Wednesday, 6 May 2015

A Tale of Two Tokamaks - Post the Second

By James Edwards

See the story about the first tokamak in our tale – JET – here.

Culham Science Centre by Abingdon, Oxfordshire was an unusual place, even in the year one thousand nine hundred and ninety nine. It was very blocky, very industrial and mostly concrete. Unusual, in the sense that the 50s & 80s architecture of the buildings was starting to house the beginnings of the most recent developments of the high-tech types of power generation: spherical tokamak fusion.

Large changes were afoot, with the approach of the turn of the millennium not only bringing the not-quite-so-apocalyptic-as-the-media-made-out Millennium Bug, but also a new, larger type of spherical tokamak (ST, for short) than had ever been built before (funnily enough, named for its approximate shape).

The UK has its own fusion research and development programme, entirely independent of the Joint European Torus (JET). Over the years, dozens of experimental machines had been built at Culham to test out an array of different fusion concepts.

In 1997, construction had begun of the Mega-Ampere Spherical Tokamak (or MAST, for fans of the less wordy version, like myself). Its mission: to investigate how larger spherical tokamaks worked and affected the conditions for fusion.

MAST was built as a follow-on to the first purpose-built full size spherical tokamak called START (also at Culham). Although mostly assembled from spare parts, START had shown very promising results, so the UK government agreed to provide funding for the construction of MAST.

“What’s the difference?” you might (hopefully) be asking (and even if you’re not, I’m going to explain anyway). The concept of a spherical tokamak was introduced by Martin Peng at Oak Ridge National Laboratory in 1984, who suggested that the coils could be wired up in such a way as to reduce what is known as the aspect ratio of the tokamak, making it easier to get plasma cross sections that look like the capital letter “D”. (It had recently been found that plasma in this shape often had a better performance than other shapes that had been used before.)

In practice, the difference in shape means that STs can be just as good at confining the plasma as toroidal machines but with approximately ten times less magnetic field required and hence, more efficient. One downside of this is that in an ST, several magnets are typically (though not always) placed inside the machine, rather than outside (which is the usual design in a toroidal tokamak), so different aspects of the design, such as placement of magnetic coils and other components, need to be considered carefully.

Due to the research into STs and how they work still being at quite an early stage, it’s very unlikely they will be used for the first generation of fusion power plants, which will be more like JET and ITER in structure. They may have potential uses elsewhere though before that. For example, they could potentially be used as component test facilities to develop the various systems that will be required to operate a fusion power plant. Or more speculatively, it has been suggested that STs could reduce the amount of waste produced by current nuclear fission power plants through a process called nuclear transmutation.

MAST: the glow near the central column is where the fuel gets in to the vessel. You can see some of the coils winding their way around the back of the machine, seemingly segmented.
Anyway, back to MAST... operations started in 2000 and since then it has been investigating many various plasma conditions in STs as well as testing different diagnostic (experimental) systems that have been built and fitted to the machine over the years.

In particular, MAST has made advances in imaging diagnostics and techniques. Some of the more recent work that’s been done on MAST includes imaging different high speed plasma events, such as Edge Localised Modes (ELM) – a sudden release of pressure at the edge of the plasma (similar, but not the same as, a solar flare) – and getting pictures of the filament structures in the plasma during a disruption (an event that happens when control of the plasma confinement is suddenly lost).

Edge Localised Modes in MAST. You might notice they look similar to these coronal loops in the Sun’s atmosphere taken by NASA’s TRACE spacecraft. The similarity shows the magnetic field’s power for each of the two different plasmas.
ELMs are something that can be helpful to tokamaks in some situations and an annoyance in others, so other recent work has focused on analysing different techniques for controlling ELMs in MAST. This is helping to improve the plasma physics models required for future power stations, where ideally plasma events like this will be both reasonably predictable and controllable to keep the tokamak putting power on the grid steadily.

MAST continued running pulses up until October 2013, when it went into a period known as a shutdown. This one is special though, because unlike previous shutdowns, this time the machine is essentially being rebuilt to become MAST-Upgrade. One of the brand new systems being added is the Super-X divertor. (Quick note: the divertor in a tokamak was initially just meant to be used as an exhaust for the helium produced during fusion, but it turns out that with particular magnetic field shapes, it can also produce more efficient fusion plasmas by significantly improving confinement.) This particular type of divertor hasn’t been tried before, and is testing several engineering and plasma physics ideas.

Plasma in tokamaks is pretty hot stuff, so the divertor region gets a rather large amount of energy in the form of heat dumped on it during operations. One of these ideas is to test a new design in the shape of the divertor to allow the amount of power exhausted from the plasma to distribute over a larger area and provide a larger region for the heat to radiate away. This will dissipate the heat more effectively than the current divertor. You can get a rough idea of how it works in this short video.

The upgrade also allows us to improve the plasma positioning and control systems in place on MAST. This improvement in control means that the distance from the edge of the plasma to the nearest components will be about a centimetre or so (rather close, for something that is several thousand Celsius hot!).

The MAST upgrade is effectively a rebuild of the machine. The spherical part goes inside this container. All the circular sections are ports for diagnostic systems to be fitted to.

The MAST Upgrade project is expected to complete in late 2016 and we should see results from this large effort shortly after this. The results will be used when designing future STs as well as when making technology decisions during the design of a demonstration fusion power plant (particularly considering the new divertor design).

I’m sure you’ll be seeing more about the MAST upgrade project in the future on Tokamak Tales!

PS: Simultaneous thanks and apologies to Charles Dickens for inspiration.

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