Manipulating density pedestal structure to improve core-edge integration towards low collisionality*

By leveraging the benefits of low-density-gradient pedestal in the closed divertor, DIII-D has achieved a promising core-edge integrated scenario plasma which integrates a high-temperature, low-collisionality (n^*_ped<1) pedestal with a partially detached divertor. With a closed divertor and high heating power, strong gas puffing moves the pedestal peak density radially outward and reduces the density gradient in the pedestal region. These leads to a significant separation between density and temperature pedestals and result in high η_e - well above the stability threshold. Concomitantly, measured electron turbulence (kr_s>1) is enhanced at pedestal and strongly correlated with the high η_e. Nonlinear CGYRO simulations found a similar instability as seen in experiment driving significant heat transport that could broaden the pedestal to be wider than the EPED scaling. The wide temperature pedestal facilitates the achievement of high-temperature low-collisionality pedestal. Simultaneously, the outward shift of the density pedestal facilitates access to detached divertor conditions with low temperature and low heat flux towards target plate. Experiments also found that higher plasma current, strong shaping and higher heating power are beneficial for this promising core-edge scenario plasmas.