In the heart of Oxfordshire, Tokamak Energy is carrying out Research&Development within the scope of nuclear fusion. He recently published an article on his site that includes a video which portrays the plasma and its various colors. Let’s see together why this video is interesting and what impact it can have for research and the future of fusion in the future. Tokamak Energy is a British company, founded in 2009 as a spin-off of UK Atomic Energy Authority (UKAEA), which it aims to achieve compact and commercial fusion reactors in the coming years. The main device it works in is lo ST40a spherical tokamak, designed to reach high magnetic fields, very high temperatures and test technologies that could become central in the future generation of reactors. Over the years it has already achieved noteworthy results: for example, the ST40 has exceeded the threshold of 100 million degrees Celsius ion temperature, a key target for commercial fusion.
The color of the plasma in the ST40 Tokamak for nuclear fusion
The article published by Tokamak Energy on October 15, 2025 “Seeing plasma in colour: new imaging from ST40” describes footage captured with a very high speed color camera (with 16,000 frames per second) of the plasma in the ST40. Plasma is composed of deuterium (an isotope of hydrogen) which is ionized and heated to very high temperatures and confined by a magnetic field. The heart of the plasma is so hot that it does not emit visible light: so what we see in the X video below are mainly the outermost, “coldest” regions.
One appears in the video pink-purple luminescence on the edge of the plasma: it is the emission from the deuterium that emits visible in the outermost area. At the top right of the video, you can see them lithium granules introduced into the plasma. These granules start from neutral (and therefore non-ionized) conditions, emit a intense red when they are still intact in the colder edge of the plasma cloud. Then, as they penetrate the hotter, denser plasma, they lose an electron and they become Li⁺ ions; in this state they emit a green-yellow light which appears as streaks that follow magnetic field lines.
The fact that lithium tracers follow the magnetic field lines visually it helps to see the magnetic boundary, the position of the plasma and lithium-plasma interactions, all useful aspects for understanding stability, impurities and performance. The videos last about 0.3 secondsbut the shot was slowed down to clearly show the dynamic phenomena in an understandable way.
Why the nuclear fusion experiment is important
This experiment allows us to have a direct vision of the plasma edges and provides feedback to help understand how the process evolves over time. Lithium injection serves to improve plasma performance, control impurities and develop advanced operating modes. In the video, the red/green-yellow lights allow you to track the movement of lithium, to understand how it ionizes and how it follows magnetic field lines. This helps validate models, simulations and understand whether lithium diffusion is happening as expected. Every improvement in understanding plasma behavior, walls, impurities and stability is a step towards commercial fusion.
