Error analysis and calibration of Langmuir probes embedded in ITER-like tungsten divertor on EAST

Abstract The divertor Langmuir probes (Div-LPs) system is a key diagnostic for measuring divertor plasma parameters in tokamaks. The accuracy of the measurement is the basis of physical researches. However, the excessive heat load on the target plates may lead to serious ablation of graphite probe tips, especially during long-pulse operation or unmitigated disruptions. In order to evaluate the data uncertainty caused by ablation, a statistical study has been made based on the Ohmic discharges in the EAST (Experimental Advanced Superconducting Tokamak) 2018 experiment campaign. The results show that for the discharges with the same plasma parameters, the peak particle flux of upper outer divertor measured by Div-LPs becomes several times smaller as the discharge progresses in a long experimental campaign, which is mainly due to that the effective collection area of the ablated probe tip becomes smaller and smaller. To quantitatively calculate the decrease of the effective area of the probe tip, the physical quantity of accumulated injection energy is introduced. A simple model is proposed and used to characterize the relationship between the effective collection area and the total accumulated injection energy. In addition, arbitrary H-mode discharges are selected to verify the model. By comparing the calibrated data of Div-LPs with other diagnostics, it is found that the model is also suitable for H-mode discharges with the strike point near the frequently-used-SP, and thereby the uncertainty after calibration is significantly reduced. In the future, the method will be employed for Div-LPs data processing near the strike point on EAST, which will facilitate further researches on divertor physics.