ITER-Relevant Turbulence Broadening of the Divertor Heat Flux Width in DIII-D Quiescent H-Mode Plasmas Featuring Turbulence-Limited Pedestals*

Multi-machine scaling predicts a narrow heat exhaust layer in future high magnetic field tokamaks, producing extreme power densities that require mitigation. In our experiments, the width of the exhaust layer is nearly doubled using actuators to increase turbulent transport in the plasma edge. This is achieved in low collisionality, high confinement Quiescent H-Mode edge pedestals with their gradients limited by turbulence, instead of coherent MHD modes or Edge Localized Modes, consistent with predictions that ITER pedestal turbulent transport will greatly exceed that of present machines. The exhaust heat flux profile width and divertor leg diffusive spreading both double as a high frequency band of turbulent fluctuations propagating in the electron diamagnetic direction doubles in amplitude [1]. The results are quantitatively reproduced in electromagnetic XGC particle-in-cell simulations which show the heat flux carried by electrons emerges to broaden the heat flux profile, directly supported by Langmuir probe measurements and infra-red thermography. The broadening of the total heat flux profile is measured by infra-red thermography. The XGC simulations show pedestal fluctuations characteristic of trapped electron modes (TEMs), consistent with the increased electron heat flux.

[1] D. R. Ernst et al., Phys. Rev. Lett. 132, 235102 (2024). https://doi.org/10.1103/PhysRevLett.132.235102