Study On The Temporal And Spatial Evolution Of High-Z And Low-Z Elements Of LIBS Plasma

Fuel retention measurement on plasma-facing components(PFCs)has been an active research field in tokamak devices.The issues about fuel retention which is low-Z element remained in the first wall can influence the performance of the material and the particles recycling of edge plasma.Laser-induced breakdown spectroscopy(LIBS)is an important technique based on the laser-ablation on sample to detect the elemental composition.Fuel retention and element distribution in PFCs can be well diagnosed by this technique.However,there are still several challenging issues about the fuel retention measurement with plasma-wall interaction(PWI)processes by LIBS.Due to the vacuum environment of tokamak,the lifetime of the laser-ablation plasma is short and the expansion of the laser-ablation plasma is fast.This results in that the temporal and spatial evolution of the plasma,especially for the mixed plasma with high-Z and low-Z elements,is very nonuniformity.The nonuniformity elemental evolution will affect not only the quantitative accuracy of LIBS,but also the signal collection system and the elemental qualitative measurement.In these cases,it is very significant to study the temporal and spatial dynamic evolution of the LIBS plasma for high accurate quantitative measurement on fuel retention.In this thesis,an upgraded co-axis LIBS system based on the linear fibers bundle collection system has been established to study the temporal and spatial dynamic evolution of high-Z and low-Z elements in LIBS plasma in vacuum condition.The distribution and characteristic of temporal and spatial evolution of H,Ta,W,Th elements as well as the electron excitation temperature and electron density have been achieved by analyzing the LIBS spectra.The plasma emission sizes with different elements under various delay times have been also obtained.The detailed contents of this thesis are structured as follows:1.Based on the linear fibers bundle collection system,a LIBS system for temporal and spatial dynamic evolution research has been well-established.The LIBS system consists with laser,timing and triggering system,optical system and vacuum system.The different fibers in the linear fibers bundle are corresponding to the different positions of the LIBS plasma.The space distributions of plasma with various delay times can be achieved by this co-axial LIBS system.The results show that the LIBS system can be used for the study on the temporal and spatial measurement of LIBS plasma.The space-resolution and the time-resolution of the system are 1 mm and 100 ns-200 ns,respectively.2.In vacuum condition,the Ta sample and the H element which is retention elements on the surface of the sample is used for the temporal and spatial evolution of LIBS plasma.The intensities of both high-Z element Ta and low-Z element H have maximum values in the center region of plasma and decrease from center to the edge of the plasma.The intensities also decrease with increasing of the delay time.By using the fitting method,the Ta plasma size with light emission is about 6 mm.The H plasma size with light emission is much larger than Ta plasma.This is due to that the atom masses of Ta and H are different.The expansion of H atoms is much faster than Ta atoms.In addition,the electron excitation temperature and electron density have been achieved by the Boltzmann slope method and Stark broadening method.Both electron excitation temperature and electron density decrease from center to the edge of the plasma,as well as decrease with the delay time.3.The laser-ablation pure W sample and W alloy(WTh)sample are used to study the temporal and spatial evolution of LIBS plasma in vacuum condition.Temporal and spatial evolution of several different elements such as W,Th and H are obtained in pure sample and alloy sample.The results show that the H plasma size is much larger than W and Th plasma due to the light mass and fast expansion of H atoms.At the beginning of the plasma expansion with delay time from 0 to 200 ns,the evolution behaviors of W and Th are similar.After 200 ns,the W plasma size is larger than Th plasma.The electron excitation temperature of pure W sample is high than that of WTh alloy sample.By comparing between the space-resolution electron excitation temperatures and space-integration electron excitation temperature,the space-integration electron excitation temperature is close to the electron excitation temperature in the center of the plasma.This means that the regions with high plasma intensity has more contribution for the space-integration electron excitation temperature.