A multi-pulse, multi-species laser blow-off system on DIII-D for impurity transport studies.

Abstract

A Laser Blow-Off (LBO) impurity injection system was developed and installed on the DIII-D tokamak to enable controlled, repeatable studies of impurity and heat transport under reactor-relevant plasma conditions. The system employs a high-energy (up to 1.2 J), pulsed Nd:YAG laser, operating typically at 10 Hz and up to 50 Hz, coupled to a fast piezoelectric steering mirror assembly, allowing multiple, precisely timed impurity injections within a single plasma discharge, with accurate control of the injection location. Independent control of laser energy, beam size, and focal position enables selection of the target energy density and ablated particle number, facilitating controlled injection of trace to non-trace low- and high-Z materials, such as calcium and tungsten, with typical injected particle numbers spanning ∼1017-1019 (low-Z) and ∼1014-1016 (high-Z). The system supports injection of a wide range of materials (over 30 elements is available), enabling both non-perturbative trace impurity injections that nevertheless remain detectable in the plasma and perturbative impurity injections suitable for validation studies of impurity radiation effects on plasma performance. The injected impurities are monitored using DIII-D's comprehensive suite of spectroscopic diagnostics to characterize impurity evolution and radial transport. This work details the design and integration of the LBO system on DIII-D and presents impurity and thermal transport studies enabled by controlled impurity injection in quiescent H-mode, hybrid, negative triangularity, and resonant magnetic perturbation-induced edge-localized modes-suppressed plasmas.