Impact of drifts on divertor power exhaust in DIII-D

A.E Jaervinen,T.D Rognlien,C.J Lasnier,D.N Hill,G.D Porter,A.W Leonard,M Groth,S.L Allen,J.G Watkins,C.M Samuell,M.E Fenstermacher,H.Q Wang,A.G Mclean,D Eldon

doi:10.1016/j.nme.2019.02.023

Abstract

Abstract Radiative divertor experiments and 2D fluid simulations show strong impact of cross-field drifts on the low field side (LFS) divertor target heat flux and volumetric radiation profiles when evolving to detached conditions in DIII-D high confinement mode (H-mode) plasmas with forward and reversed toroidal field configurations. In both field configurations, the peak heat flux is reduced by about a factor of 2 in detachment by D2-injection and by factor of 3 – 4 in detachment by N2-injection. Operating with the B × ∇B-drift towards the X-point (fwd. B T ), the LFS divertor radiation front is observed to shift step-like from the target to near to the X-point at the onset of detachment. In contrast, operating with the B × ∇B-drift away from the X-point (rev. B T ), the radiation front is observed to remain closer to the target plate and to be radially shifted towards the far SOL. These phenomena occur with detachment induced both with N2- and D2-injection. The step-like detachment onset is consistent with recently published theory of the role of poloidal E × B-drift in the private flux region in driving highly non-linear detachment onset in fwd. B T [22].

Highlights

To avoid overheating of the plasma facing materials, a controlled state of partially to fully detached divertor conditions with power exhaust dominated by radiation is foreseen to be mandatory in reactor scale fusion devices [1,2]
Radiative divertor experiments and 2D fluid simulations show strong impact of cross-field drifts on the low field side (LFS) divertor target heat flux and volumetric radiation profiles when evolving to detached conditions in DIII-D high confinement mode (H-mode) plasmas with forward and reversed toroidal field configurations
At the onset of detachment, the E × B-drift in the private flux region (PFR) is diminished leading to a factor of 6 increase of recycling neutral production in the PFR in the LFS divertor leg, increasing ionization, ne and radiation near the separatrix pushing the plasma into strong detachment [22]

Summary

Introduction

To avoid overheating of the plasma facing materials, a controlled state of partially to fully detached divertor conditions with power exhaust dominated by radiation is foreseen to be mandatory in reactor scale fusion devices [1,2]. Poloidal drifts are observed to have a substantial impact on the characteristics of the LFS divertor plasmas, including transition to detached conditions as well as poloidal and radial structure of density, temperature and radiation profiles. These impacts are large enough that they cannot be considered as corrections to a divertor solution primarily driven by sources and sinks not related to drift transport.

Setup of the experiments and simulations

Detachment with D2-injection

Detachment with N2-injection

Findings

Discussion and summary