Investigation of Density Fluctuation Influence on ECH Microwave Propagation in LHD*

Electron cyclotron heating (ECH) is an essential electron heating and current drive method for magnetically confined fusion plasmas and this method uses a high-power focused microwave beam, which is absorbed in plasma via electron cyclotron resonance. Recent studies suggested the ECH microwave beam is scattered by turbulent density fluctuations in plasmas and the beam width evaluated by a Gaussian profile is broadened. The beam broadening caused by the density fluctuations will deteriorate the heating locality and the current drive efficiency by ECH in plasmas. Therefore, it is important to evaluate the influence of the density fluctuations on the ECH beam. The Large Helical Device (LHD) is equipped with 1 MW class ECH systems of 77 GHz and 154 GHz. A two-dimensional phase contrast imaging (PCI) system is operational in LHD and can measure the absolute value of electron density fluctuations caused by ion-scale turbulence. This PCI system provides the time evolution of the radial profile of the density fluctuations including the wavenumber spectrum. We carry out the electromagnetic simulation to analyze the effect of the density fluctuations on the 77 GHz microwave beam based on the ECH system in LHD. We will present the influence of the spatial structure of the density fluctuations on the ECH beam and the estimation of the 77 GHz ECH beam broadening caused by density fluctuations measured by the PCI in the LHD.