It seems that currently, only two types of fusion reactor styles are being used for large scale testing: the Stellarator and the Tokamak. Both of these and a whole bunch of other designs have been developed in the 1950s and '60s. They are based on the idea of confining very hot plasma in a magnetic field, shaped just weirdly enough to not let the plasma escape.

Shaping such a field seems to be extremely complicated and could apparently only recently be achieved using computational optimization on the coil shape. On a side note, these reactors are also massively huge and expensive, probably not unrelated to the design.

Why are we still relying on these half a century old designs? What is stopping us from creating a very simple coil arrangement and just using a control loop to keep the plasma confined? Surely, somebody already thought of this. The designs mentioned before rely on the magnet field to create an inherently stable confinement. But today we can use inherently unstable processes and artificially make them stable by using a control loop.

A viable coil arrangement could potentially consist of 6 coils, distributed around a cube shaped container. The plasma could be induced and heated using light or radiation. If necessary, the coils could be made superconductive. The plasma cloud's shape, size and orientation could be tracked and used to change the field's shape in a way to contain it. The plasma could be compressed by collectively increasing the field strength, further heating it up.

Why doesn't this work?

I'm posting this in the Physics StackExchange because, despite being somewhat of an engineering question, I'm quite sure the underlying physics is what makes the idea impractical.

  • $\begingroup$ x5444, in addition to the other answers that you got, note that there are limits to what a control loop can do. The loop must be tuned, which requires some process stability to do properly. In addition, many control loop configurations work best with processes that are approximately linear in the range of applicability, as great variations in process gain from nonlinear processes give the control loop difficulties. There are also problems if the process demonstrates inverse response of any sort. $\endgroup$ – David White Jan 18 at 22:25

What do you mean by control loop? If you mean some sort of diagnostics to check when confinement gets lost and quickly reacting against it, then a) this is being done, b) confinement can fail in ways you can't even imagine without knowledge of plasma physics.

I suggest you check up on the history of the development of plasma reactors, that should clarify many of your questions and their follow-up questions. There are excellent youtube talks by high-ranking plasma people.

What you're describing in your question is a polywell reactor. Doesn't work due to massive radiative losses in the center of the polywell, if my memory serves.

These 'half-century-old' designs (of which only Tokamaks count, because Stellarators only existed as ideas) were the first ones to break the 1 million Kelvin bareer, and research effort into Tokamaks intensified.

Stellarators came later, as they are more complex to build, but they can overcome many plasma stability issues that Tokamaks have.

Those concepts are leading in a) stability b) simplicity c) the plasma triple-product. No other design can compete with those, although there's a bunch of secret military operations and private industries that try to develop competitors. As a last word, the plasma-triple product is the main measure of energy effectivity of a plasma fusion reactor. Because it is simple to produce some fusion reactions in hand-held devices even, but it is hard to produce enough fusion reactions for the nuclear fire to be self-sustaining and economical.

  • $\begingroup$ The Polywell is pretty much the answer I was looking for, thank you! While I can't claim I thought of confining only electrons magnetically to create a virtual cathode, the magnetic field seems to look reasonably close to what I imagined. It seems that the design mostly has issues with efficiency, as you said. $\endgroup$ – x5444 Jan 16 at 7:42
  • $\begingroup$ @x5444: Yeah, essentially all the simple designs have already been tried. The successful ones give us the ancestors of what we have today. The technology is being really evolved in this way. The polywell would look like what a next gen-type reactor should be capable of, namely direct harvesting of charged, energetic reaction products via an external conducting grid instead of using steam and water. But if it doesn't work, then it doesn't :/ $\endgroup$ – AtmosphericPrisonEscape Jan 16 at 8:56
  • $\begingroup$ I'm sure they have, it's just a bit difficult to find the particular design I'm looking for. Directly harvesting electrical energy is something that would likely be quite simple on a reactor that confines the plasma magnetically like that. You would probably heat it and then compress it magnetically. The energy produced by the fusion would then force the plasma to expand, performing work in the magnetic field, available at the coils. But as you said, if it doesn't work then it doesn't work. $\endgroup$ – x5444 Jan 16 at 11:45

The reason your arrangement isn't used is that it does not work for confining a plasma, because there's nothing that can be done to prevent leakage at the corners of your cube where the different magnetic fields come together. A toroid has no corners like that and hence stands a better chance of not leaking.

This is a complex subject, as you note, and it has a long history. Most of your questions have answers in Charles Seife's book "Sun In A Bottle", which I recommend you have a look at.

  • $\begingroup$ I wonder if spinning the cube rapidly and quasi-randomly in two axes-- maybe at 100KHz or so-- could help. This could be done by increasing the number of coils by a factor of 6 or so, and activating them in appropriate groups. $\endgroup$ – S. McGrew Jan 15 at 22:57
  • $\begingroup$ @S. McGrew This is also something I considered. Conceptually, it may be sufficient to add another set of 6 coils, overlapping the original coils. The new ones would be put exactly at the corners of the original cube. The fields could then be alternated or electronically controlled (be measuring leakage and adjusting appropriately). However, I think the magnetic fields cannot be constructed and destructed quickly enough to react to particles about to escape. You would likely end up with the sum magnetic field of all coils, creating new (possibly thinner) "edges" where the fields meet. $\endgroup$ – x5444 Jan 16 at 7:50
  • $\begingroup$ There are a bunch of articles like this: [physicstoday.scitation.org/do/10.1063/PT.4.0676/full/], describing the benefits of causing the plasma to spin. Spinning the field should be equivalent. Only collective motion of relatively large numbers of particles would need to be responded to. Switching the fields to cause them to rotate could be done more or less the way fields are switched in an electric motor. $\endgroup$ – S. McGrew Jan 16 at 12:05

Why are we still relying on these half a century old designs? What is stopping us from creating a very simple coil arrangement and just using a control loop to keep the plasma confined?

The thermal velocity of an ion at fusion temperatures is on the order of kilometers per second. Modern computers are fast. They aren't that fast. And then there's hysteresis.

The complex shaping seen in the stellarator and tokamak are designed, basically, to continually mix the fuel so that any drift in one direction gets evened out. It's the same basic concept as rifling in a gun, or missiles that are spin stabilized.


Active stabilization of a fusion plasma is possible. This article describes a recently developed system for disrupting certain kinds of plasma instabilities in a Tokamak, by detecting an instability as it begins to form, then disrupting it using a "sabot" fired into the forming instability. The "sabot" is a tiny pellet containing smaller pellets of low-z granules, and needs to be fired accurately within roughly 1 millisecond to a velocity of ~150 meters/sec. I think the result is that the plasma instability is rapidly locally cooled, draining its energy.

It's not exactly electronic control, but it is feedback control; and it- or an analogous technique- is likely to be important in future fusion reactors.


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