High-speed colour imaging unveils plasma dynamics in fusion tokamak

Current experiments on Tokamak Energy’s ST40 spherical tokamak have produced new high-speed colour imaging that reveals detailed plasma behaviour within the reactor. Captured at 16,000 frames per second, the footage shows plasma pulses lasting around one-fifth of a second, offering engineers a clearer view of phenomena occurring at the plasma edge that are otherwise invisible in monochrome or lower frame rates. The plasma emits a characteristic pink glow primarily due to visible light from the outer zone where ionised hydrogen isotopes like deuterium radiate red and blue wavelengths combined into pink. The core plasma remains too hot to emit visible light.

An innovative feature of the recent ST40 experiments is the injection of lithium granules via a newly installed Impurity Powder Dropper system. These sand-sized lithium particles enter the plasma and initially glow crimson-red as neutral lithium atoms are excited in the cooler plasma periphery. As the lithium becomes ionised and loses electrons, it transitions to a greenish-yellow glow, tracing the magnetic field lines within the tokamak and visibly outlining the plasma’s spiral trajectory around the device. This detailed visualisation underscores the role of lithium in plasma performance enhancement.

The lithium injection ties into ST40’s $52 million upgrade program, supported by UK government energy departments, aimed at exploring X-point radiator operating regimes. These regimes seek to cool the plasma before it contacts the plasma-facing components, thereby reducing material wear and prolonging component life without sacrificing plasma performance. Lithium plays a vital role by interacting with impurities and moderating heat loads, facilitating more stable and efficient reactor operation.

These colour plasma videos offer an unprecedented real-time view of dynamic plasma behaviour, enhancing understanding of impurity migration, magnetic confinement, and heat dissipation in a compact spherical tokamak environment. For fusion engineers, this represents a significant advance in diagnostic capabilities, providing richer data to optimise the path toward practical fusion energy generation. The ST40’s colourful plasma pulses demonstrate both the complexity and the promise of engineering magnetic fusion systems with improved plasma control techniques, driven by innovative plasma-facing materials and impurity management strategies.

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