Axisymmetric Eigenmodes Excited by Alpha Particle Energy Gradients in JET D-T Plasmas

Axisymmetric Alfvén eigenmodes have been observed at the plasma edge in deuterium-tritium (D-T) tokamak plasmas externally heated only by neutral beam injection in JET. The modes were detected only in D-T plasmas, not in pure D plasmas, indicating excitation by fusion-born α particles. The presence of the axisymmetric mode suggests that the modes were driven by positive energy gradients in the α particle distribution rather than radial gradients. The modes are driven by counter-current passing α particles with large orbit widths, allowing core-born α particles to interact with modes at the plasma edge. This reveals an excitation mechanism arising from positive energy gradients produced by the minimum energy required to confine particles at the edge, relevant to all burning plasmas.