Where does the plasma colour come from?

The colour comes from the fuel, which is basically hydrogen. Hydrogen gives out two different colours, a strong red and aqua-blue, which combine to give the pink colour that shows up in most pictures. The two colours come from specific transitions of the hydrogen atom’s electron between different energy levels (for more info look up the Balmer series). It’s interesting to note that in the centre of the plasma it is so hot that the electrons are able to escape from their atoms, and so there is less light emitted as no transitions can occur. Around the edges where the plasma is cooler the electrons are able to reunite with their nuclei thereby emitting light.

Each element has its own unique set of electron levels, and hence the light emitted when the electrons jump between levels gives a unique signature for that element, called a spectrum (the study of it is called spectroscopy). So if there are any contaminants in the plasma they can show up by changing the colour. So the colour of a pure plasma is pink, if there is too much beryllium from the walls, that adds a blue green tinge, Oxygen is blue (that’s a bad sign, means there’s a leak) etc etc.

Our expert on cameras  pointed out that from the brightest point to the darkest point inside the vessel is a huge dynamic range – the cameras JET is using don’t cope with this very well (cameras that do, don’t survive the harsh environment inside the vessel!). It may be that different cameras give different colours for the same plasma, or we had turned the gain up to bring out a darker part, which made it look a lighter colour. Also some of the cameras have different optical components (filters  etc) in front of them to cut out particular parts of the radiation, so that may have affected the apparent colour too.