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

Sharapov S.; Oliver H.; Garcia J.; Keeling D.; Dreval M.; Goloborod’Ko V.; Kazakov Y.O.; Kiptily V.; Štancar I.; Bonofiglo P.; Coelho R.; Craciunescu T.; Ferreira J.; Figueiredo A.; Fil N.; Fitzgerald M.; Nabais F.; Nocente M.; Puglia P.; Rivero-Rodriguez J.; Rodrigues P.; Salewski M.; Tinguely R.; Zakharov L.

Key information

Authors:

Sharapov S.; Oliver H.; Garcia J.; Keeling D.; Dreval M.; Goloborod’Ko V.; Kazakov Y.O.; Kiptily V.; Štancar I.; Bonofiglo P.; Coelho R.; Craciunescu T.; Ferreira J. (Jorge Miguel de Sousa Ferreira); Figueiredo A.; Fil N.; Fitzgerald M.; Nabais F. (Fernando José Robalo Nabais); Nocente M.; Puglia P.; Rivero-Rodriguez J.; Rodrigues P.; Salewski M.; Tinguely R.; Zakharov L.

Published in

11/01/2023

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). Neutral beam injection-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 toroidicity-induced Alfven eigenmode (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 BOT 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 magneto-hydrodynamic (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 (Oliver et al 2023 Nucl. Fusion submitted). The observed multiple short-lived modes in a wider frequency range are identified as the on-axis kinetic AEs predicted in Rosenbluth and Rutherford (1975 Phys. Rev. Lett.34 1428).