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

Full-wave Analysis for Estimation of Microwave Beam Broadening by TurbulentDensity Fluctuations in LHD
Ryoma Yanai1, Toshiki Kinoshita2, Kenji Tanaka1,2, Hikona Sakai2, Shin Kubo3
1
National Institute for Fusion Science, 2 Kyushu University, 3 Chubu University
e-mail (speaker): yanai@nifs.ac.jp
Electron cyclotron heating (ECH) is an essential auxiliary
heating method for magnetically confined fusion plasmas.
A high-power microwave beam is used and the power isabsorbed in plasma via electron cyclotron resonance. ECH
realizes local high-power density heating by using a
focused beam and is used for electron heating, plasma
initiation, and current drive. Recent simulation andexperiment studies suggested that the ECH microwave
beam is scattered by turbulent density fluctuations inplasmas and the beam width is broadened.[1, 2] The
important characteristics of ECH such as heating locality
and the current drive efficiency can be deteriorated bybeam width broadening caused by the turbulent density
fluctuations. Therefore, it is necessary to investigate andevaluate the effect of the density fluctuations on the ECH
beam quality. The Large Helical Device (LHD) has 1
MW-class ECH systems using 77 GHz and 154 GHz
gyrotrons. A two-dimensional phase contrast imaging(PCI) system is in operation at the LHD[3] and can provide
data on the absolute value of electron density fluctuations
caused by ion-scale turbulence. This PCI system also
provides the time evolution of the radial profile of thedensity fluctuations including the wavenumber spectrum.
We carry out the electromagnetic simulation with a cold
plasma model to analyze the effect of the density
fluctuations on the 77 GHz microwave beam based on theECH system in the LHD. Firstly, we conducted a twodimensional simulation to investigate the influence of thescale of the density fluctuations by introducing a

monochromatic wavenumber structure into the ambient
electron density 𝑛!" = 3 × 10#$ m%& . The amplitude ofthe density fluctuations is 2% of 𝑛!" . Figure. 1 shows anexample of the two-dimensional profile of electron
density for the simulation and the wave electric field of a
Gaussian beam propagating in a plasma model with amonochromatic density fluctuation, where thewavenumber of the fluctuation was set to one-tenth of thatof a 77 GHz electromagnetic wave in free space (𝑘'' )*+ ).
The wave electric field was distorted, and the profile
deviated from the Gaussian profile by the density
fluctuations. The simulation result showed that thewavenumber less than one-third of 𝑘'' )*+ had relatively
large influences on the beam quality. We will show thedetail of the effect of the fluctuation size and the result
using a precise model based on the experimental data ofLHD plasma in the presentation.
This work was supported by JSPS KAKENHI Grant
Number JP19K14688.
References[1] A. Köhn et al., Phys. Control. Fusion 60, 075006(2018).
[2] M. W. Brookman et al, Phys. Plasmas 28, 042507(2021).
[3] K. Tanaka et al., Rev. Sci. Instrum. 79 10E702(2008).

Figure. 1. Two-dimensional profile of electron density with monochromatic density fluctuations for fullwave simulation (left), and the absolute value of the wave electric field propagating in the positive xdirection from 𝑥 = 0 m (right).