DOE Data Explorer title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Mode-conversion of the extraordinary wave at the upper hybrid resonance in the presence of small-amplitude density fluctuations

Abstract

In spherical tokamaks, the electron plasma frequency is greater than the electron cyclotron frequency. Electromagnetic waves in the electron cyclotron range of frequencies are unsuitable for directly heating such plasmas due to their reduced accessibility. However, mode-conversion of the extraordinary wave to the electron Bernstein wave (X-B mode-conversion) at the upper hybrid resonance makes it possible to efficiently couple externally-launched electromagnetic wave energy into an overdense plasma core. Traditional mode-conversion models describe an X-mode wave propagating in a potential containing two cutoffs that bracket a single wave resonance. Often, however, the mode-conversion region is in the edge, where turbulent fluctuations and blobs can generate abrupt cutoffs and scattering of the incident X-mode wave. We present a new framework for studying the X-B mode-conversion which makes the inclusion of these fluctuations analytically tractable. In the new approach, the high- field cutoff is modelled as an infinite barrier, which manifests as a boundary condition applied to a wave equation involving only one cutoff adjacent to the resonance on the low-field side. The new model reproduces the main features of the previous approach, yet is more suitable for analyzing experimental observations and extrapolating to higher dimensions. We then develop an analytical estimate formore » the effect of small-amplitude, quasi-monochromatic density fluctuations on the X-B mode-conversion efficiency using perturbation theory. We find that Bragg backscattering of the launched X-mode wave reduces the mode-conversion efficiency significantly when the fluctuation wavenumber is resonant with the wavenumber of the incident X-mode wave. These analytical results are corroborated by numerically integrating the mode-conversion equations.« less

Authors:
;
Publication Date:
DOE Contract Number:  
FG02-91ER54109; FG02-99ER54525; FC02-01ER54648
Research Org.:
Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center
Sponsoring Org.:
USDOE Office of Science (SC), Fusion Energy Sciences (FES)
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY
OSTI Identifier:
1881328
DOI:
https://doi.org/10.7910/DVN/BMYBAV

Citation Formats

Lopez, N. A., and Ram, A. K. Mode-conversion of the extraordinary wave at the upper hybrid resonance in the presence of small-amplitude density fluctuations. United States: N. p., 2019. Web. doi:10.7910/DVN/BMYBAV.
Lopez, N. A., & Ram, A. K. Mode-conversion of the extraordinary wave at the upper hybrid resonance in the presence of small-amplitude density fluctuations. United States. doi:https://doi.org/10.7910/DVN/BMYBAV
Lopez, N. A., and Ram, A. K. 2019. "Mode-conversion of the extraordinary wave at the upper hybrid resonance in the presence of small-amplitude density fluctuations". United States. doi:https://doi.org/10.7910/DVN/BMYBAV. https://www.osti.gov/servlets/purl/1881328. Pub date:Mon Mar 18 00:00:00 EDT 2019
@article{osti_1881328,
title = {Mode-conversion of the extraordinary wave at the upper hybrid resonance in the presence of small-amplitude density fluctuations},
author = {Lopez, N. A. and Ram, A. K.},
abstractNote = {In spherical tokamaks, the electron plasma frequency is greater than the electron cyclotron frequency. Electromagnetic waves in the electron cyclotron range of frequencies are unsuitable for directly heating such plasmas due to their reduced accessibility. However, mode-conversion of the extraordinary wave to the electron Bernstein wave (X-B mode-conversion) at the upper hybrid resonance makes it possible to efficiently couple externally-launched electromagnetic wave energy into an overdense plasma core. Traditional mode-conversion models describe an X-mode wave propagating in a potential containing two cutoffs that bracket a single wave resonance. Often, however, the mode-conversion region is in the edge, where turbulent fluctuations and blobs can generate abrupt cutoffs and scattering of the incident X-mode wave. We present a new framework for studying the X-B mode-conversion which makes the inclusion of these fluctuations analytically tractable. In the new approach, the high- field cutoff is modelled as an infinite barrier, which manifests as a boundary condition applied to a wave equation involving only one cutoff adjacent to the resonance on the low-field side. The new model reproduces the main features of the previous approach, yet is more suitable for analyzing experimental observations and extrapolating to higher dimensions. We then develop an analytical estimate for the effect of small-amplitude, quasi-monochromatic density fluctuations on the X-B mode-conversion efficiency using perturbation theory. We find that Bragg backscattering of the launched X-mode wave reduces the mode-conversion efficiency significantly when the fluctuation wavenumber is resonant with the wavenumber of the incident X-mode wave. These analytical results are corroborated by numerically integrating the mode-conversion equations.},
doi = {10.7910/DVN/BMYBAV},
journal = {},
number = ,
volume = ,
place = {United States},
year = {2019},
month = {3}
}

Works referencing / citing this record: