Section 1 of 5The Barrier to Fusion
Positive charges
- All nuclei carry a positive charge, so they repel each other with a strong electrostatic force.
Strong force
- Only when nuclei are extremely close, about the size of a nucleus, can the attractive strong nuclear force take over.
Energy needed
- The nuclei need very high kinetic energy to overcome the repulsion and get close enough to fuse.
Result
- High kinetic energy means very high temperature, so fusion needs temperatures of millions of degrees.
Section 2 of 5Conditions in Stars
1
In a star the mass of hydrogen is pulled together by gravity, forming a dense, hot core.
2
Gravity creates very high pressure in the core, which squeezes the nuclei close together.
3
The core temperature reaches millions of degrees Celsius, so nuclei move fast enough to overcome repulsion.
4
Hydrogen nuclei fuse into helium and the energy released keeps the core hot, balancing the inward pull of gravity.
5
The star remains stable while the fusion supply lasts, and the Sun has been doing this for billions of years.
Section 3 of 5Attempts on Earth
Attempts on Earth
Temperature
- Deuterium-tritium fusion needs a temperature of the order of 100 million degrees Celsius in a reactor.
Plasma
- At these temperatures the gas is a plasma of ions and electrons, which would melt any solid container.
Magnetic confinement
- Strong magnetic fields hold the plasma away from the walls of a doughnut-shaped chamber called a tokamak.
Energy balance
- So far experimental reactors have found it hard to produce much more energy than they use to heat the plasma.
| Place | Typical core temperature |
|---|---|
| Sun | about 15 million °C |
| Fusion reactor (planned) | about 100 million °C |
Section 4 of 5Why It Is Hard
Containment
- No material can touch a plasma that hot, so magnetic fields are needed to hold it in place.
Stability
- The plasma tends to become unstable and cool, which stops the fusion reactions.
Energy input
- A lot of energy must be supplied to heat and confine the plasma, which must be less than the output.
Cost
- Experimental reactors are huge and expensive, requiring international cooperation.
Section 5 of 5Exam Checklist & Tips
6.4.7 – Key Points for the Exam
- Electrostatic repulsion The force that pushes nuclei with the same charge apart.
- Strong nuclear force The short-range attractive force that holds protons and neutrons together in a nucleus.
- Plasma A state of matter in which the atoms are ionised.
- Tokamak A device that confines hot plasma using magnetic fields.
- Confinement Keeping the hot plasma away from the walls and together long enough for fusion.
Common Mistakes
- Saying fusion needs high temperature to melt the nuclei;
- it is needed to give nuclei enough kinetic energy to overcome repulsion.
- Thinking that magnetic fields heat the plasma rather than hold it in place.
- Saying that fusion happens in a power station once the plasma reaches a high temperature;
- it must also be held at high density for long enough.
- Claiming that we already have a fusion power station feeding electricity into the grid.