Evidence for Alfvén Eigenmodes Driven by Alpha Particles in D-3He Fusion Experiments on JET

Evidence for Alfvén Eigenmodes Driven by Alpha Particles in D-3He Fusion Experiments on JET

Evidence for Alfvén Eigenmodes Driven by Alpha Particles in D-3He Fusion Experiments on JET 150 150 UKAEA Opendata
UKAEA-CCFE-PR(20)100

Evidence for Alfvén Eigenmodes Driven by Alpha Particles in D-3He Fusion Experiments on JET

Dedicated experiments to generate high-energy D ions and D-3He fusion-born alpha particles have been performed at the Joint European Torus (JET) with the ITER-like wall (ILW). Deuterium ions from neutral beam injection (NBI) with acceleration voltage of 100 keV were accelerated to higher energies in the core of mixed D-3He plasmas by applying the 3‑ion radio frequency (RF) scenario. A large variety of fast-ion driven magnetohydrodynamic (MHD) modes with different toroidal mode numbers n were observed in these JET-ILW experiments, including the n = ‒1 and n = 0 modes in the frequency range of the elliptical Alfvén eigenmodes (EAEs). The simultaneous observation of these modes implies the presence of a fast-ion population in the plasma with a large number of highly energetic counter-passing ions, in addition to having a positive gradient in the particle energy distribution function. The combined fast-ion and MHD analysis shows that the population of RF-generated fast D ions does not include particles capable to excite effectively n = ‒1 EAEs. We demonstrate that the fast-ion observations are consistent with the hypothesis that D-3He fusion-born alpha particles are the drive for these modes. For the first time, we propose a theoretical mechanism and provide experimental evidence for a bump-on-tail distribution of fusion-born alphas, sustained by a periodic modulation of the fusion source due to sawtooth crashes. A possible application of these results to planned future D-T plasma experiments in JET-ILW is presented.

Collection:
Journals
Journal:
Physical Review Letters
Publisher:
APS (American Physical Society)