7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Two-dimensional full wave analysis of O-X-B mode conversionof electron cyclotron waves in tokamak plasmas
1Shabbir A. Khan1, Atsushi Fukuyama2
National Centre for Physics, Pakistan, 2Department of Nuclear Engineering, Kyoto University
e-mail (speaker): sakhan@ncp.edu.pk

Electron cyclotron (EC) waves have been extensively
employed for heating and current drive in tokamak
plasmas because of good coupling with antenna,
accessibility to high-density plasma, localized andcontrollable power deposition profile, and recent
availability of high power sources. When the EC wave islaunched from the low-field-side of a tokamak, both theordinary (O) mode and the extra-ordinary (X) mode may
encounter the cutoff layer at a certain density and cannot
penetrate into high-density region. It has been shown byHansen et al. [1], however, that the O mode with
optimum injection angle is mode-converted to the X
mode which penetrates into the higher-density region
after converted to the electron Bernstein (B) wave.
In order to describe the O-X-B mode conversion in
tokamak configuration, the one-dimensional kinetic fullwave code (TASK/W1) was developed [2]. The codesolves the integro-differential equation for the wave
electric field with frequency ω with the integral form of
the dielectric tensor where the electron gyro motion is
fully take into account. This model describes thetunneling of the wave with non-optimum injection angle
over the evanescent layer and the excitation of theshort-wavelength Bernstein waves.
In a realistic plasma configuration, two-dimensionalbehavior of the wave in horizontal plane and poloidal
plane has to be considered. First the wave structure inhorizontal plane of the tokamak configuration isconsidered. Figure 1 shows the wave structure for typicalspherical tokamak configuration (R0=1.76m, a=1.28m,B0=0.08T, ne0=1.2x1017m-3, f=2.45GHz).
The O mode excited by the waveguide is converted to
the X mode, reflected, further converted to the Bernsteinwave, and absorbed near the electron cyclotron
resonance. It has been shown that, with a small amountof collisions, ν/ω ~ 10-5, the collisional absorption
exceeds the cyclotron absorption.
The second analysis on the poloidal plane requires thetwo dimensional code (TASK/WF2D) with thefinite-element-method and the integral form of thedielectric tensor including the effect of inhomogeneous
cyclotron resonance. The results will be presented in theconference.
References[1] F.R. Hansen J.P. Lynov, P. Michelsen., Plasma Phys.
Control. Fusion 27 1077 (1985)[2] S.A. Khan, A. Fukuyama, H.Idei, H.Igami, Plasma
Fus. Res. 11 2403070 (2016)

Figure 1. Wave electric field on the mid plane of the tokamak configuration. The horizontal axis is in the direction ofmajor radius, and the vertical axis the toroidal direction. The figures show the imaginary part of Ex (the electronBernstein wave,) real part of Ey (the X mode component), and the imaginary part of Ez (the O mode,) respectively fromthe left. The O mode is excited by the waveguide located near the lower right corner, mode-converted to the X modenear the cutoff (x ~ 1.0m), reflected near x ~ 0.7m, mode-converted to the Bernstein wave near the UHR (x ~ 0.9m) andabsorbed near the electron cyclotron resonance (x ~ 0.0m).