Mitigation of plasma-wall interactions with low-Z powders in DIII-D high confinement plasmas

TL;DR

This study demonstrates that low-Z powder injection in high-confinement plasmas enhances divertor detachment, reduces wall impurities, and maintains core confinement, offering a promising method for plasma-wall interaction mitigation.

Contribution

It introduces the use of multi-species low-Z powders, including boron nitride, as an effective technique to improve divertor and wall conditions without significantly degrading core plasma performance.

Findings

BN powders increased divertor neutral compression by over an order of magnitude

High powder injection rates reduced ELM-fluxes but triggered tearing modes and confinement loss

Wall impurity levels decreased with cumulative powder injection

Abstract

Experiments with low-Z powder injection in DIII-D high confinement discharges demonstrated increased divertor dissipation and detachment while maintaining good core energy confinement. Lithium (Li), boron (B), and boron nitride (BN) powders were injected in high-confinement mode plasmas ($I_p=$1 MA, $B_t=$2 T, $P_{NB}=$6 MW, $\langle n_e\rangle=3.6-5.0\cdot10^{19}$ m$^{-3}$) into the upper small-angle slot (SAS) divertor for 2-s intervals at constant rates of 3-204 mg/s. The multi-species BN powders at a rate of 54 mg/s showed the most substantial increase in divertor neutral compression by more than an order of magnitude and lasting detachment with minor degradation of the stored magnetic energy $W_{mhd}$ by 5%. Rates of 204 mg/s of boron nitride powder further reduce ELM-fluxes on the divertor but also cause a drop in confinement performance by 24% due to the onset of an $n=2$ tearing mode. The application of powders also showed a substantial improvement of wall conditions manifesting in reduced wall fueling source and intrinsic carbon and oxygen content in response to the cumulative injection of non-recycling materials. The results suggest that low-Z powder injection, including mixed element compounds, is a promising new core-edge compatible technique that simultaneously enables divertor detachment and improves wall conditions during high confinement operation.