The Conditions for Fusion (Edexcel IGCSE Physics: Double Science)

• Stars are huge balls of (mostly) hydrogen gas
• In the centre of star, hydrogen atoms undergo nuclear fusion to form helium
• The equation for the reaction is shown here:

Deuterium and Tritium are both isotopes of hydrogen. They can be formed through other fission reactions in the star

• A huge amount of energy is released in the reaction
• This provides a pressure that prevents the star from collapsing under its gravity

The fusion of deuterium and tritium to form helium with the release of energy

• In larger stars the helium that has been produced can then be fused to form heavier elements

• Within stars the atoms are ionised
• This means that the electron that is normally on a hydrogen atom would be lost
• A hydrogen ion, or a proton, is therefore positively charged
• There exists a repulsive force between protons that has to be overcome for hydrogen fusion to happen

Hydrogen ions are protons, and their positive charge makes them repel one another

• In order to overcome this repulsion, the protons must have very high kinetic energy in order to be travelling towards each other at very high speeds
• In order to make the molecules of a gas travel at such speeds, the gas has to be heated to millions of degrees Celsius – a temperature that is usually only reached at the centre of a star
• In regular conditions, ie. on Earth, the possibility of collisions between nuclei which result in fusion is very low
  • In order to increase the number of collisions (and hence fusions) that occur between nuclei, high densities (and hence pressures) are also needed

• The difficulties in achieving and maintaining such high temperatures and pressures are the main reasons why fusion is not currently used as a source of power on Earth

• The conditions for fusion therefore are:
  • Very high temperature of fuel
  • Very high kinetic energy / speed of nuclei to overcome repulsion
  • Very high density / pressure to increase the possibility of suitable collisions