Tokamak Energy films fusion plasma in color for first time at ST40

Tokamak Energy has filmed plasma inside ST40 in full color at 16,000 frames per second for the first time, giving researchers a new way to watch how injected material moves through the machine.

The images were taken as part of X-point radiator, or XPR, research, a regime Tokamak Energy is studying as a way to cool plasma before it reaches plasma-facing components. In the video, deuterium gas appears bright pink, while lithium granules introduced with the Impurity Powder Dropper glow crimson-red when neutral lithium is excited in the cooler outer plasma. As the lithium becomes ionized, it shifts to a greenish-yellow color and follows magnetic field lines, a visual cue that helps researchers see whether the material is penetrating as intended.

Tokamak Energy physicist Laura Zhang called the color camera especially useful because it lets researchers quickly tell whether introduced impurities are radiating in the right place and whether lithium powders are reaching the core. The footage also backs up spectroscopy-based diagnostics by visually confirming where the emitted wavelengths originate.

The color footage is part of the broader LEAPS upgrade program, short for Lithium Evaporations to Advance PFCs in ST40. The $52 million program involves Tokamak Energy, the U.S. Department of Energy and the UK Department for Energy Security and Net Zero. On Dec. 5, 2024, the Department of Energy linked the upgrade to advancing fusion science and technology needed for a future pilot plant. It valued ST40 at more than $100 million.

ST40 has already stacked up a series of firsts. Tokamak Energy put the machine at 100 million degrees Celsius in 2022, the threshold required for commercial fusion energy, and achieved first plasma after an upgrade on Dec. 14, 2022. A 2024 Nuclear Fusion paper put central deuterium ion temperatures at 9.6 ± 0.4 keV in 2022, showing pilot-plant-relevant conditions in a compact high-field spherical tokamak. Tokamak Energy also said a 2023 campaign improved diverted plasma scenarios with an infrared camera installed alongside Oak Ridge National Laboratory.