Description
Elucidating the influence of turbulence on plasma confinement is a crucial step toward achieving nuclear fusion in magnetically confined plasmas. Turbulence-driven fluctuations in electron density, electron temperature, and plasma potential generate additional electron heat flux and are believed to play a dominant role in anomalous transport. Consequently, experimental measurements of electron temperature fluctuations are essential for understanding turbulence-induced electron heat transport and for improving predictive models of plasma confinement.
The correlation electron cyclotron emission (CECE) technique is the primary diagnostic method for measuring small, turbulence-induced electron temperature fluctuations in high-temperature plasmas. The radial CECE approach, which employs correlations between electron cyclotron emission (ECE) signals in adjacent frequency bands, has been widely implemented in many magnetically confined plasma devices. By correlating signals from closely spaced frequency channels, the CECE method suppresses uncorrelated thermal noise in the ECE signals and isolates coherent fluctuation components, thereby achieving higher sensitivity to small-amplitude electron temperature fluctuations.
Recently, two different radial CECE systems have been installed and operated to investigate the behaviour of electron temperature fluctuations in the Large Helical Device (LHD). One system employs a fixed in-vessel metallic antenna with a radial line of sight. It utilizes YIG filters in the intermediate-frequency (IF) section to radially scan the observation location using W-band and D-band ECE signals. The other system uses an electron cyclotron heating (ECH) antenna and transmission line for 154 GHz ECH. This system employs fixed band-pass filters in the IF section and allows changes in the observation location by adjusting the antenna direction. In this presentation, we will describe the configurations of these CECE systems and present recent measurement results obtained in the LHD.