Heating a fusion device How hot A plasma

Heating a fusion device |

How hot? A plasma is the name given to the cloud of superheated charged particles in a nuclear fusion device. The plasma needs to be heated to a temperature around one hundred million degrees C (100, 000⁰ Celsius) to cause the particles to fuse and release energy. Inside JET, the world’s biggest fusion machine, shown opposite, temperatures have reached 300 million degrees Celsius – 20 times hotter than the Sun’s core. 2 | Heating a fusion device Image: EUROfusion

Why so hot? Plasma is an ionised gas made up of positively-charged nuclei and negativelycharged electrons. In a fusion device we need to fuse the nuclei of hydrogen isotopes to release energy. As the nuclei have a positive charge, they naturally repel each other. The way to overcome this electrostatic force is to heat the plasma to over 100, 000 degrees C. At this temperature, the nuclei are moving so fast they come close enough for the strong nuclear force to bring them together – fusion then occurs. 3 | Heating a fusion device Plasma e e + + + e e e +

Three ways to heat a plasma Three methods work together to create the incredible temperatures needed for fusion reactions to begin to take place. These are: • Neutral beam injection • Radio frequency wave heating • Ohmic heating 4 | Heating a fusion device

Neutral beam injectors Charged particle beams are accelerated outside the reactor and then neutralised and fired at high speed into the plasma in the reactor chamber. As the particles crash together, energy is transferred and the temperature rapidly goes up. It’s a similar idea to how steam is used to heat milk in a coffee maker. 5 | Heating a fusion device Neutral beam injectors on the ITER international fusion experiment, currently being built in France

Wave heating Image: EUROfusion Powerful radio frequency or micro waves, tuned to the rotation frequency of the particles, make the particles move quicker, increasing the temperature as the particles begin to collide with each other. Microwave ovens work in the same way by using waves to increase the motion and energy of the water molecules and hence heating the food. You would need about 20, 000 microwave ovens to generate the same amount of power as the wave heating system on a big fusion machine! 6 | Heating a fusion device Wave-launching antennae in JET

Ohmic heating A plasma is made up of charged particles which can carry an electric current induced by a large transformer coil. Electrons and ions collide, increasing resistance and increasing the temperature, similar to how the resistance of the wire in an electric fire causes it to warm up when electricity is passed through it. 7 | Heating a fusion device

The goal: a ‘burning plasma’ We need to get more energy out than we put in to make fusion useful as a source of energy. At high plasma temperatures, fusion reactions begin releasing more energy than is put in. The plasma largely heats itself and the external energy sources can be reduced or switched off. This state is called a ‘burning plasma’. The Sun is a burning plasma. 8 | Heating a fusion device Image: CC 0 Creative Commons

Summary Extremely high temperatures are needed to heat plasma to a point where selfsustaining fusion reactions take place. Image: EUROfusion Injection of high energy beams, radio frequency and micro waves and the heating effect of electricity passing through the plasma, all combine to raise the temperature of the plasma to 100, 000 degrees. Image: CC 0 Creative Commons 9 | Heating a fusion device

Subscribe to the UKAEA channel on Dendrite. me for more fusion energy educational resources and STEM career information. Find out more about fusion: www. gov. uk/ukaea www. ccfe. ac. uk 10 | Heating a fusion device