Fast-particle driven Alfv´en Eigenmodes (AEs) have been observed in low-collisionality discharges with off-axis neutral beam injection (NBI), electron cyclotron resonance heating (ECRH) and a reduced toroidal magnetic field. During NBI and ECRH, Toroidicity induced Alfv´en Eigenmodes (TAEs) appear in frequency bands close to 200 kHz, and chirping modes are observed at about 40 kHz and 80 kHz that are likely Energetic-Particle-Induced Geodesic Acoustic Modes (EGAMs). When turning off ECRH in the experiment, those beam-driven modes disappear which can be explained by a modification of the fast-ion slowing down distribution. In contrast, coherent fluctu- ations close to the frequency of the beam driven TAEs are present throughout the experiment. The modes have the same toroidal mode number as the beam-driven ones and are even observed during ohmic plasma conditions. This clearly demonstrates that they are not caused by fast particles and suggests an alternative drive, such as turbulence. The mode-induced fast-ion transport has been found to be weak and marginal in terms of the fast-ion diagnostic sensitivities. Measurements of the plasma stored energy, neutron rates, neutral particle fluxes and fast-ion D-alpha spectroscopy show good agreement with neoclassical modelling result from TRANSP. This is further supported by a similarly good agreement between measurement and modelling in cases with and without
Observation of Alfv{\'e}n Eigenmodes driven by off-axis neutral beam injection in the TCV tokamak
Dal Molin, A.;Nocente, M.;
2020
Abstract
Fast-particle driven Alfv´en Eigenmodes (AEs) have been observed in low-collisionality discharges with off-axis neutral beam injection (NBI), electron cyclotron resonance heating (ECRH) and a reduced toroidal magnetic field. During NBI and ECRH, Toroidicity induced Alfv´en Eigenmodes (TAEs) appear in frequency bands close to 200 kHz, and chirping modes are observed at about 40 kHz and 80 kHz that are likely Energetic-Particle-Induced Geodesic Acoustic Modes (EGAMs). When turning off ECRH in the experiment, those beam-driven modes disappear which can be explained by a modification of the fast-ion slowing down distribution. In contrast, coherent fluctu- ations close to the frequency of the beam driven TAEs are present throughout the experiment. The modes have the same toroidal mode number as the beam-driven ones and are even observed during ohmic plasma conditions. This clearly demonstrates that they are not caused by fast particles and suggests an alternative drive, such as turbulence. The mode-induced fast-ion transport has been found to be weak and marginal in terms of the fast-ion diagnostic sensitivities. Measurements of the plasma stored energy, neutron rates, neutral particle fluxes and fast-ion D-alpha spectroscopy show good agreement with neoclassical modelling result from TRANSP. This is further supported by a similarly good agreement between measurement and modelling in cases with and withoutI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
