Evidence of turbulence spreading in the inner shear layer of wide-pedestal QH-mode discharges of DIII-D*

Trapped Electron Mode (TEM)-scale density turbulence (ñ) has been found to be first excited near the bottom of the E_r well and then seemingly propagates towards the pedestal top of wide-pedestal [1] QH-mode discharges. This radially inward propagation is evidenced by a delay in the limit cycle oscillation (LCO) [2] modulated ñ amplitude evolution. This ñ amplitude evolution correlates with a radially inward propagation of an increased electron temperature (T_e) gradient front. The turbulence front is found to be followed by a correlated decrease in local T_e (and its gradient). This phenomenon of local cooling in T_e resembles that of an inward cold pulse propagation and maybe related to the turbulence driven local transport. As these processes evolve (i.e., increase in local T_e gradient leads to increase in ñ amplitude followed by a cold pulse propagation) at LCO timescale, the pedestal structure (height and width) is also found to be modulated [2] at LCO timescales indicating role of turbulence spreading and related local transport on pedestal structure.

[1] K. H. Burrell et al, Phys. Plasmas 23, 056103 (2016).

[2] K. Barada et al, Phys. Plasmas 26, 092501 (2019).