Double Pulse Laser-induced Breakdown Spectroscopic Diagnosis Of Plasma Facing Materials In Tokamak

During tokamak devices operation, plasma-facing material (PFM) has to withstand harsh radiation, intense particle bombardments and steady/transient high heat loads coming from the core plasma. As a cumulative result, the composition and micro-structure of PFM would be changed by a series of complicated plasma wall interaction (PWI), such as sputtering, erosion, fuel retention, blistering, re-deposition and so on. The damaged surface could shorten the lifetime of PFM, and the radioactive tritium retention could obstruct tokamak devices for steady-state and safe operations. Currently, PWI issues are among the most important problems to be solved along the way towards the controlled nuclear fusion. An effective in-situ diagnostic method of monitoring elemental impurity deposition and fuel retention in PFM is desired.Laser Induced Breakdown Spectroscopy (LIBS) is an in-situ and micro-destructive diagnostic tool that can provide distinct spectral signatures characteristic of all chemical species without sample preparation. LIBS has been regarded as "a future superstar" for chemical analysis, and has been applied to many studies on various subjects successfully over the past several years. LIBS would be an ideal candidate for in-situ monitoring the PFM, however, relevant research papers are quite few that presented the investigation about the feasibility of LIBS diagnosis proposed for tokamak device under vacuum conditions as well as with a magnetic field. The relatively low sensitivity is the main challenge of the future in-situ LIBS system used to identify the trace deposited impurities and retained fuel in PFM. In my thesis, the effects of background pressure and strong magnetic field on the spectral emission features has been investigated, and a novel double pulse LIBS method has been developed to improve the analytical accuracy, precision, and sensitivity of in-situ LIBS system under the tokamak environment. The thesis is organized as follows:In chapter 2, the basics of LIBS plasma were investigated. This contained the fundamental nanosecond laser ablation processes, plasma dynamics and plasma parameters (the spectral emission features, excitation temperature and electron density). The surface compositions of the original divertor graphite tile and the exposed divertor graphite tile (installed in EAST during the 2010 campaign) were characterized by the LIBS from 200 to 980 nm. The results revealed that the deposition layer on the exposed tiles was mainly composed of Li, Si, Fe, Cr, K, Na and so on, which were consistent with the results of XPS analysis. LIBS depth profile analysis and three-dimensional compositional mapping can be used to investigate the structural features of the exposed divertor tiles. The measured thickness of the co-deposition layer was about 60?m.In chapter 3, a systematic study of the effect of pressure on the emission intensity of LIBS has been conducted in the laboratory. The pressure can be changed in the range of 6× 10-6 mbar to 1000 mbar. The line emission intensity initially increases with a decrease in pressure; a further decrease in pressure causes a decrease in the LIBS signal intensity. The maximum intensity achieved at the range of 1 mbar to 100 mbar. This behavior can be explained by the competing processes of collisional excitation of species in the plasma and ablation mass from the tile. Though the signals at low pressures (p<< 10-2 mbar) are weaker, the LIBS system located at the H port of the EAST tokamak successfully in-situ captured the signals of the rentained fuel in the co-deposited impurities on the surface of Mo tiles and in the gaps between Mo tiles at 10-8mbar.In chapter 4, the collinear laser-induced breakdown spectroscopy (DP-LIBS) configuration was developed with the aim of overcoming the sensitivity shortcomings of the conventional single pulse laser-induced breakdown spectroscopy (SP-LIBS) technique in vacuum conditions. A systematic study of plasma emission signal dependence on the inter-pulse delay at 3.5× 10-5 mbar was carried out, and the results were compared with the ones obtained with a single laser pulse of energy corresponding to the sum of the two pulses. For a molybdenum tile, it was found that the atomic spectral lines of Mo were enhanced by a factor of 6.5 when an inter-delay time of 1.5 ?s was installed. For an exposed divertor tile (DM2-2012), significant increases in the emission line intensities of various minor elements (such as 0.14% Ti,0.05% Cr,0.08% Ca,0.26% Fe,0.05% Ni and 0.13% Mo) were observed in DP-LIBS, while no spectra signal was achieved in SP-LIBS. The capability of this collinear DP-LIBS technique was consistent with the performance of ex-situ XPS and EDX methods.In chapter 5, investigations are presented to clarify that the orthogonal DP-LIBS would have great potential in non-destructive analysis and high resolution depth profiling (80nm/pulse) of the ultra thin co-deposition layer (0.8?m) on the first mirrors of HL-2A tokamak. The spectroscopic study of the plasma emission can be used to determine the elemental composition of the ablated materials. The real-time monitoring and accurate identification the interface boundary can provide important information regarding the cleaned mirror surface in order to avoid under-cleaning. The reflectivity of the polluted first mirror (at 1064 nm) was recorded from 4% to 80% after 10 cleaning laser pulses. These DP-LIBS techniques would help us to develop a more promising system with high resolution, sensitivity, precision and accuracy to in-situ monitor the fuel retention and impurity deposition on PFM in tokamak device.