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Modelling of sheath effects on radio-frequency antennas

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Abstract
The large voltages on radio frequency (RF) antennas that are used for heating of fusion plasmas, can create a thin sheath layer with largely negative potential and thus strong electric near-fields that attract and accelerate positively charged ions. The possible damage to antenna and in-vessel components due to local overheating and sputtering, is one of the main concerns for high power antennas in future fusion reactors. Good predictive simulation tools that take these sheath effects into account are still lacking. A practical implementation for modelling codes was proposed in [1], where sheath properties are introduced by means of a non-linear sheath boundary condition (SBC) on antenna surfaces. The sheath is represented by a scalar dielectric medium with relative permittivity sh = 1 + ish, i.e. a lossy vacuum layer. It is assumed that the electrons are inertia-free and therefore accelerated immediately into the metal surface, and that the power lost in the sheath is purely coming from ions accelerated in the rectified sheath potential. The sheath width (sh) is determined by the Child-Langmuir law, and the sheath potential depends on the electric field component normal to the surface. Continuity of the normal component of the displacement vector at the sheath plasma interface leads to the general description of the sheath as boundary condition Et = t ((sh/sh) n·pl·E) = t ((sh/sh) Dn), where Et is the tangential component of electric field and Dn the normal component of the displacement vector, all with respect to the sheath surface. For pl a cold plasma [2] description is used. Due to the Dn dependence of the sheath width the SBC is a non-linear equation, preventing a direct inversion of the underlying set of equations. A hybrid implementation of a SBC in the TOPICA code [3] was reported in [4], plasma properties were introduced for the calculation of the sheath parameters (sh, pl and sh), but the wave propagation was calculated using a vacuum Green's function. In the present paper a realistic finite density plasma is assumed to surround the antenna, and a cold plasma description assesses the impact of a magnetized dielectric medium on the antenna near-fields. The COMSOL Multiphysics [5] package was used for the RF modelling.

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MLA
Crombé, Kristel, et al. “Modelling of Sheath Effects on Radio-Frequency Antennas.” Europhysics Conference Abstracts (ECA) Series, vol. 36.F, European Physical Society, 2012.
APA
Crombé, K., Kyrytsya, V., & Van Eester, D. (2012). Modelling of sheath effects on radio-frequency antennas. Europhysics Conference Abstracts (ECA) Series, 36.F. European Physical Society.
Chicago author-date
Crombé, Kristel, V Kyrytsya, and D Van Eester. 2012. “Modelling of Sheath Effects on Radio-Frequency Antennas.” In Europhysics Conference Abstracts (ECA) Series. Vol. 36.F. European Physical Society.
Chicago author-date (all authors)
Crombé, Kristel, V Kyrytsya, and D Van Eester. 2012. “Modelling of Sheath Effects on Radio-Frequency Antennas.” In Europhysics Conference Abstracts (ECA) Series. Vol. 36.F. European Physical Society.
Vancouver
1.
Crombé K, Kyrytsya V, Van Eester D. Modelling of sheath effects on radio-frequency antennas. In: Europhysics Conference Abstracts (ECA) series. European Physical Society; 2012.
IEEE
[1]
K. Crombé, V. Kyrytsya, and D. Van Eester, “Modelling of sheath effects on radio-frequency antennas,” in Europhysics Conference Abstracts (ECA) series, Stockholm, Sweden, 2012, vol. 36.F.
@inproceedings{3006085,
  abstract     = {{The large voltages on radio frequency (RF) antennas that are used for heating of fusion plasmas, can create a thin sheath layer with largely negative potential and thus strong electric near-fields that attract and accelerate positively charged ions. The possible damage to antenna and in-vessel components due to local overheating and sputtering, is one of the main concerns for high power antennas in future fusion reactors. Good predictive simulation tools that take these sheath effects into account are still lacking. A practical implementation for modelling codes was proposed in [1], where sheath properties are introduced by means of a non-linear sheath boundary condition (SBC) on antenna surfaces. The sheath is represented by a scalar dielectric medium with relative permittivity sh = 1 + ish, i.e. a lossy vacuum layer. It is assumed that the electrons are inertia-free and therefore accelerated immediately into the metal surface, and that the power lost in the sheath is purely coming from ions accelerated in the rectified sheath potential. The sheath width (sh) is determined by the Child-Langmuir law, and the sheath potential depends on the electric field component normal to the surface. Continuity of the normal component of the displacement vector at the sheath plasma interface leads to the general description of the sheath as boundary condition Et = t ((sh/sh) n·pl·E) = t ((sh/sh) Dn), where Et is the tangential component of electric field and Dn the normal component of the displacement vector, all with respect to the sheath surface. For pl a cold plasma [2] description is used. Due to the Dn dependence of the sheath width the SBC is a non-linear equation, preventing a direct inversion of the underlying set of equations. A hybrid implementation of a SBC in the TOPICA code [3] was reported in [4], plasma properties were introduced for the calculation of the sheath parameters (sh, pl and sh), but the wave propagation was calculated using a vacuum Green's function. In the present paper a realistic finite density plasma is assumed to surround the antenna, and a cold plasma description assesses the impact of a magnetized dielectric medium on the antenna near-fields. The COMSOL Multiphysics [5] package was used for the RF modelling.}},
  articleno    = {{P4.045}},
  author       = {{Crombé, Kristel and Kyrytsya, V and Van Eester, D}},
  booktitle    = {{Europhysics Conference Abstracts (ECA) series}},
  isbn         = {{2914771797}},
  language     = {{eng}},
  location     = {{Stockholm, Sweden}},
  pages        = {{4}},
  publisher    = {{European Physical Society}},
  title        = {{Modelling of sheath effects on radio-frequency antennas}},
  url          = {{http://ocs.ciemat.es/EPSICPP2012PAP/html/}},
  volume       = {{36.F}},
  year         = {{2012}},
}