2010 Schools Lecture – part 5

Our 2010 School Lecture presenter Dr Melanie Windridge on her lecture tour and fusion reaction.

Last time we were talking about how we can use magnetic fields to trap charged particles.  There are a variety of machines that can be used to do this, such as the tokamak, the stellerator and the reversed-field pinch.  The tokamak is the most successful and prevalent of the different devices, and it is a tokamak design that is currently the most likely candidate for a fusion power plant.  So today I’m going to tell you a bit about the basics of tokamaks.

Now you might be wondering where the word “tokamak” comes from.  It’s actually a Russian acronym.  Tokamak stands for toroidalnaya camera magnitsnaya katusha, which means “toroidal chamber magnetic coils”.  This describes exactly what the machine is.

A torus is the mathematical name for a ring-doughnut shape.  So a tokamak is a ring-doughnut-shaped (toroidal) vessel surrounded by magnetic coils that make the trap for the hot plasma and keep it from touching the walls. 

Fusion experiments started as linear (straight line) machines with magnetic field lines that went through cylinders.  But while this trapped the charged particles along the field lines and stopped them escaping through the sides, particles could easily escape at the ends.  The simple solution to this is to bend the cylinder round into a torus so the ends join up.  This way the particles keep circling. 
Before we discuss the main features of tokamaks, it’s useful to introduce some terminology.  The main directions used in tokamaks are toroidal (the long way round the doughnut ring) and poloidal (the short way round), as shown in Picture 1.
 
Picture 2 shows the configuration of a typical tokamak.  To make a magnetic field go toroidally around in the centre of the torus requires a current to flow through magnetic coils that encircle the vessel in the poloidal direction.  These are shown as the blue coils in the picture.  It’s a bit like a solenoid wrapped round on itself.  The magnetic field created partially traps the plasma, but not very well.  With only a toroidal field the whole plasma will drift outwards. 

The way to stop this drift is to induce a current flowing in the plasma in the same direction (remember that plasmas can conduct electricity).  This current then generates a magnetic field surrounding it in the poloidal direction, the green arrows in Picture 2.  The two magnetic fields in combination create a resultant magnetic field in a helical shape, so the plasma particles spiral around the doughnut-shaped vessel and remain trapped.  
 
This is a simple explanation of what’s going on, but building a tokamak is quite complicated.  You have to get all the currents and magnetic fields right to hold the plasma in place, and even then it moves! The grey Outer Poloidal field coils in Picture 2 are used to shape and move the plasma.  Currents flowing in these coils will generate their own magnetic fields around them, which push against the magnetic field of the plasma because magnetic fields can’t cross.  Complex feedback systems are designed to detect where the plasma moves and push it back into a central position.

So that’s a bit about tokamaks and how they trap hot plasma.  Tokamaks come in many different shapes and sizes, all contributing to the overall fusion programme.  Now tokamaks are being used for research all around the world.  More than half the world is working together to make fusion happen.