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  4. Observation of Alfvén Eigenmodes driven by off-axis neutral beam injection in the TCV tokamak
 
research article

Observation of Alfvén Eigenmodes driven by off-axis neutral beam injection in the TCV tokamak

Geiger, B
•
Karpushov, A N  
•
Lauber, P
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July 31, 2020
Plasma Physics and Controlled Fusion

Fast-particle driven Alfv´en Eigenmodes 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 energetic-particle-induced geodesic acoustic modes (EGAMs) are observed at about 40 and 80 kHz. 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 fluctuations close to the frequency of the beam-driven TAEs are present throughout the experiment. The modes are even observed during ohmic plasma conditions, which 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 results from TRANSP. This is further supported by a similarly good agreement between measurement and modelling in cases with and without ECRH and therefore with and without the modes. Instead, a significant level of charge exchange losses are predicted and observed which generate a bump-on-tail fast-ion distribution function that can provide the necessary free energy to EGAMs.

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Type
research article
DOI
10.1088/1361-6587/aba19e
Author(s)
Geiger, B
Karpushov, A N  
Lauber, P
Sharapov, S
Dreval, M
Bagnato, F
Baquero-Ruiz, M
Dal Molin, A
Duval, B P
Garcia-Munoz, M
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Date Issued

2020-07-31

Published in
Plasma Physics and Controlled Fusion
Volume

62

Issue

9

Start page

1

End page

11, 095017

Subjects

fast particles

•

tokamak

•

Alfvén Eigenmodes

•

off-axis NBI

Editorial or Peer reviewed

REVIEWED

Written at

OTHER

EPFL units
SPC  
Available on Infoscience
August 28, 2020
Use this identifier to reference this record
https://infoscience.epfl.ch/handle/20.500.14299/171200
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