E×B Flux Driven Detachment Bifurcation in the DIII-D Tokamak

Abstract

A bifurcative step transition from low-density, high-temperature, attached divertor conditions to high-density, low-temperature, detached divertor conditions is experimentally observed in DIII-D tokamak plasmas as density is increased. The step transition is only observed in the high confinement mode and only when the B×∇B drift is directed towards the divertor. This work reports for the first time a theoretical explanation and numerical simulations that qualitatively reproduce this bifurcation and its dependence on the toroidal field direction. According to the model, the bifurcation is primarily driven by the interdependence of the E×B-drift fluxes, divertor electric potential structure, and divertor conditions. In the attached conditions, strong potential gradients in the low field side (LFS) divertor drive E×B-drift flux towards the high field side divertor, reinforcing low density, high temperature conditions in the LFS divertor leg. At the onset of detachment, reduction in the potential gradients in the LFS divertor leg reduce the E×B-drift flux as well, such that the divertor plasma evolves nonlinearly to high density, strongly detached conditions. Experimental estimates of the E×B-drift fluxes, based on divertor Thomson scattering measurements, and their dependence on the divertor conditions are qualitatively consistent with the numerical predictions. The implications for divertor power exhaust and detachment control in the next step fusion devices are discussed.

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

“…However, since the formed potential hill is not physically connected to wall structures, it leads to mostly self-closing E × B particle circulation around the potential hill. This private flux region (PFR) potential hill formation was previously discussed [15]. However, even though the particle flow loop is mostly self-closing, the heat carried with the particle flow is dissipated as the flow propagates through the divertor leg, such that for heat transport this circulation is not divergence free.…”

X-point potential well formation in diverted tokamaks with unfavorable magnetic field direction

“…Previous works on drifts often focused on in-out asymmetries of ion flux and heat loads on the divertor targets for various tokamaks: EAST [7,8], C-Mod [9], DIII-D [2,10], ASDEX Upgrade [11], TCV [12] with less attention to the underlying electric potential. Recent studies in forward magnetic field direction for detached divertor conditions show strong poloidal electric fields [3,13] and the formation of a "potential hill" below the X-point [14], whereas experiments with reversed magnetic field show a significant reduction of the X-point potential [15]. For the first time, we predict a reversal of the poloidal electric field in deeply detached, reversed field conditions on the basis of 2D fluid simulations.…”

SOLPS-ITER simulations of the TCV divertor upgrade

“…Drift effects are not included in the present study. The effect of drifts on detachment has been demonstrated to give rise to bifurcation and redistribution of power and particles between the target plates [11]. This should not affect strongly the trends observed during the presented scans but may affect the absolute values.…”