Development of the Q-band ECE imaging system in the large helical device

In this study, we developed the Electron Cyclotron Emission Imaging (ECEI) system with the Q-band in the Large Helical Device (LHD). ECEI measurement makes it possible to obtain the spatiotemporal structure of magnetohydrodynamics (MHD) instabilities in the high-β plasma. Although there were several difficulties for realizing the multi-channelization, such as local oscillator (LO) optics and an expensive high-power LO source, we have solved these problems by developing a Local Integrated Antenna array (LIA) which has an internal LO supply, using a frequency doubler integrated circuit on each channel, instead of a conventional Horn-antenna Mixer Array (HMA) with common LOs. In addition, we have made some improvements to enhance the quality of the measurement signal. First, we developed and introduced notch filters that prevent the strong Electron Cyclotron Resonance Heating (ECRH) stray signal from being mixed into the measurement circuit. Second, the position of the doubler built in the printed circuit board was reconsidered to prevent the mixing of higher harmonic components into the mixer. Also, we have adopted the Logarithmic detector (LOG detector) to deal with the wide dynamic range of the plasma fluctuation. After these improvements, for the first time, we could successfully obtain the initial results of the two-dimensional temperature distribution and its fluctuation distribution in the LHD.

Development of high-power, long-pulse, multi-frequency ECH/CD system for JT-60SA

A gyrotron and a matching optics unit (MOU) for the multi-frequency electron cyclotron heating and current drive (ECH/CD) system in JT-60SA have been developed successfully. The gyrotron demonstrated stable operation at high powers of 1.5 MW for 5 s and 1.9 MW for 1 s at 110 GHz. To obtain high HE11 mode purity (> 90%) at the outlet of the waveguide in the ECH/CD launcher, a MOU for operating at three frequencies of 82 GHz, 110 GHz, and 138 GHz that includes three pairs of water-cooled phase correcting mirrors has been developed, which allows the mirrors to be changed without opening the evacuated MOU. The mode purity at the inlet of a dummy load in the transmission line was evaluated at high-power and an HE11 mode purity of > 90% was obtained at three frequencies.

Suppression of toroidal Alfvén eigenmodes by the electron cyclotron current drive in KSTAR plasmas

Advanced operation scenarios such as high poloidal beta (βP) or high q min are promising concepts to achieve the steady-state high-performance fusion plasmas. However, those scenarios are prone to substantial Alfvénic activity, causing fast-ion transport and losses. Recent experiments with the advanced operation scenario on KSTAR tokamak have shown that the electron cyclotron current drive (ECCD) is able to mitigate and suppress the beam-ion driven toroidal Alfvén eigenmodes (TAEs) for over several tens of global energy confinement time. Co-current directional intermediate off-axis ECCD lowers the central safety factor slightly and tilts the central q-profile shape so that the continuum damping in the core region increases. Besides, the rise of central plasma pressure and increased thermal-ion Landau damping contribute to TAE stabilization. While the TAEs are suppressed, neutron emission rate and total stored energy increase by approximately 45% and 25%, respectively. Fast-ion transport estimated by TRANSP calculations approaches the classical level during the TAE suppression period. Substantial reduction in fast-ion loss and neutron deficit is also observed. Enhancement of fast-ion confinement by suppressing the TAEs leads to an increase of non-inductive current fraction and will benefit the sustainment of the long-pulse high-performance discharges.