Axisymmetric eigenmodes excited by alpha particle energy gradients in JET D-T plasmas
Alfven eigenmodes driven by positive energy gradients in the alpha particle distribution have been observed in JET deuterium-tritium hybrid JET pulses heated only by neutral beam injection. Coherent low frequency magnetic fluctuations were detected in D-T pulses, but were absent in pure D reference pulses, indicating that the modes were destabilised by fusion-born alpha particles. The presence of the axisymmetric mode suggests that the modes were driven by positive energy gradients in the alpha particle distribution. This mode is identified as a global Alfven eigenmode located at the top of the density pedestal due to modulation by type III edge localised modes. The mode is driven by counter-passing particles, where the free energy source is provided by positive energy gradients that are present in all plasmas at the separatrix. The alpha particles have sufficiently large orbit widths to access the mode location, while beam ions, which have narrower orbits, cannot reach the plasma edge to sufficiently damp the mode.