Magnetic shear effect on plasma transport in ECH injected DIII-D plasma*

The effect of magnetic shear on plasma transport for T_e/T_i near unity has been explored in DIII-D during ECH. Previous reports showed that significant confinement degradation occurs at T_e/T_i~1 in positive shear (PS) plasmas in DIII-D and JT-60U [1,2], whereas confinement degradation is reduced at T_e/T_i~1 in negative central shear (NCS) plasmas [1]. In this study, plasma transport in weak magnetic shear (WS) plasmas with ECH is investigated and compared with that in NCS and PS plasmas. Weak magnetic shear is found to be effective in minimizing degradation of ion thermal confinement as T_e/T_i increases through ECH application, and an improved confinement factor of H_98y2~1.2 is maintained, similar to NCS plasmas. During ECH, the ion thermal diffusivity in the core region becomes smaller with decreasing magnetic shear and the density and toroidal rotation in the core region tend to be steeper in the negative shear region. The variation in plasma transport will be investigated using gyro-kinetic simulations.

[1]M. Yoshida et al Nucl. Fusion 57 (2017) 056027

[2]G.R. McKee et al 25th IAEA FEC (St Petersburg, Russia, 2014) EX/2-2