Cause of transport-channel decoupling in TJ-II, examined from its sensitivity to the relative phase between density and temperature fluctuations

Uncoupling transport channels is a key goal for reactor-scenario optimization. We seek insight from a new retarding-field energy analyzer method, measuring the plasma ion temperature T fluctuations [1], combined with a new Langmuir probe method [2]. In TJ-II, T evolves during the transition from the electron-root to the ion-root in the Scrape-Off-Layer (SOL) region. As line-averaged density n increases above a threshold, the edge radial-electric-field E_r reverses from + to -- values, as predicted by the neoclassical electron-to-ion root transition. Results in the SOL show a decrease in T, concomitant with reduced edge-SOL turbulence spreading, controlled by edge E_r. A helium-line-ratio technique in TJ-II [3] measures n of turbulent coherent structures in the plasma edge using fast (15 usec), few-mm-resolution, 2-dim imaging of n -- an upgrade uses a triple-bundle technique which 2-dim-maps n,T fluctuations and relative phase. Spatially overlapped heavy-ion-beam probe (HIBP) [4] profiles (from core to edge) and Langmuir probe profiles (at plasma edge), which agree very well, allow novel baffle-probe configurations to be tested, and provide plasma-boundary conditions for HIBP calibration. [1] RSI 82 (2011) 043505; [2] Contr. Plasma Phys. 44 (2004) 689; [3] NF 56 (2016) 106031; [4] NF 51 (2011) 083043.