James does Japan

by James Simpson  
 
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.

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: 

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:

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:

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