Influence of expanding and contracting magnetic field configurations on detached plasma formation in a linear plasma device

Y Hayashi,S Kajita,N Ohno,H Tanaka

doi:10.1063/1.4986100

Abstract

We investigated the effects of magnetic field structure on detached plasma formation by simulating magnetically expanding and contracting plasma in a linear plasma device. The present study helps to characterize the geometries of a conventional poloidal divertor and advanced divertors, e.g., super-X divertor. The total ion particle flux measured with a large-diameter target plate dramatically changed under the detached plasma condition compared to that in attached plasma. Under the detached plasma condition, the magnetically expanding plasma clearly exhibited a significant influence on the degradation of detached plasma formation. Further, the magnetically contracting plasma slightly enhanced the electron-ion recombination (EIR) processes. By changing the magnetic field structure from contraction to expansion, the electron density (ne) decreased and the electron temperature (Te) increased upstream from the recombination front, leading to the degradation of the EIR processes. The effect of the decrease in parallel flow velocity under the magnetically contracting plasma on the plasma detachment was not observed because the driven flow due to pressure gradient compensated the effect.

Highlights

For divertor plate protection in future fusion devices, such as ITER and DEMO, it is necessary to reduce the electron temperature (Te) in the scrape-off layer (SOL) due to radiation enhancement achieved by seeding impurities [1, 2]
The reduction could not be explained by the enhancement of the electron-ion recombination (EIR) processes, because Te was more than 1 eV at the plasma center
In order to understand the influence of magnetic field structure on the detached plasma formation, magnetically expanding and contracting plasmas to the target plate were simulated in a linear plasma device NAGDIS-II

Summary

Introduction

For divertor plate protection in future fusion devices, such as ITER and DEMO, it is necessary to reduce the electron temperature (Te) in the scrape-off layer (SOL) due to radiation enhancement achieved by seeding impurities [1, 2]. Large heat and particle fluxes interact with these impurities and recycling particles during their transport towards the divertor. The operation with the large radiation and the momentum losses leads to a detached divertor. A volumetric recombination in the detached plasma plays an important role in the strong decrease of the ion particle flux to the divertor plate [3,4,5]. In a simple one-dimensional model, ne and Te are considered by assuming the magnetic field strength to be constant during the transport towards the divertor. The magnetic field strength two-dimensionally varies in the divertor region

Methods

Results

Conclusion