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Current experiments on our ST40 spherical tokamak are offering new visual insights into plasma behaviour, thanks to a high-speed colour camera capturing footage at 16,000 frames per second.

What the colours reveal

The image shows visible light emitted from the plasma’s edge, where temperatures are lower. The core of the plasma is too hot to emit visible light.

One of the most recognisable features is the bright pink glow from deuterium gas injection, visible in the upper left of the image. A pure hydrogen plasma, or any of its isotopes – deuterium or tritium – typically produces a light shade of pink, as it emits wavelengths of both red and blue light.

In the upper right, lithium granules are introduced using our newly installed Impurity Powder Dropper (IPD). As these sand-sized grains fall into the plasma, they emit crimson-red light when neutral lithium is excited in the cooler outer regions.

As the lithium penetrates deeper into the hotter, denser plasma, the atoms lose an electron and become singly ionised lithium (Li⁺). Once ionised, Li⁺ emits greenish-yellow light and begins to follow the confining magnetic field lines, visible in the footage as greenish-yellow streaks tracing the field around the tokamak.

The images from the colour camera help researchers trace the movement and behaviour of lithium within the plasma, and provide visual confirmation of more detailed data gathered through spectroscopy, which analyses the exact wavelengths of light emitted by the plasma.

Why lithium matters

This experiment is part of ongoing research into X-point radiator (XPR) regimes, a promising operating mode for future fusion power plants. XPR regimes aim to cool the plasma before it reaches plasma-facing components (PFCs), helping to reduce wear without compromising performance.

The coloured imaging is proving to be a valuable tool in these studies. As physicist Laura Zhang explains:

“The colour camera is especially helpful for experiments like these. It helps us immediately identify whether the gaseous impurities we’re introducing are radiating at the expected place, and whether lithium powders are penetrating to the plasma core.”

Lithium is also central to the $52 million ST40 upgrade programme, known as LEAPS (Lithium Evaporations to Advance PFCs in ST40), which is being undertaken in partnership with the U.S. Department of Energy (DOE) and the UK Department for Energy Security and Net Zero (DESNZ). The programme will apply lithium coatings to all PFCs using a lithium evaporation technique, building on pioneering work by Princeton Plasma Physics Laboratory and others that has shown lithium PFCs can significantly improve plasma performance.

The programme also includes the replacement of carbon armour tiles with molybdenum – a more power-plant-relevant refractory metal – and the addition of new diagnostics to measure the plasma with greater fidelity.

By incorporating lithium into ST40, the world’s highest field spherical tokamak, we will enhance our understanding of this critical enabling technology.

This work is advancing our understanding of plasma behaviour as we scale up to energy producing fusion devices. The addition of colour imaging is already providing valuable insights into how materials interact within the plasma.

-- Tokamak Energy