Abstract
Correlation ECE (CECE) is a diagnostic technique that allows measurement of small amplitude electron temperature, Te, fluctuations through standard cross-correlation analysis methods. In Alcator C-Mod, a new CECE diagnostic has been installed[Sung RSI 2012], and interesting phenomena have been observed in various plasma conditions. We find that local Te fluctuations near the edge (ρ ~ 0:8) decrease across the linearto- saturated ohmic confinement transition, with fluctuations decreasing with increasing plasma density[Sung NF 2013], which occurs simultaneously with rotation reversals[Rice NF 2011]. Te fluctuations are also reduced across core rotation reversals with an increase of plasma density in RF heated L-mode plasmas, which implies that the same physics related to the reduction of Te fluctuations may be applied to both ohmic and RF heated L-mode plasmas. In I-mode plasmas, we observe the reduction of core Te fluctuations, which indicates changes of turbulence occur not only in the pedestal region but also in the core across the L/I transition[White NF 2014]. The present CECE diagnostic system in C-Mod and these experimental results are described in this paper.
Highlights
In fusion research, the cross-field transport level is usually higher than the level predicted from neo-classical theory, and it is considered that turbulent transport is responsible for this anomalous transport[1]
In order to compare Te fluctuations across the Linear Ohmic Confinement (LOC)/Saturated Ohmic Confinement (SOC) transition, we studied the dependence of fluctuations on the normalized average electron density(ne/ncrit), where ncrit is the rotation reversal density[14], which defines the LOC/SOC boundary
In addition to the ohmic LOC/SOC experiments, we have used the new Correlation ECE (CECE) system to study changes in turbulence occurring across spontaneous rotation reversals in RF heated L-mode plasmas
Summary
The cross-field transport level is usually higher than the level predicted from neo-classical theory, and it is considered that turbulent transport is responsible for this anomalous transport[1]. Since turbulent transport is determined by fluctuations of potential, density, temperature and magnetic field and their phase relations, fluctuation measurements are required to understand the turbulent transport behavior. Correlation ECE (CECE) is a diagnostic technique that can resolve small amplitude Te fluctuations. It is hard to measure turbulent Te fluctuations, whose level is ∼ 1%, with ECE radiometry due to inherent thermal noise. If we have two channels with uncorrelated thermal noise and common Te fluctuations, the fluctuations can be resolved by correlating these two channels. More details about the principles of CECE can be found in [2,3,4]
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