The Super X Divertor

What limits the power of fusion reactors at the moment is how to make an exhaust system that can cope with the extreme super-heated gases that need...
18 July 2010

Interview with 

Professor Swadesh Mahajan, University of Texas at Austin


Split image showing interior view of the JET vacuum vessel with a superimposed image of an actual JET plasma, taken with an infra-red camera, 2005


Chris - One of the obstacles of having a hybrid reactor is that you need a fusion reactor that's small enough to fit inside the fission reactor and yet at the same time, be sufficiently powerful to drive the nuclear chain reaction. In fact, to work in a hybrid reactor, you'd need a fusion reactor that's about five times more powerful than the ones we currently have. What limits the power of fusion reactors at the moment is how to make an exhaust system that can cope with the extreme super-heated gases that need to be vented from the reaction process.  But researchers at the Institute for Fusion Studies at the University of Texas in Austin have come up with a new way of handling the exhaust. They're calling it the Super X Divertor and Swadesh Mahajan is here to explain how it works and partly also what the name means. Swadesh, hello.

Swadesh -   Hi.

Chris -    Tell us first of all, before we get into the Super X Divertor, what's the structure of a fusion reactor? How does it work and what's the problem that you're trying to solve?

Swadesh -   Typically, what you have is superheated plasma, as you said, and when certain conditions are satisfied, it can have enough fusion reactions to produce the much-wanted neutrons. Now of course, you raised the question that when there is so much doubt about fusion being a reality, how could we really talk about the fusion reactors being stuck inside a fission reactor? Although producing direct energy from fusion may be in the very distant future, there is another aspect of fusion that is its ability to produce neutrons which could go and aid the fission reaction. That particular goal is well within our site, and in fact, we are going to simply depend on that. This Super X Divertor that one is talking about essentially became a necessity because we need to make a fusion reactor which produces a copious supply of neutrons and at the same time, it has to be sufficiently compact that we can load it in and take it out of the standard fission reactor. For that, you require enormous energy densities. It's not necessarily that the total power is more than ITER, but that power densities are about five times more than that of ITER.JET vacuum vessel

Chris -   So it's going to be a material science isn't it: the demands you're placing on the reactor infrastructure are far and away beyond what the materials we have today are capable of delivering?

Swadesh -   You said it absolutely correctly. What we are going to do is develop a scheme so that we do not depend on wonder materials. We're going to just learn to live with the materials that we have. The important thing, then, is that a better confined plasma has to have a better concentration of magnetic fields outside the plasma which is connected to the external world (and the external world means walls). The divertor has a special wall which is designed so that the hard plasma goes and finds a resting place there and if we use the standard divertor which for instance ITER uses then of course, it'll be far too inadequate in being able to handle the enormous heat and particle fluxes which a high power density compact machine will produce.

Chris -   I get it. Just to translate a little bit: at the moment, a fusion reactor has got the fuel in the centre of the reaction vessel at very high temperature, 100 million degrees or more, and this is being held in situ by very powerful magnetic fields.

Swadesh -   Correct.

Chris -   It's going to produce exhaust gases which have got to be vented. At the moment, trying to vent those through existing materials, the demands are too high for those materials, so you've come up with this very clever way of taking the exhaust in a sort of circuitous route around the wall of the containment vessel so that it loses some of its heat into that wall, bringing it down to a temperature that the materials can handle.

Swadesh -   Right. In the process of its transport from the main plasma to the wall, we do two things to it. We make it travel a very long distance along the magnetic lines so that it loses a fair amount of energy in this process. Furthermore, we do what's called flux expansion so that they expand in area so that the impact on a square metre of the wall material becomes considerably less in this case which is confirmed by state of the art [analysis] by a factor of five less. Therefore, we just cross that threshold so that the powerful fusion module which is replaceable, which can be taken in and out of a fission reactor, can actually be thought of as a near-term possibility.

Chris -   So it's looking like it's genuinely real. Just to finally finish off Swadesh, what do you think is the ultimate benefit of moving into the fusion regime, rather than just doing nuclear fission like we do at the moment?

Swadesh -   Actually, I would say that what we have right now is an intermediate regime where fusion and fission are cooperating. In order to make them cooperate, one has to work very hard because these two are extremely complicated technologies, and the progeny, to be beautiful, you have to work extremely hard to get it there. I believe that with the Super X Divertor, as I've called it, possibilities have come into existence. I believe that within the next 15 to 20 years, a real hybrid can be actually put together if there is a sufficient amount of funding and interest. Once you have that, I can imagine an era of nuclear energy which is quite green and quite abundant, and in fact, the fuel for this is produced in a reasonably proliferation-resistant manner - much more proliferation-resistant than, for instance, centrifugation.


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