Mitigation of plasma-material interactions with low-Z powders in DIII-D H-mode discharges

First experiments with multiple low-Z powder injections in DIII-D H-mode discharges demonstrated enhanced divertor dissipation with a rapid reduction of divertor electron temperature from 30 eV to below 10 eV, increasing divertor neutral pressure from 0.9 to 9 mTorr, and transition into stable detachment during the injection. In some cases, the plasma was observed to detach during powder injection, and in all cases, good energy confinement (W_mhd=0.7 MJ, τ_E=0.11 s) was maintained. 

Lithium (Li), boron (B), and boron nitride (BN) powders (5-100 μm) were injected in upper-single-null H‑mode plasmas (I_p~1 MA, B_t=2 T, P_NB~6 MW, n_e~3.6-5.0x10¹⁹ m^-3) in 2-s intervals at rates of 1-50 mg/s. Data reveal that the low Z powder injection increased line emission in the plasma boundary and near the divertor. EMC3-EIRENE modeling was used to resolve the species-dependent magnitude and spatial distribution of the radiative power losses. It is found that Li radiates more from the separatrix region and thus has a more substantial cooling effect on the core-edge. On the other hand, B and BN radiation is more concentrated in the divertor. Lithium powder injection at a rate of 3.3 mg/s causes an increase of divertor pressure by a factor of 3, while it was increased by only 1.5 in the case of B despite a higher rate of 35 mg/s¹.

Synergistic use of boron and nitrogen in the form of BN powder at a rate of 81 mg/s showed the most substantial increase in divertor neutral pressure by one order of magnitude and lasting detachment without a significant change in core density and energy confinement.

The concomitant increase in fueling rates to maintain density and the reduction of O, C, and neutral D line emission in the plasma edge and divertor suggest additional wall conditioning effects from the cumulative injection of low-Z non-recycling materials².