Posted on: 11th February 2013

Something that surprises many people when they see their first plasma pulse on a screen in the control room, is that the plasma is invisible. There is a bit of glow around the edges, and the divertor – the bottom area of the vessel where the plasma touches the tiles – glows red hot. But the core of the plasma, at something like 100 million degrees, is completely transparent.

This is a desirable characteristic – it means that there is no energy being lost via radiation. It comes about because the atoms of the hydrogen fuel have been completely stripped of their electrons, or ionised. When attached to a nucleus at lower temperatures, these electrons absorb and emit light as they jump between the energy levels, but once they are detached that mechanism is disabled, so no light is absorbed or emitted.

To become this transparent, of course all the electrons must be detached. There is a pink glow around the edges because the plasma is cooler and so some electrons are attached, but generally for deuterium and tritium atoms, their single electron is easily removed. But for all other elements, with more electrons, it is harder to remove every last one and therefore to completely prevent energy leaking out through in the form of radiation.

Each element has its own colour, corresponding to the gaps between its electrons’ energy levels. The pink glow is a characteristic colour of hydrogen (and its isotopes, deuterium and tritium), which is made up mostly of a deep red and two weaker blue shades. When impurities creep in, with their own set of colours, the pink changes slightly. But exactly how each scientist’s eye interprets it varies. Some say that a blueish plasma comes from oxygen, others say helium. Others detect a greenish or yellowish tone, and ascribe it to the argon or nitrogen that has been puffed in to stabilise the plasma at a given moment.

And there are other causes of radiation too. When the plasma gets too dense, the number of collisions between electrons and nuclei increases, which leads to broad spectrum radiation called bremsstrahlung – a blob in the core of the plasma which to some scientists seems yellow, to others it looks brown.

Inevitably over thirty years some folklore will have built up around JET and it seems the colours of the plasma is one of those stories. Everyone’s heard it, but not quite sure whether it’s true and whether they can trust what they saw with their own eyes.

But of course the science doesn’t rely on these fanciful discussions in the control room – there are instruments measuring the radiation from the plasma and dividing it up into its component colours and they tell us exactly how much of each element is present. The human classification of today’s plasma colour as salmon, or peach, or burnt sienna is quite irrelevant. But a fun discussion to have, nonetheless.