Diagnostic study of impurity deposition in fusion device by calibration-free laser-induced breakdown spectroscopy

Abstract

The quantification of impurities in fusion devices is important to improve understanding of all impurity-related issues and their solutions. In this study, the deposition of the impurity layer on the lower hybrid wave gas puffing pipe (LHW-GPP) used in the Experimental Advanced Superconducting Tokamak (EAST) was characterized. The calibration-free laser-induced breakdown spectroscopy (CF-LIBS) was employed to resolve the impurity deposition of the unknown standard by focusing the laser at various spot positions on the sample surface. The impurity elements that appeared in the spectrum of deposition are copper (Cu), tungsten (W), molybdenum (Mo), lithium (Li), silicon (Si), and carbon (C). The laser ablation of impurity showed an uneven deposition pattern. Each laser pulse depicted the different concentrations of the impurity elements. The results obtained from the quantification and the depth profiling showed that the impurity deposition was composed of eroded materials caused by the plasma-surface interactions with the divertors, main wall, limiters, and supporting structure materials. The exponential decrease in impurity signals and constant behavior of substrate intensity were also observed during consecutive laser shots. The study provided the estimation of impurity contents in the fusion plasma.