Measurement of absolute carbon impurity concentration via CXRS and BES diagnostics in EAST tokamak

Investigating impurity transport properties in tokamaks is crucial for understanding plasma behavior and improving confinement performance. Carbon, a common low-Z impurity in fusion devices, is particularly prevalent in those using plasma-facing materials containing graphite. Hence, precise measurement of its impurity concentration is highly necessary. A combined diagnostic method utilizing Charge eXchange Recombination Spectroscopy (CXRS) and Direct Current Beam Emission Spectroscopy (DC-BES) has been applied on Experimental Advanced Superconducting Tokamak (EAST) to derive absolute radial profiles of carbon impurity concentrations. This approach eliminates systematic uncertainties from line of sight differences between the two systems and the neutral beam attenuation calculations. Additionally, as the charge exchange and beam emission rates depend on the beam energy, a forward modeling technique was employed to extract the fraction of full (E), one-half (E/2), and one-third (E/3) beam energy components from the BES spectral intensities. The obtained carbon concentrations range from approximately 0.3%-0.8%, indicating that the application of lithium wall conditioning effectively reduces the carbon impurity concentration in the plasma.