As part of a new initiative into trying to simulate tokamaks more accurately using 3D models I had the chance to travel to Japan to learn how to use a stellarator equilibrium code. After an 18-hour trip including a 1-hour trip on a Japanese-only train (which totally confused me) my travels took me to a place called the National Institute for Fusion Studies (or NIFS for short) located in a rural town called Tajimi.
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| Main Entrance to NIFS |
Here the largest currently operating superconducting
stellarator in the world is housed– the Large Helical device (LHD). Stellarators
are fairly similar to tokamaks with a more complex shape. There are many
different types of stellarator configurations and LHD is a helitron, a bit like
a DNA helix shaped into a ring, so it looks like this:
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| Diagram of a stellarator: the red is the plasma and the other colours are the magnets |
This means that, in order to generate the magnetic
bottle to confine the plasma, rather than using the a set of toroidal and poloidal
magnets LHD actually has helically shaped magnetic fields, so the magnets look
like this:
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| Note: this is actually the vacuum vessel, the grooves are where the superconducting magnets would sit, and there's me for scale! |
With a 8m major radius this is a pretty huge machine, amazingly the entire thing was built by Hitachi.
The next holiday snap is of the inner and outer coils
(with me as a scale again! and the cryostat tower in the background) which
Hitachi made as a test rig for the superconductors:
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| The inner and outer coils test rig, it's difficult to get across how massive these actually are! |
LHD can run plasmas for almost half an hour! However
the core temperature even in their high power runs is much lower than that of a
tokamak. The main plasma heating comes from three neutral beam boxes and electron
cyclotron resonance heating (ECRH). Even more impressively because of using
superconducting magnets, they only need turning on and off once a day, and so
there’s no need to wait for them to cool
down after each plasma discharge. Above is an aerial view of the stellarator
just to give you an idea of its sheer size.
You may have noticed that in some of the pictures of
me I am wearing a pink hard hat. Now, this is because LHD has its own mascot, named
Plasma Boy (he’s the little guy in the centre of
the hat). They also have a full sized version:
Every year they have an open day for children, in
which someone suits up in this and becomes plasma boy for the day.
I hope you have enjoyed a quick whistle stop tour of
LHD! I would just like to take a moment to thank Dr Byron Peterson San for
taking me on the tour of LHD and Dr Yasuhiro Suzuki San for hosting me whilst
in Japan and organising the tour for me.
For further information about LHD please see their website
located at: http://www.lhd.nifs.ac.jp/en/home/lhd.html