Speaker
Description
The Joint European Torus (JET) recently completed historic deuterium-tritium (D-T) campaigns, which provided unique opportunities to study burning plasma physics. Alfvén eigenmodes were excited by fusion-born α particles [1] in several high-performance plasmas with fusion power > 8 MW externally heated only by neutral beam injection at up to 32 MW [2]. These modes were absent in pure deuterium reference plasmas, indicating that α particles were the source of instability. Magnetic probe signals were sufficiently strong to determine the toroidal mode numbers n, revealing that both axisymmetric ($n=0$) and non-axisymmetric ($n=1$) harmonics were destabilised. The excitation of the axisymmetric mode indicates drive from positive energy gradients in the α particle distribution, distinct from the expected drive from radial gradients.
Modelling reveals that the electric field of both modes peaked at the high-field-side edge of the plasma. The modes were driven by counter-current passing α particles with large orbit widths that enabled α particles born in the core to interact with modes at the plasma edge. The drift of the counter-passing α particles led to significant spatial overlap between the high-field-side orbit excursion of the α particle and the electric field of the eigenmodes, enabling the efficient power transfer that destabilised the Alfvén eigenmodes. Conversely, damping from beams ions was relatively weak because the co-passing beam ions had narrow orbit widths and drifted toward the low-field-side of the tokamak, away from the eigenmode. The high magnetic field in this plasma (3.85 T on the magnetic axis) and mode location on the high-field-side increased the Alfvén velocity and limited Landau damping by the thermal and beam ions to inefficient sideband resonances.
The drive from α particles came from positive energy gradients at the edge of the plasma – a universal feature of all magnetically confined plasmas due to the minimum energy required to confine particles. To explore the implications for future burning plasmas, this presentation will compare the α particle drive of the axisymmetric mode, which was entirely from energy gradients, with the drive of the non-axisymmetric mode, which accessed energy from gradients in the toroidal angular momentum $P_\phi$ in addition to energy gradients.
References
[1] H. J. C. Oliver et al., PRL, 136, 055101 (2026)
[2] D. B. King et al., PPCF, 68, 045023 (2026)