2010 Schools Lecture – part 6

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

Another short week of lectures this week – just two.  Tuesday saw me up very early and braving the M40 traffic to get into London for my talks in Camden.  Then it was north into Hertfordshire for talks in Hatfield.  Both venues had good audiences asking lots of questions.  In Camden some of the students came out to the front afterwards to try some of the demonstrations – a chaotic ten minutes with sticky fingers on the plasma ball, neodymium magnets getting stuck together and variously-sized balls bouncing around!

In Part 5 of this blog we discussed the basic principles of tokamaks and this time I’m going to tell you a bit about how we heat the plasma to temperatures of around 100 million degrees.
There are three main forms of plasma heating: ohmic heating, neutral beam injection, and resonance heating.

You are already all familiar with ohmic heating.  This is where conductors with a current running through them heat up due to their own resistance.  A good example of this is an old fashioned light bulb.  The thin tungsten filament heats up so much that it gives off light.  It gets hot because current-carrying electrons hit against atomic ions in the conductor as they move through, and during each collision energy is transferred to the ions.  Temperature is a measure of how fast the component particles are moving, so the more the ions get hit the hotter the conductor becomes.    

In tokamaks, the current that is induced in the toroidal direction (flowing the long way around the ring, see picture in Part 5) heats the plasma by ohmic heating.  However, this method is limited because as the plasma heats up to extremely high temperatures (millions of degrees) it becomes too good at conducting electricity and there is very little resistance to generate heat.  After that, additional heating is required to increase the temperature to hundreds of millions of degrees.  By targeting where the additional heating energy is deposited it can also be used to improve the plasma performance, that is how well the turbulent plasma is trapped.

Neutral Beam Injection (NBI) is a way of heating the plasma by firing very fast particles into the plasma.  Once inside, they crash into lots of the plasma particles and give up some of their energy, making the plasma hotter.  The particles have to be neutral or they wouldn’t get through the magnetic fields that trap the plasma (remember magnetic fields affect charged particles).  You can think of NBI a bit like heating milk for a cappuccino.  Steam is fired into the milk and it gets hot.  Steam is high-energy water, and all the fast steam particles give up some of their energy to the milk.

The neutral beams are usually atoms of hydrogen or deuterium.  They have to be going very fast because they need enough energy to get them into the centre of the plasma and still have extra energy to give to the plasma particles.  If they were going too slowly they would give up all their energy before they reached the centre.  The atoms in the neutral beam used on JET, which is the biggest tokamak in the world, can travel at more than 3000 km/s.  That’s ten thousand times faster than the speed of sound!

Neutral beams are made by first taking charged particles (ions) and using an electric field to accelerate them to high speeds.  At JET they use a voltage of more than 100,000V.  The fast ions then pass through a neutral gas where some recombine to form neutral atoms that continue into the plasma.  Any particles that remain charged are deflected by a magnetic field to an ion dump that absorbs their energy.  Picture 1 is a diagram of the NBI system on JET.
 

The final method we use to heat the plasma is resonance heating.  This uses electromagnetic waves, specifically radio waves.  Many types of waves can travel though plasma.  Depending on the local conditions in the plasma, sometimes they pass right through, sometimes they are reflected and sometimes they are absorbed.  If they are absorbed, energy is transferred from the wave to the plasma particles, increasing their kinetic (movement) energy and heating them up. 

We talked in Part 4 about how the magnetic fields trap the charged plasma particles – they gyrate around the magnetic field lines.  They do this with a particular frequency (the number of circles made per second) that is called the cyclotron frequency. 
 

The heating is most efficient if the frequency of the wave (how many times it goes up and down in one second) resonates with the cyclotron frequency of the particles to be heated.  In other words, the frequency of the radio wave must be the same as, or a multiple of, the cyclotron frequency.  In JET the radio waves have a frequency of 23-57 MHz (FM radio frequencies are around 100 MHz). 
So, heating will occur when the radio frequency and the cyclotron frequency resonate.  The cyclotron frequency only depends on the particles’ charge and mass and the magnetic field strength.  The particles’ charge and mass are constant, but the strength of the magnetic field decreases outwards across the torus, so the cyclotron frequency also changes across the torus.  This means we can heat very specific regions in the plasma just by changing the frequency of the radio wave.

Next time we’ll be talking about JET – the biggest tokamak in the world.  My next part of the tour takes me to Scotland in September, but I may try to blog before that, sometime over the summer holidays.

images courtesy of EFTA-JET, www.jet.efda.org