Change of fluctuation properties during non-local temperature rise in LHD from 2d phase contrast imaging

Abstract

Fluctuation properties are analyzed in the Large Helical Device (LHD) for discharges exhibiting a non-local core temperature rise in response to edge cooling by injection of a Tracer Encapsulated Solid Pellet (TESPEL). Edge ion gyro-scale density fluctuation amplitudes, measured with a 2D CO2 phase contrast imaging system, are found to increase as the core temperature is rising and is therefore anti-correlated with a reduction of the total heat flux. This suggests that another mechanism, such as turbulence de-correlation has a stronger role on the heat flux than change of the density fluctuation level. The core fluctuation properties, in particular the phase velocity, change immediately in response to the edge temperature reduction, and possibly change of gradient, unresponsive to core gradients, suggesting a possible non-local transfer mechanism between the edge and core. Comparing different cases, it appears that the excitation of a fluctuation branch with higher core frequency and phase velocity (∼6km/s) is an essential ingredient for the core Te rise. After pellet injection, the amplitude of higher k components increases, implying a reduction of correlation length. One possible interpretation is that there may be increased E × B shear.