This is the third installment in Asia Times’ Science Editor Jonathan Tennenbaum’s series “Fusion Diary.” For an introduction to the series, readers are encouraged to start with “US abandoning its leadership in fusion energy,” by Matthew Moynihan and Alfred B Bortz. Then read part 1 of the series here and part 2 here.
In its national program to build a prototype fusion power plant, Britain has decided upon a reactor type that differs radically from the conventional tokamak design, exemplified by JET and giant ITER. Instead, the UK is placing its main bet on the so-called spherical tokamak.
In the spherical tokamak, the fat central column of a conventional tokamak is replaced by a narrow post, giving the vacuum chamber a roughly spherical form rather than the conventional doughnut-like shape. The 100-150 million-degree plasma inside behaves differently and there are big differences in the technical design of the reactor.
Why is the UK going against the mainstream in the fusion race? This has a fascinating history.
Like many of the alternative concepts in magnetic confinement fusion, the spherical tokamak was born in a US national lab. In the period 1982-87, fusion researchers at the Oak Ridge National Laboratory (ORNL) hypothesized that a tokamak with a compact spherical form would be significantly more efficient, in terms of its ability to confine the hot plasma, than a conventional tokamak.
Let me explain briefly what the problem is.
As with an ordinary gas in a container, when we heat a plasma to ever higher temperatures, the pressure goes up. As one might imagine, even at the low densities employed in tokamaks, a 100 million-degree plasma will have a very strong tendency to expand.
The role of “container” is played in a tokamak by powerful magnetic fields that counteract the pressure exerted by the hot plasma, confining the plasma to the interior of the reactor chamber and keeping it away from the chamber walls.
How does this work? The particles constituting the plasma – rapidly moving electrons and ions – are electrically charged. According to the laws of electromagnetism, charged particles moving in a magnetic field will tend to spiral around the field lines and are thereby trapped by them to a greater or lesser extent. (Here is an elementary YouTube introduction to these concepts.)
It turns out that to produce the field strengths needed to confine a plasma at 100-150 million degrees, the magnetic coils in a tokamak reactor must be run with electric currents of millions of amperes. This requirement imposes extremely severe engineering constraints on the reactor, and is a major factor in determining its cost and economic viability.
Needless to say, it would be a great advantage to be able to reduce the strength of the magnetic field needed to confine a plasma at a given density and temperature – or, alternatively, to increase the density and/or temperature of the plasma that can be confined using a given magnetic field strength.
In fusion science, the efficiency of confinement is commonly expressed in terms of a parameter called the “plasma beta,” which is technically proportional to the ratio of the plasma pressure to the square of the magnetic field strength. Reactor designers endeavor to make beta as large as possible.
Not surprisingly, the value of beta depends on a great many factors. Unlike the molecules of a gas in a bottle, where the molecules move around and bump into each other in random fashion, the ions and electrons constituting the plasma whizz around the toroidal vacuum chamber in a complicated pattern of spiraling trajectories.
It should thus be no surprise to find that the shape of the vacuum chamber has a major impact on the possible patterns of particle motions, and thereby also on the ability of the reactor to confine the plasma.
Analyzing these relationships is a very difficult physics problem.
In 1982-1987 the plasma physicists Alan Sykes, Martin Peng and Dennis Strickler, working at Oak Ridge National Laboratory, calculated that higher values of beta could be reached by greatly reducing the so-called aspect ratio of the vacuum chamber – the lengthwise radius of the toroid divided by its cross-section radius. Their studies also indicated that the plasma would behave better if the cross-section of the chamber were made oval – or D-shaped.
The result is an overall spherical – or apple-like – shape with a narrow column in the center.
Why would the spherical shape work better?
The analysis is complicated. Asked for a simple intuitive explanation, a physicist I spoke with told me that in the spherical tokamak, the magnetic field lines wind tightly around the narrow central column, holding particles there for relatively long periods before their spiral trajectories carry them toward the outside region and quickly back in again. The longer sojourn of the particles in the inner region, as opposed to the outer region, adds up to a more effective confinement of the plasma.
Creating a tokamak with a spherical shape poses major challenges, however. In a conventional tokamak, the bulk of the external magnetic field is provided by identical ring-shaped vertical coils equally spaced around the vacuum chamber.
JET has 32 of them. These flat coils all bunch together in the middle of the device, taking up a lot of room. In addition there must be space in the middle for the central solenoid coil, needed to drive current through the plasma.
In 1984, Martin Peng came up with an elegant solution: to replace the closed-ring magnets with half-rings sharing a single conducting rod in the middle of the reactor. (As I understand, in later practice the toroidal coils are closed but wedge-shaped along the central axis, fitting together to form a narrow column.)
Martin Peng proposed building a first spherical tokamak, christened STX, at Oak Ridge. The cost of this small first device was estimated at about US$6 million. Coming at a time when the US fusion budget was undergoing drastic cuts, the STX proposal was rejected. The spherical tokamak idea subsequently “emigrated” to Culham, England, with Peng accompanying it.
Peng had met with significant resistance from plasma physicists who thought his idea was crazy. But the directors at Culham gave Peng the green light to build a small spherical tokamak using mainly spare components left over from other experiments. The whole project cost only about $125 000, a minuscule amount even for a small tokamak experiment.
In 1991, after two years of construction time, the world’s first spherical tokamak, START (“Small Tight Aspect Ratio Tokamak”), began operation. The whole device was only about two meters across.
START’s performance exceeded all expectations. START achieved a beta value more than three times that of any conventional tokamak. It also showed superior plasma stability and other favorable features. Laboratories around the world rushed to build small spherical tokamaks, including even countries not well-known for their fusion research, including Australia, Brazil, Egypt, Kazakhstan, Pakistan and Turkey.
START operated until 1998. In the meantime, the revolutionary results with START immediately suggested building a larger spherical tokamak with higher fields, higher plasma currents, more powerful heating systems and other features providing for a broader scope of investigations.
This became the Multi-Ampere Spherical Tokamak (MAST) at Culham. The volume of the MAST plasma was three times larger than in START, but three times smaller than in the JET reactor.
MAST operated from 1999 until 2013. It confirmed the results of START, demonstrating a number of additional advantages of the spherical tokamak and providing a large store of knowledge and experience for further devices.
In the same year as MAST, a similar reactor went into operation in the United States, called the National Spherical Torus Experiment (NSTX). Situated at the Princeton Plasma Physics Laboratory, NSTX is still operating successfully today. It has further confirmed the high beta and other advantages of the spherical tokamak.
In view of the successes of the spherical tokamak experiments, it was decided in Culham to upgrade MAST into a device with greater heating power, plasma current, magnetic field and pulse length. Most urgently, however, MAST-U was to provide a first-ever platform to test a revolutionary design for the so-called divertor, called the “Super-X divertor.”
As I explained in an earlier article, the divertor is a key component in any tokamak reactor; it provides the means to remove, continuously, impurities from the plasma, as well as helium ions produced by the fusion reactions (frequently referred to as the fusion “ash”). At the same time, the divertor removes a substantial portion of the heat energy generated by the fusion process.
The divertor is a slot-like structure, installed at the lower (and sometimes also the upper) extreme of the chamber. The magnetic field is configured in such a way, that a thin stream of plasma is effectively “scrapped off” the core of the plasma and directed at a set of metal “target” plates, where the plasma is neutralized into a gas and pumped away. At the same time, fresh DT fuel can be injected into the reactor, renewing the plasma.
The main problem with divertors comes from the fact, that the target plates are in direct contact with the 100-150 million-degree plasma, and exposed to intense bombardment by neutrons and other “hot” particles.
The divertor plates must withstand an overall energy flux 10 or more times larger than the heat shields of space capsules during reentry into the atmosphere. While reentry lasts only minutes, the divertor plates must last for periods of at least months, preferably years, before being replaced.
The lifetimes of the divertors will be a significant factor in the economic viability of future tokamak fusion power plants.
The Super-X divertor design promises to drastically reduce the heat and power load on the divertor plates. With the help of additional magnetic coils, Super-X bends the pathway taken by the plasma, making it travel a longer distance before striking the target plates. This gives the plasma the chance to cool down significantly before contact occurs. There are other important differences, relative to conventional designs, that I cannot go into here.
During my August visit to Culham, I had the opportunity to speak with a young scientist working on the design of Super-X divertors for MAST-U. I learned that there are many variations on the Super-X concept, involving different configurations of the plates, magnetic coils and field strengths. Finding the optimal ones is a major task. MAST-U was explicitly designed for trying out various divertor designs.
Like all the other reactor designs being pursued in the fusion race, the spherical tokamak design has its advantages and disadvantages. One cannot be sure at this point which one will ultimately come out ahead.
Britain is currently betting on the spherical tokamak, having chosen this design as the basis for the UK national program to build the first electricity-producing fusion reactor. Hence the name Spherical Tokamak for Energy Production (STEP).
In his interview with me — to be published as the next installment of this series – STEP Director Paul Methven emphasizes that Britain will continue to participate in the ITER project, a giant tokamak of the conventional type, while also supporting a variety of alternative approaches to both magnetic and inertial confinement fusion. I shall report about one of the most exciting of those alternatives later in this series.
Regardless of who wins the current fusion race, I am convinced that in the future fusion energy will take many forms and will involve a variety of reactor types, even more so than has been the case for fission.
NEXT: An Apollo program for fusion
Jonathan Tennenbaum, PhD (mathematics), is a former editor of FUSION magazine and has written on a wide variety of topics in science and technology, including several books on nuclear energy.