Long-lived predator-prey dynamics in the pedestal of near-zero torque high performance DIII-D plasmas

Novel turbulence dynamics are found to regulate ELM-free pedestal gradients of near-zero applied torque, wide-pedestal quiescent H-mode plasmas. ELM-free, low torque operation of future burning plasmas, such as in ITER, make this an important research area. In these plasmas, a long-lived predator-prey type limit cycle oscillation (LCO) regime is sustained for 3-12 energy confinement times immediately following suppression of edge harmonic oscillations (EHOs). This LCO regime is characterized by coupled time dependent dynamics in E×B velocity shear (V'), electron temperature gradient (∇ T_e), and density fluctuation amplitude (ñ). A pulse-like radially inward propagation of perturbations in E×B velocity is found to be critical for the existence of this unique system. The temporal dynamics can be described as follows: ñ modifies transport, transport modifies gradients, gradients modify E×B velocity. The spatiotemporally varying E×B velocity due to the above mentioned pulse like propagation modifies E×B shear which modifies ñ, and the cycle repeats. These quasi-stationary E×B velocity perturbations are observed to have toroidal and poloidal symmetry but are found inconsistent with turbulence driven zonal flows. LCO modulated transport, as evidenced by increased levels of divertor ion saturation current and divertor D_α intensity on LCO timescales, can serve to partially replace missing EHO related particle transport and allow continued ELM-free operation. This LCO transport is hypothesized to prevent the pressure gradient increases necessary to trigger ELMs. The unique LCO dynamics discussed here is a potentially important mechanism to control and regulate pedestal structure in future fusion plasma devices.