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Title: Computational studies on scattering of radio frequency waves by density filaments in fusion plasmas

Abstract

In modern magnetic fusion devices, such as tokamaks and stellarators, radio frequency (RF) waves are commonly used for plasma heating and current profile control, as well as for certain diagnostics. The frequencies of the RF waves range from ion cyclotron frequency to the electron cyclotron frequency. The RF waves are launched from structures, like waveguides and current straps, placed near the wall in a very low density, tenuous plasma region of a fusion device. The RF electromagnetic fields have to propagate through this scrape-off layer before coupling power to the core of the plasma. The scrape-off layer is characterized by turbulent plasmas fluctuations and by blobs and filaments. The variations in the edge density due to these fluctuations and filaments can affect the propagation characteristics of the RF waves—changes in density leading to regions with differing plasma permittivity. Analytical full-wave theories have shown that scattering by blobs and filaments can alter the RF power flow into the core of the plasma in a variety of ways, such as through reflection, refraction, diffraction, and shadowing [see, for example, Ram and Hizanidis, Phys. Plasmas 23, 022504 (2016), and references therein]. There are changes in the wave vectors and the distribution of power–scatteringmore » leading to coupling of the incident RF wave to other plasma waves, side-scattering, surface waves, and fragmentation of the Poynting flux in the direction towards the core. However, these theoretical models are somewhat idealized. In particular, it is assumed that there is step-function discontinuity in the density between the plasma inside the filament and the background plasma. In this paper, results from numerical simulations of RF scattering by filaments using a commercial full-wave code are described. The filaments are taken to be cylindrical with the axis of the cylinder aligned along the direction of the ambient magnetic field. The plasma inside and outside the filament is assumed to be cold. There are three primary objectives of these studies. The first objective is to validate the numerical simulations by comparing with the analytical results for the same plasma description—a step-function discontinuity in density. A detailed comparison of the Poynting flux shows that numerical simulations lead to the same results as those from the theoretical model. The second objective is to extend the simulations to take into account a smooth transition in density from the background plasma to the interior of the filament. The ensuing comparison shows that the deviations from the results of the theoretical model are quite small. The third objective is to consider the scattering process for situations well beyond a reasonable theoretical analysis. This includes scattering off multiple filaments with different densities and sizes. In conclusion, simulations for these complex arrangements of filaments show that, in spite of the obvious limitations, the essential physics of RF scattering is captured by the analytical theory for a single filament« less

Authors:
ORCiD logo [1];  [2]; ORCiD logo [3];  [4]
+ Show Author Affiliations
  1. Karlsruhe Inst. of Technology (KIT) (Germany). Inst. for Pulsed Power and Microwave Technology
  2. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center
  3. National Technical Univ. of Athens, Athens (Greece). School of Electrical and Computer Engineering
  4. National and Kapodistrian Univ. of Athens, Athens (Greece). Faculty of Physics
Publication Date:
Research Org.:
Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1524588
Alternate Identifier(s):
OSTI ID: 1394860
Grant/Contract Number:  
FC02-01ER54648; FG02-91ER54109; FG02-99ER54525; FG02-91ER-54109
Resource Type:
Accepted Manuscript
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 24; Journal Issue: 10; Journal ID: ISSN 1070-664X
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY

Citation Formats

Ioannidis, Zisis C., Ram, Abhay K., Hizanidis, Kyriakos, and Tigelis, Ioannis G. Computational studies on scattering of radio frequency waves by density filaments in fusion plasmas. United States: N. p., 2017. Web. doi:10.1063/1.4992032.
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Ioannidis, Zisis C., Ram, Abhay K., Hizanidis, Kyriakos, & Tigelis, Ioannis G. Computational studies on scattering of radio frequency waves by density filaments in fusion plasmas. United States. https://doi.org/10.1063/1.4992032
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Ioannidis, Zisis C., Ram, Abhay K., Hizanidis, Kyriakos, and Tigelis, Ioannis G. Tue . "Computational studies on scattering of radio frequency waves by density filaments in fusion plasmas". United States. https://doi.org/10.1063/1.4992032. https://www.osti.gov/servlets/purl/1524588.
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@article{osti_1524588,
title = {Computational studies on scattering of radio frequency waves by density filaments in fusion plasmas},
author = {Ioannidis, Zisis C. and Ram, Abhay K. and Hizanidis, Kyriakos and Tigelis, Ioannis G.},
abstractNote = {In modern magnetic fusion devices, such as tokamaks and stellarators, radio frequency (RF) waves are commonly used for plasma heating and current profile control, as well as for certain diagnostics. The frequencies of the RF waves range from ion cyclotron frequency to the electron cyclotron frequency. The RF waves are launched from structures, like waveguides and current straps, placed near the wall in a very low density, tenuous plasma region of a fusion device. The RF electromagnetic fields have to propagate through this scrape-off layer before coupling power to the core of the plasma. The scrape-off layer is characterized by turbulent plasmas fluctuations and by blobs and filaments. The variations in the edge density due to these fluctuations and filaments can affect the propagation characteristics of the RF waves—changes in density leading to regions with differing plasma permittivity. Analytical full-wave theories have shown that scattering by blobs and filaments can alter the RF power flow into the core of the plasma in a variety of ways, such as through reflection, refraction, diffraction, and shadowing [see, for example, Ram and Hizanidis, Phys. Plasmas 23, 022504 (2016), and references therein]. There are changes in the wave vectors and the distribution of power–scattering leading to coupling of the incident RF wave to other plasma waves, side-scattering, surface waves, and fragmentation of the Poynting flux in the direction towards the core. However, these theoretical models are somewhat idealized. In particular, it is assumed that there is step-function discontinuity in the density between the plasma inside the filament and the background plasma. In this paper, results from numerical simulations of RF scattering by filaments using a commercial full-wave code are described. The filaments are taken to be cylindrical with the axis of the cylinder aligned along the direction of the ambient magnetic field. The plasma inside and outside the filament is assumed to be cold. There are three primary objectives of these studies. The first objective is to validate the numerical simulations by comparing with the analytical results for the same plasma description—a step-function discontinuity in density. A detailed comparison of the Poynting flux shows that numerical simulations lead to the same results as those from the theoretical model. The second objective is to extend the simulations to take into account a smooth transition in density from the background plasma to the interior of the filament. The ensuing comparison shows that the deviations from the results of the theoretical model are quite small. The third objective is to consider the scattering process for situations well beyond a reasonable theoretical analysis. This includes scattering off multiple filaments with different densities and sizes. In conclusion, simulations for these complex arrangements of filaments show that, in spite of the obvious limitations, the essential physics of RF scattering is captured by the analytical theory for a single filament},
doi = {10.1063/1.4992032},
journal = {Physics of Plasmas},
number = 10,
volume = 24,
place = {United States},
year = {Tue Sep 26 00:00:00 EDT 2017},
month = {Tue Sep 26 00:00:00 EDT 2017}
}
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Journal Article:
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Publisher's Version of Record
https://doi.org/10.1063/1.4992032
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Cited by: 20 works
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Figures / Tables:

FIG. 1 FIG. 1: The origin of the coordinate system is at the center of the cylindrical filament of radiusmore » $a$. The ambient magnetic field is along the axial direction $\hat{z}$, and $\hat{ρ}$ and $\hat{\phi}$are the unit vectors along the radial and azimuthal directions, respectively. The effect of the ends of the filament are ignored by assuming that $L$$\rightarrow$ ∞.« less
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Works referenced in this record:

Radially propagating fluctuation structures in the scrape-off layer of Alcator C-Mod
journal, January 2006

  • Grulke, O.; Terry, J. L.; LaBombard, B.
  • Physics of Plasmas, Vol. 13, Issue 1
  • DOI: 10.1063/1.2164991

Scattering of radio frequency waves by blobs in tokamak plasmas
journal, May 2013

  • Ram, Abhay K.; Hizanidis, Kyriakos; Kominis, Yannis
  • Physics of Plasmas, Vol. 20, Issue 5
  • DOI: 10.1063/1.4803898

On scrape off layer plasma transport
journal, May 2001

Scattering of electromagnetic waves by a plasma sphere embedded in a magnetized plasma
journal, October 2013

Edge turbulence measurements in toroidal fusion devices
journal, June 2007

Scattering of radio frequency waves by cylindrical density filaments in tokamak plasmas
journal, February 2016

  • Ram, Abhay K.; Hizanidis, Kyriakos
  • Physics of Plasmas, Vol. 23, Issue 2
  • DOI: 10.1063/1.4941588

High-speed imaging of edge turbulence in NSTX
journal, December 2003

Blob birth and transport in the tokamak edge plasma: Analysis of imaging data
journal, September 2006

  • Myra, J. R.; D’Ippolito, D. A.; Stotler, D. P.
  • Physics of Plasmas, Vol. 13, Issue 9
  • DOI: 10.1063/1.2355668
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    Works referencing / citing this record:

    Diffraction of radio frequency waves by spatially modulated interfaces in the plasma edge in tokamaks
    journal, June 2019

    Resonance scattering of an extraordinary wave by a cylindrical density depletion in a magnetoplasma
    journal, October 2018

    • Ivoninsky, Alexander V.; Kudrin, Alexander V.
    • Physics of Plasmas, Vol. 25, Issue 10
    • DOI: 10.1063/1.5054687

    Microwave beam broadening due to turbulent plasma density fluctuations within the limit of the Born approximation and beyond
    journal, May 2018

    • Köhn, A.; Guidi, L.; Holzhauer, E.
    • Plasma Physics and Controlled Fusion, Vol. 60, Issue 7
    • DOI: 10.1088/1361-6587/aac000

    From a reflectrometry code to a ‘standard’ EC code to investigate the impact of the edge density fluctuations on the EC waves propagation
    journal, September 2019

    • Bertelli, N.; Kramer, G. J.; Valeo, E. J.
    • Plasma Physics and Controlled Fusion, Vol. 61, Issue 10
    • DOI: 10.1088/1361-6587/ab4106

    Full-wave model for the lower hybrid wave electric field vector with synthetic turbulence on Alcator C-Mod
    journal, January 2020

    Redirection of radio-frequency power flow by filaments
    journal, February 2020

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      Figures / Tables found in this record:

      FIG. 1 (p. 3)figure
      FIG. 2 (p. 5)figure
      FIG. 3 (p. 6)figure
      FIG. 4 (p. 7)figure
      FIG. 5 (p. 7)figure
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      FIG. 10 (p. 9)figure
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      FIG. 17 (p. 12)figure
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      FIG. 19 (p. 12)figure
      TABLE I (p. 12)table
      FIG. 20 (p. 13)figure
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