Correlation ECE diagnostic in Alcator C-Mod

Abstract

Abstract. Correlation ECE (CECE) is a diagnostic technique that allows measurement of small amplitude electron temperature, T e , 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 T e 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]. T e 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 T e fluctuations may be applied to both ohmic and RF heated L-mode plasmas. In I-mode plasmas, we observe the reduction of core T e 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.