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Title: Measurements of the parallel wavenumber of lower hybrid waves in the scrape-off layer of a high-density tokamak

Abstract

In lower hybrid current drive (LHCD) experiments on tokamaks, the parallel wavenumber of lower hybrid waves is an important physics parameter that governs the wave propagation and absorption physics. However, this parameter has not been experimentally well-characterized in the present-day high density tokamaks, despite the advances in the wave physics modeling. In this paper, we present the first measurement of the dominant parallel wavenumber of lower hybrid waves in the scrape-off layer (SOL) of the Alcator C-Mod tokamak with an array of magnetic loop probes. The electric field strength measured with the probe in typical C-Mod plasmas is about one-fifth of that of the electric field at the mouth of the grill antenna. The amplitude and phase responses of the measured signals on the applied power spectrum are consistent with the expected wave energy propagation. At higher density, the observed k{sub ||} increases for the fixed launched k{sub ||}, and the wave amplitude decreases rapidly. This decrease is correlated with the loss of LHCD efficiency at high density, suggesting the presence of loss mechanisms. Evidence of the spectral broadening mechanisms is observed in the frequency spectra. However, no clear modifications in the dominant k{sub ||} are observed in the spectrallymore » broadened wave components, as compared to the measured k{sub ||} at the applied frequency. It could be due to (1) the probe being in the SOL and (2) the limited k{sub ||} resolution of the diagnostic. Future experiments are planned to investigate the roles of the observed spectral broadening mechanisms on the LH density limit problem in the strong single pass damping regime.« less

Authors:
; ; ; ; ; ; ; ;  [1]; ;  [2]
  1. MIT Plasma Science and Fusion Center, Cambridge, Massachusetts 02139 (United States)
  2. Univeristy of Tokyo, Kashiwa 277-8561 (Japan)
Publication Date:
OSTI Identifier:
22600169
Resource Type:
Journal Article
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 23; Journal Issue: 5; Other Information: (c) 2016 Author(s); Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 1070-664X
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ABSORPTION; ALCATOR DEVICE; AMPLITUDES; ANTENNAS; COMPARATIVE EVALUATIONS; COMPUTERIZED SIMULATION; DAMPING; ELECTRIC FIELDS; LINE BROADENING; LOWER HYBRID CURRENT DRIVE; LOWER HYBRID HEATING; PLASMA; PLASMA DENSITY; PLASMA SCRAPE-OFF LAYER; PROBES; RESOLUTION; SIGNALS; SPECTRA; WAVE PROPAGATION

Citation Formats

Baek, S. G., Wallace, G. M., Parker, R. R., Shiraiwa, S., Bonoli, P. T., Brunner, D., Faust, I., LaBombard, B. L., Wukitch, S., Shinya, T., and Takase, Y. Measurements of the parallel wavenumber of lower hybrid waves in the scrape-off layer of a high-density tokamak. United States: N. p., 2016. Web. doi:10.1063/1.4948555.
Baek, S. G., Wallace, G. M., Parker, R. R., Shiraiwa, S., Bonoli, P. T., Brunner, D., Faust, I., LaBombard, B. L., Wukitch, S., Shinya, T., & Takase, Y. Measurements of the parallel wavenumber of lower hybrid waves in the scrape-off layer of a high-density tokamak. United States. doi:10.1063/1.4948555.
Baek, S. G., Wallace, G. M., Parker, R. R., Shiraiwa, S., Bonoli, P. T., Brunner, D., Faust, I., LaBombard, B. L., Wukitch, S., Shinya, T., and Takase, Y. Sun . "Measurements of the parallel wavenumber of lower hybrid waves in the scrape-off layer of a high-density tokamak". United States. doi:10.1063/1.4948555.
@article{osti_22600169,
title = {Measurements of the parallel wavenumber of lower hybrid waves in the scrape-off layer of a high-density tokamak},
author = {Baek, S. G. and Wallace, G. M. and Parker, R. R. and Shiraiwa, S. and Bonoli, P. T. and Brunner, D. and Faust, I. and LaBombard, B. L. and Wukitch, S. and Shinya, T. and Takase, Y.},
abstractNote = {In lower hybrid current drive (LHCD) experiments on tokamaks, the parallel wavenumber of lower hybrid waves is an important physics parameter that governs the wave propagation and absorption physics. However, this parameter has not been experimentally well-characterized in the present-day high density tokamaks, despite the advances in the wave physics modeling. In this paper, we present the first measurement of the dominant parallel wavenumber of lower hybrid waves in the scrape-off layer (SOL) of the Alcator C-Mod tokamak with an array of magnetic loop probes. The electric field strength measured with the probe in typical C-Mod plasmas is about one-fifth of that of the electric field at the mouth of the grill antenna. The amplitude and phase responses of the measured signals on the applied power spectrum are consistent with the expected wave energy propagation. At higher density, the observed k{sub ||} increases for the fixed launched k{sub ||}, and the wave amplitude decreases rapidly. This decrease is correlated with the loss of LHCD efficiency at high density, suggesting the presence of loss mechanisms. Evidence of the spectral broadening mechanisms is observed in the frequency spectra. However, no clear modifications in the dominant k{sub ||} are observed in the spectrally broadened wave components, as compared to the measured k{sub ||} at the applied frequency. It could be due to (1) the probe being in the SOL and (2) the limited k{sub ||} resolution of the diagnostic. Future experiments are planned to investigate the roles of the observed spectral broadening mechanisms on the LH density limit problem in the strong single pass damping regime.},
doi = {10.1063/1.4948555},
journal = {Physics of Plasmas},
issn = {1070-664X},
number = 5,
volume = 23,
place = {United States},
year = {2016},
month = {5}
}