Characterization and removal of co-deposition on the first mirror of HL-2A by excimer laser cleaning

In this study, an excimer laser ablation has was proposed for the first time for removing the co-deposition layer of the first mirrors of HL-2A tokamak. It was found that this approach to be much more effective and homogeneous in the removal of the impurity layer than using Nd:YAG lasers at 532 nm and 355 nm wavelengths. Time-of-flight mass spectrometry has been used for characterization of the released impurities during laser cleaning. X-ray photoelectron spectroscopy and secondary ion mass-spectrometry was used to evaluate the depth profile and chemical state of the compositions in the deposited layer. The results indicated that the ablation process can be characterized by two well-defined threshold fluences: (a) ablation threshold ≈0.01 J/cm2 and (b) a damage threshold ≈0.55 J/cm2. SEM has been used to characterize the ablation spot after cleaning at the different laser fluences. The results showed that the deposited layer was almost completely removed after multiple pulses and the repeated spatial scans at 0.3-0.55 J/cm2. After the cleaning, the reflectivity of the first mirror was restored to 80% of full capacity in the wavelength range of 600-1100 nm.