Showing papers by "T.D. Rognlien published in 2020"

Progress in DIII-D towards validating divertor power exhaust predictions

Abstract: Radiative power exhaust experiments in DIII-D and 2D fluid simulations have addressed critical physics issues that advance our understanding of divertor heat load control in fusion reactors. Operating with the ∇B-drift towards the X-point (forward BT) in high confinement (H-mode) plasmas, the low field side (LFS) divertor is observed to undergo a step-like transition from attached (Te,target ~ 20 eV) to well detached (Te,target < 2 eV) conditions with increasing upstream plasma density. This step-like transition is not observed in reversed BT or in low confinement plasmas, indicating that both the direction and the magnitude of drifts impacts the detachment transition. UEDGE simulations with drifts reproduce qualitatively the experimentally observed step transition to detachment by predicting a bifurcation of the LFS divertor solution to attached and detached branches in forward BT in H-mode like conditions [1]. In forward BT in H-mode, the LFS divertor radiation front is measured and predicted to move abruptly from the LFS target to next to the X-point at the onset of the step-like detachment. In contrast, in reversed BT, electron density and the radiated power front in the LFS divertor leg are measured and predicted to be shifted radially away from the strike point region, consistent with the radial E×B-drift direction in the LFS divertor leg. At the onset of detachment in reversed BT, the radiation front expands towards the separatrix, while the peak radiation remains located away from the strike point. A new divertor ultraviolet spectrometer (DivSPRED) [2] has been used to measure the constituents of radiation in detached conditions in forward and reversed BT. In forward BT, CIV (1550 Å) resonant line emission is measured and predicted to dominate the radiated power, consistent with previous measurements [3, 4]. In reversed BT, Ly-α (1215 Å) radiation is measured and predicted to contribute significantly to the peak radiation. While UEDGE simulations capture the relative hierarchy of dominant radiators in the divertor, the spatial width of the

Role of poloidal E × B drift in divertor heat transport in DIII‐D

Abstract: Simulations for DIII-D high confinement mode plasmas with the multifluid code UEDGE show a strong role of poloidal $\mathbf{E}\times\mathbf{B}$ drifts on divertor heat transport, challenging the paradigm of conduction limited scrape-off layer (SOL) transport. While simulations with reduced drift magnitude are well aligned with the assumption that electron heat conduction dominates the SOL heat transport, simulations with drifts predict that the poloidal convective $\mathbf{E}\times\mathbf{B}$ heat transport dominates over electron heat conduction in both attached and detached conditions. Since poloidal $\mathbf{E}\times\mathbf{B}$ flow propagates across magnetic field lines, poloidal transport with shallow magnetic pitch angles can reach values that are of the same order as would be provided by sonic flows parallel to the field lines. These flows can lead to strongly convection dominated divertor heat transport, increasing the poloidal volume of radiative power front, consistent with previous measurements at DIII-D. Due to these convective flows, the Lengyel integral approach, assuming zero convective fraction, is expected to provide a pessimistic estimate for radiative capability of impurities in the divertor. For the DIII-D simulations shown here, the Lengyel integral approach underestimates the radiated power by a factor of 6, indicating that for reliable DIII-D divertor power exhaust predictions, full 2D calculations, including drifts, would be necessary.

Potential Impacts of Liquid-Metal Plasma-Facing Components on Heating and Current Drive Actuators for a Fusion Nuclear Science Facility

Abstract: This paper addresses the potential impact of liquid metal (LM) plasma-facing components (PFCs) for the heating and current drive (H&CD) actuators on the Fusion Nuclear Science Facility (FNS...