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Title: Phase coherence of parametric-decay modes during high-harmonic fast-wave heating in the National Spherical Torus Experiment

Abstract

Third-order spectral analysis, in particular, the auto bicoherence, was applied to probe signals from high-harmonic fast-wave heating experiments in the National Spherical Torus Experiment. Strong evidence was found for parametric decay of the 30 MHz radio-frequency (RF) pump wave, with a low-frequency daughter wave at 2.7 MHz, the local majority-ion cyclotron frequency. The primary decay modes have auto bicoherence values around 0.85, very close to the theoretical value of one, which corresponds to total phase coherence with the pump wave. The threshold RF pump power for onset of parametric decay was found to be between 200 kW and 400 kW.

Authors:
 [1]; ; ; ;  [2]
  1. Crow Radio and Plasma Science, Princeton, New Jersey 08540 (United States)
  2. Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, New Jersey 08543-0451 (United States)
Publication Date:
OSTI Identifier:
22598925
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 23; Journal Issue: 6; Other Information: (c) 2016 Author(s); Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; CYCLOTRON FREQUENCY; CYCLOTRONS; DECAY; HEATING; IONS; MHZ RANGE 01-100; NSTX DEVICE; RADIOWAVE RADIATION; SPHERICAL CONFIGURATION

Citation Formats

Carlsson, J. A., E-mail: carlsson@pppl.gov, Wilson, J. R., Hosea, J. C., Greenough, N. L., and Perkins, R. J.. Phase coherence of parametric-decay modes during high-harmonic fast-wave heating in the National Spherical Torus Experiment. United States: N. p., 2016. Web. doi:10.1063/1.4954825.
Carlsson, J. A., E-mail: carlsson@pppl.gov, Wilson, J. R., Hosea, J. C., Greenough, N. L., & Perkins, R. J.. Phase coherence of parametric-decay modes during high-harmonic fast-wave heating in the National Spherical Torus Experiment. United States. doi:10.1063/1.4954825.
Carlsson, J. A., E-mail: carlsson@pppl.gov, Wilson, J. R., Hosea, J. C., Greenough, N. L., and Perkins, R. J.. Wed . "Phase coherence of parametric-decay modes during high-harmonic fast-wave heating in the National Spherical Torus Experiment". United States. doi:10.1063/1.4954825.
@article{osti_22598925,
title = {Phase coherence of parametric-decay modes during high-harmonic fast-wave heating in the National Spherical Torus Experiment},
author = {Carlsson, J. A., E-mail: carlsson@pppl.gov and Wilson, J. R. and Hosea, J. C. and Greenough, N. L. and Perkins, R. J.},
abstractNote = {Third-order spectral analysis, in particular, the auto bicoherence, was applied to probe signals from high-harmonic fast-wave heating experiments in the National Spherical Torus Experiment. Strong evidence was found for parametric decay of the 30 MHz radio-frequency (RF) pump wave, with a low-frequency daughter wave at 2.7 MHz, the local majority-ion cyclotron frequency. The primary decay modes have auto bicoherence values around 0.85, very close to the theoretical value of one, which corresponds to total phase coherence with the pump wave. The threshold RF pump power for onset of parametric decay was found to be between 200 kW and 400 kW.},
doi = {10.1063/1.4954825},
journal = {Physics of Plasmas},
number = 6,
volume = 23,
place = {United States},
year = {Wed Jun 15 00:00:00 EDT 2016},
month = {Wed Jun 15 00:00:00 EDT 2016}
}
  • A deuterium H-mode discharge with a plasma current of 300 kA, an axial toroidal magnetic field of 0.55 T, and a calculated non-inductive plasma current fraction of 0.7 1 has been generated in the National Spherical Torus Experiment by 1.4MW of 30MHz high-harmonic fast wave (HHFW) heating and current drive. Seventy-five percent of the non-inductive current was generated inside an internal transport barrier that formed at a normalized minor radius 0.4. Three quarters of the non-inductive current was bootstrap current, and the remaining non-inductive current was generated directly by HHFW power inside a normalized minor radius 0.2. VC 2012 Americanmore » Institute of Physics.« less
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  • High harmonic fast wave heating and current drive (CD) are being developed on the National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 41, 1435 (2001)] for supporting startup and sustainment of the spherical torus plasma. Considerable enhancement of the core heating efficiency ({eta}) from 44% to 65% has been obtained for CD phasing of the antenna (strap-to-strap {phi}=-90 deg., k{sub {phi}}=-8 m{sup -1}) by increasing the magnetic field from 4.5 to 5.5 kG. This increase in efficiency is strongly correlated to moving the location of the onset density for perpendicular fast wave propagation (n{sub onset}{proportional_to}Bxk{sub parallel}{sup 2}/{omega}) awaymore » from the antenna face and wall, and hence reducing the propagating surface wave fields. Radio frequency (RF) waves propagating close to the wall at lower B and k{sub parallel} can enhance power losses from both the parametric decay instability (PDI) and wave dissipation in sheaths and structures around the machine. The improved efficiency found here is attributed to a reduction in the latter, as PDI losses are little changed at the higher magnetic field. Under these conditions of higher coupling efficiency, initial measurements of localized CD effects have been made and compared with advanced RF code simulations.« less