Posted on: 16th July 2012
To create fusion, you need a very different world to the one we live in. It needs to be six orders of magnitude hotter and six orders of magnitude less dense – and the result is six orders of magnitude more energy than most everyday processes.
The temperature required for fusion is over 100 million degrees. In the heat of Oxfordshire summer outside JET it might even reach 25 degrees Celsius, or 298 degrees Kelvin – which some people find hot – but it is still 6 orders of magnitude colder than the core of a fusion plasma.
One might expect that the pressure in a fusion tokamak must be very high – similar to the core of the sun. In fact it’s the opposite: the optimum density for fusion is about 106 times less than that of our atmosphere. The reason for this surprising fact is that higher density means more collisions between particles. Of course collisions between ions are good, because they are what leads to fusion. However when the ions in the plasma collide with electrons, energy is lost. This is because of a process known as bremsstrahlung, or “braking radiation”, which refers to radiation that is caused by charged particles changing direction – for example during a collision. In a plasma at the extreme temperatures required for fusion, the high speed of the electrons means they emit X-rays when they collide with a nucleus; if the density gets too high, there are so many collisions that the energy of the plasma is lost due to all the resulting X-rays.
Having created an environment six orders of magnitude hotter and six orders of magnitude less dense than our everyday world, the result is energy in quantities six orders of magnitude greater than most common processes. Plasma physicists measure energy in electron volts – in other words the energy that one electron would acquire if powered by a one volt battery. Batteries are powered by chemical processes – by combining a number of electrochemical cells we can create powerful batteries such as those in a car, which have a voltage of 12 volts. Burning coal creates about four electron volts. But the products of the fusion, powered by nuclear processes, have a staggering 17.6 million electron volts. In other words you would need to connect well over a million car batteries to give particles the same amount of energy as that released when deuterium and tritium fuse.
So it seems the conclusion to draw is that, although achieving fusion sometimes seems six orders of magnitude harder than burning fossil fuel, it will be a million times better in the long run!