Physics in perspective lecture 2012

Powering the Future

By Dr Melanie Windridge

The following video is a version of the lecture done in 2010.

Plasma is an ionised gas. Our Sun is a huge ball of plasma undergoing fusion with the result that energy is released. We are aware of this in the summertime when we feel the warmth. It provides clean energy which we would like to replicate on Earth. Unfortunately we can’t produce the pressure necessary.

In Britain the joint European Torus (JET) investigates the potential of fusion as a safe, clean and virtually limitless energy source for the future. See http://www.efda.org/ for more information.

Why do we need fusion energy?

We are running out of fossil fuels. Fossil fuels are inefficient and cause pollution.

Renewable energy resources don’t provide enough energy. They have a low energy density. You would need 1000s of wind turbines to replace one coal fired power station. Wind is intermittent.

An increasing population increases the need for energy.

Fusion produces no pollution. 1kg of fusion material per day would produce the same amount of energy as 10 million kg of coal (100s of truckloads) per day.

To understand how fusion can occur we first need to know what an atom is made up of.

The atom consists of a nucleus containing protons and neutrons with electrons orbiting it.

The neutron has no charge. The proton is positive and the electron is negative.

To illustrate the empty space in an atom think of  the nucleus as a pin head placed in the centre of a football pitch then the electron would be right at the back of the stand. If you could remove all the space from the atom then the whole human race would fit into a sugar cube.

In the Sun, fusion is a multi-stage process and takes millions of years. This would take too long on Earth. We need to be able squash deuterium and tritium together. This is very difficult as you have to overcome repulsion between positive charges. A way round this problem is to use very high temperatures and produce a plasma.

Magnets are used to contain the plasma as they provide a centripetal force on the charged particles.

The magnetic field produced by a current carrying solenoid traps the plasma. To prevent the plasma escaping the solenoid forms a loop. Plasma is too hot to see.

Robots are needed to maintain the apparatus due to the presence of high energy neutrons bashing into things causing some radioactivity (not long lasting). The robot look likes a jointed snake.

Why does fusion release energy.

The most stable nucleus has a high nuclear binding energy per nucleon. A big nucleus can achieve this by fission and a small nucleus can achieve this by fusion. In both cases energy is released. E = Δmc2 is Einstein’s famous equation that shows that energy and mass are connected.

At the moment JET has to have more energy put into it than can be released by fusion.

How does fusion give us electricity? The energy released during fusion is used to turn water into high pressure steam and the following process is exactly the same as in a coal fired power station.

Lithium reacts with neutrons to give tritium and energy is released. More neutrons are produced and these carry the energy away.

What has to be done in the future?

To produce more fusion energy. ITER is expected to produce twice the energy of JET. Hopefully in the future we will be able to produce ten times the energy of JET. We want bigger plasmas that will stay hotter for longer.

Disruptions are the losing the plasma. This happens when the plasma hits the wall. Energy is transferred producing currents and forces.

Fusion is a lot safer than fission as there are no chain reactions and the levels of radioactivity produced are lower.

The following video is a version of the lecture done in 2010.

http://www.youtube.com/watch?v=KujVtrjW3fU

Dr Windridge has also produced an excellent book on the subject: Star Chambers – The race for fusion power.

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