Experiments on excitation of Alfvén eigenmodes by alpha-particles with bump-on-tail distribution in JET DTE2 plasmas

Abstract

Dedicated experiments were performed in JET DTE2 plasmas for obtaining an α-particle bump-on-tail (BOT) distribution aiming at exciting Alfvén Eigenmodes (AEs). NBI-only heating with modulated power was used so that fusion-born α-particles were the only ions present in the MeV energy range in these DT plasmas. The beam power modulation on a time scale shorter than the α-particle slowing down time was chosen for modulating the α-particle source and thus sustaining a BOT in the α-particle distribution. High-frequency modes in the TAE frequency range and multiple short-lived modes in a wider frequency range have been detected in these DT discharges with interferometry, soft X-ray cameras, and reflectometry. The modes observed were localised close to the magnetic axis, and were not seen in the Mirnov coils. Analysis with the TRANSP and Fokker-Planck FIDIT codes confirms that α-particle distributions with bump-on-tail in energy were achieved during some time intervals in these discharges though no clear correlation was found between the times of the high-frequency mode excitation and the BOT time intervals. The combined MHD and kinetic modelling studies show that the high-frequency mode in the TAE frequency range is best fitted with a TAE of toroidal mode number n= 9. This mode is driven mostly by the on-axis beam ions while the smaller drive due to the pressure gradient of α-particles allows overcoming the marginal stability and exciting the mode [H.J.C. Oliver et al. Toroidal Alfvén eigenmodes observed in low power JET deuterium-tritium plasmas, to be submitted to Nuclear Fusion (2023)]. The observed multiple short-lived modes in a wider frequency range are identified as the on-axis kinetic Alfvén eigenmodes predicted in [M.N. Rosenbluth, P.H. Rutherford, Phys. Rev. Lett. 34 (1975) 1428].