of funding you get is ridiculously low. You should convince people that fusion is a renewable energy.


Merkel has a Doctorate in physics, doesn’t she think it’s worth more funding?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

What she personally thinks doesn’t matter that much in political reality, the chancellor in Germany can set accents but not single-handedly decide on budgets! (avs)


When I’m president or a billionaire, I will give you whatever you need for funding.

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

/u/Seventytvvo for president! (This is not an official endorsement by the Max Planck Society) (avs)


You know the president can’t just allocate billions to whatever he wants right?


How would you compare your approach to Lockheed Martin’s?

What are the pros and cons of each?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

I have listened to a talk given by on of the Lockheed physicists. His main argument regarding the timeline was a management argument: they are a commercial company and can not afford to do research for decades since they have to make money. As a consequence they have to achieve fusion in about 5 years. He did not talk about the physical problems involved and how to get fusion in 5 years. The whole talk was just ridiculous. (rk)


Thank you very much for pointing that out. 5 years…


Do you have an opinion on General Fusion’s approach? Their approach to magnetized target fusion seems to promise fusion without the extreme magnetic confinement that makes fusion so expensive and slow to develop.


Just to put things into perspective. At the APS conference that avs mentioned, they presented the current achieved parameters in the Lockheed Martin prototype. These were ~10 ev temperatures, 1017 particles per meter squared density, and confinements times of 4 to 100 microseconds. At its most simple form, fusion progress can be measured as the product of these three numbers. Current state of the art tokamaks (like JET) operate at around 5-10 keV temperatures, 1020 particles per meter cubed and have confinement times of up to 1 s. This means that Lockheed Martin is about 9 orders of magnitude behind state of the art tokamaks. They are about seven orders of magnitude behind the startup plasmas currently on W7X. The idea that they’re somehow going to improve their concept 9 orders of magnitude in a timeline of five years is insane. This would be a problem even if they didn’t have serious unanswered issues with their design, and if their lead scientist didn’t demonstrate a woefully inadequate knowledge of basic fusion science issues.

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

Honestly, we’re quite sceptical concerning the very compressed timeline that Lockheed is proposing. Having been at the APS conference last November where they presented a lot of their work, many fundamental questions were left unanswered. How will they shield their superconducting magnets against neutron radiation? How will they suppress cusp end losses?

The stellarator and tokamak concepts are much more mature and the roadmap to fusion a much clearer path for these concepts.

We’ve written a short article about this here, check it out and let us know if you have more questions! (avs)


How will they shield their superconducting magnets against neutron radiation?

Can you briefly summarize how the W7-X deals with neutron radiation? Isn’t that one of the biggest challenges with fusion reactors?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

W7-X is a research device, not a reactor. It is too small to function as a reactor, just big enough to give us lots of new physics. That is why it will not be operated with tritium. Hence, the neutron yield is tiny. The way to deal with it is a simple concrete wall. In an actual reactor the neutrons would be absorbed by a breeding blanket and used to produce new tritium.


the neutrons would be absorbed by a breeding blanket and used to produce new tritium

I’m learning so much today! Thanks for taking your time off for this!


Short: as far as i know, Lithium 7 “coat” the reactor, Neutron expulsed by fusion break lithium 7 into tritium

[–]WeaponsGradeHumanityBS|Computer Science|Data Mining and Machine Learning

What do you imagine the limits will be in terms of miniaturisation and portability?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

Regarding magnetic confinement fusion it will not be possible to do it with a small machine. The argument is roughly that we loose energy through the surface of the reactor by turbulence but energy is produced in the volume. So, we have to make the surface/volume-ratio small which can be done by making machines bigger (reducing turbulence is not possible). If a fusion reactor was to fit into a submarine we would not have to worry about money 🙂 (rk)


What is the latest work that addresses turbulence-reduction, and where it has failed or succeeded? I.E. why do you think reducing turbulence is not possible?

(I think I have a good set of these papers, but I am interested in what recent work has been done to overcome this limitation)

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

I should point out that turbulence is a limitation, especially if you want a small device. However, it is no show-stopper. It usually is a show-stopper for those promising you a tiny machine on a tiny budget :-).(md)


Why is it thought to be a limitation?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

Turbulence is driven by gradients of pressure and temperature. Hence, a smaller machine tends to have worse turbulence and experience greater difficulty maintaining its high plasma temperature.(md)


Maybe make a bigger submarine? Get some of those sweet, sweet defense dollars.

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

Fusion reactors will always be big devices, so you will unfortunately probably never see a Mr. Fusion for your car. The reason is that a fusing plasma loses energy through its surface area (residual contact with the walls) and produces energy through in its volume. The larger your device, the better the ratio of volume to surface is, just like penguins are larger near the poles than the equator to compensate for the higher heat loss there.

ITER is going to be the first reactor that clearly passes the break-even mark, producing several times more fusion power output than heating power in – look at its size! (avs)


TIL Something about penguins in a thread about nuclear fusion.


If your Stellarator got the funding and was built on the scale of ITER, what would you expect the input/output to be in comparison to the Tokamak design?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

The power output of a tokamak or stellarator plant will be the same, about 1.5GW. (rk)


Could a fusion reactor ever be a good renewable source of helium? Or is the amount generated too small for practical use?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

The amount of fuel fusion consumes, and hence the amount of helium produced, is very small. The helium we produce will be used in the reactor.


Oh? Used as fuel or would it have another use?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

Helium is a cooling agent for very low temperatures.



What are the goals of the W7-X, i.e. what would be deemed success?

What’s the next step for stellerators after W7-X? Could you guess how many iterations to a commercial reactor?


[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

The first objective is construction of the machine itself. The superconducting modular coils of this machine are a technological leap and W7-X has demonstrated that this can be done. The next important point in my opinion is the verification of the theory behind this design. W7-X is a so-called optimised stellarator and its design relies strongly on our numerical models and software. Demonstrating that our predictions are good would enable us to design the next machine. Finally, another very important point would be the investigation of steady state operation. this is one of the great advantages of the stellarator and very important for a reactor. In a project of this magnitude there are of course many other questions to be addressed, but these are, imho, the most important ones.


Thanks for the reply.

Until the W7-X hit the news there had been no emphasis placed on the pulsed operation of toroid reactors. Pulsed doesn’t seem viable long term.

Want to hazard an estimate of reactor iterations to commercial use?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

Here one has to distinguish between the tokamak and the stellarator line. For tokamaks: Currently ITER is a major step, which is a large facility build in France and will produce more fusion power than power is injected in the plasma. After ITER, the plan is to have a tokamak demonstration power plant (DEMO), which shall demonstrate the net electric power production. After this demonstration, there will be commercial fusion.

Stellarator: After W7-X a decision has not yet been made. One plan according to the European fusion roadmap is to have an intermediate step stellarator after W7-X, and after this step we go directly to commercial fusion using synergies in the development of technologies with the tokamak line. An alternative may be a direct step for W7-X to a stellarator power plant. A decision can only be made, after W7-X demonstrated its reactor capability in 2020. (fw)


Excellent, thanks.


I have very limited knowledge of fusion compared to you all, but find fusion absolutely fascinating so thanks for doing this. One of the things I am curious about is how you convert the yield into viable power? Do you aim to use a low neutron process for direct conversion? If not, how do you convert heat from inside such a delicately contained plasma field?

[–]Wendelstein7-XMax Planck Institute for Plasma Physics[S]

The plasma does not need to get out to give away its energy. The DT fusion reaction produces an alpha particle and a neutron, the latter carrying an energy of ~14MeV. The neutron is not confined by the magnetic field and is absorbed by a blanket where its energy is converted to heat. The remainder works just like a regular power plant.(md)

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