Effect of magnetic configuration on frequency of NBI-driven Alfvén modes in TJ-II

Excitation of modes in the Alfvénic frequency range, 30 kHz < fAE < 300 kHz, was observed in hydrogen plasma heated by hydrogen neutral beam injection (NBI) in the TJ-II heliac. Co-field and counter-field NBI were injected, and the components of the poloidal magnetic field were varied one by o...

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Published in:Nuclear fusion 2014-12, Vol.54 (12), p.123002-11
Main Authors: Melnikov, A.V., Ochando, M., Ascasibar, E., Castejon, F., Cappa, A., Eliseev, L.G., Hidalgo, C., Krupnik, L.I., Lopez-Fraguas, A., Liniers, M., Lysenko, S.E., de Pablos, J.L., Perfilov, S.V., Sharapov, S.E., Spong, D.A., Jimenez, J.A., Ufimtsev, M.V., Breizman, B.N.
Format: Article
Language:English
Subjects:
Alfvén modes
Density
Diagnostic systems
Dispersions
Frequency ranges
Hydrogen plasma
Magnetic fields
Plasma currents
rotational transformation
Spreads
stellarator
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Summary:Excitation of modes in the Alfvénic frequency range, 30 kHz < fAE < 300 kHz, was observed in hydrogen plasma heated by hydrogen neutral beam injection (NBI) in the TJ-II heliac. Co-field and counter-field NBI were injected, and the components of the poloidal magnetic field were varied one by one and in combinations, in order to investigate the beam-driven modes over an extended range of the rotational transform values, . Taking advantage of the unique TJ-II capabilities, a dynamic magnetic configuration experiment with variation during discharges has shown strong effects on the mode frequency via both vacuum changes and induced net plasma current. A drastic frequency increase from ∼50 to ∼250 kHz was observed for some modes when plasma current as low as ±2 kA was induced by small (10%) changes in the vertical field. A comprehensive set of diagnostics including a heavy ion beam probe, magnetic probes and a multi-chord bolometer made it possible to identify the spatial spread of the modes and deduce the internal amplitudes of their plasma density and magnetic field perturbations. A simple analytical model for fAE, based on the local Alfvén eigenmode (AE) dispersion relation, was proposed to characterize the observation. It was shown that all the observations, including vacuum iota and plasma current variations, may be fitted by the model, so the linear mode frequency dependence on (plasma current) and one over square root density dependence present the major features of the NBI-induced AEs in TJ-II, and provide the framework for further experiment-to-theory comparison.
ISSN:0029-5515
1741-4326
DOI:10.1088/0029-5515/54/12/123002