PS Flow is Found Critical in the In-out Divertor Asymmetry in H-mode with Edge Localized Modes

In the latest research on experimental advanced superconducting tokamak (EAST), researchers found the critical role of Pfirsch-Schlüter (PS) flow in driving the in-out divertor asymmetry in the high confinement mode (H-mode). This finding brings better understanding of the in-out asymmetry of divertor particle flux and may contribute to the design and operation of future high-power long-pulse tokamak devices such as the international thermonuclear experimental reactor (ITER). 

The in-out divertor asymmetry in EAST, as manifested by particle fluxes measured by the divertor triple Langmuir probe arrays, is significantly enhanced during type-I ELMs, favoring the inner divertor in lower single null(LSN) for the normal toroidal field (B_t)direction, i.e., with the ion B×▽B direction towards the lower X-point, while the in-out asymmetry is reversed when the ion B×▽B directs away from the lower X-point.

The plasma flow measured by the Mach probe at the outer midplane is in the ion PS flow direction, opposite to both B′？B and E′B drifts, i.e., towards the inner divertor for normal B_t, and the outer divertor for reverse B_t, consistent with the observed in-out divertor asymmetry in particle fluxes. 

Experiments find that the particle source from an edge localized modes (ELM) event is predominantly located near the outer midplane. This new finding suggests the critical role of the PS flow in driving the in-out divertor asymmetry. Strong in-out divertor asymmetry is also present during inter-ELM and ELM-free phases for the normal field direction, i.e., with more particle flux to the lower inner divertor target, but the peak particle flux merely becomes more symmetric, or slightly reversed for reverseB_t, i.e., reversed B×▽B drift direction.  

The related work has been published recently in Nuclear Fusion. 

Link of the article: http://iopscience.iop.org/article/10.1088/0029-5515/56/6/066006 

(Liu Jianbin Reports)