Improved Confinement Due to Boron Powder Injection in WEST L-Mode Plasmas

Alternative operating scenarios to H-mode are desired for future fusion pilot plants in order to avoid the need to suppress/mitigate ELMs. A promising scenario is to use impurity injection to achieve a discharge with improved confinement and a high radiative fraction without the creation of a pressure pedestal. This has been done successfully on a number of tokamaks using gaseous impurity injection. Experiments with impurity powder droppers, installed worldwide for active conditioning, have reported improvements in plasma confinement during particulate injection. On WEST, L-mode plasmas with dominant electron heating and no core torque source observed such improvements in confinement with B powder injection. These results are reminiscent of previous gaseous N₂ injection experiments on WEST. During powder injection, W_MHD and the neutron rate increased up to 25% and 200%, respectively, depending upon the powder drop rate. The improvements in confinement are thought to be the result of stabilized ▽T TEM turbulence through increases in Z_eff and/or collisionality leading to core density peaking at zero particle flux. The increase in density peaking stabilizes ITG-driven turbulence allowing for the achievement of increased T_i. To identify the dominant mechanisms and the causality chain behind these improvements in confinement, we employ interpretative modelling using METIS, predictive integrated modelling using a HFPS and QuaLiKiz, and stand-alone gyrokinetic simulations using GKW and QuaLiKiz.