Research On EXAFS Diagnostic Technique For Compressed Materials On Large Laser Facility

In process of strong laser and material interaction,an inward compression wave can be driven during ablation,and through laser waveform controlling,varieties of compression modes from strong impact loading to quasi isentropic loading can be achieved,which makes large laser device a potential research platform for researches of experimental material compression characteristics.However,in order to study compression properties of materials in laser device,besides high pressure material loading methods,methods of parameter diagnosis for material compression and thermodynamic state should also be developed.Extended X ray Absorption Fine Structure(EXAFS)is a diagnostic technique usually used in studies of high pressure condensed matter physics.Because of its sensitivity to atomic short range order,it can provide information for the material structures at the atomic level.Particularly,because of sensitivity to local structural disorder and thermal disorder,EXAFS can provide temperature information of studied material.EXAFS is also very useful in studying structure phase transition at atomic level.So,people consider establish this method on large laser device for diagnosis of compressed material.However,in large laser devices,there are several difficulties to adopt EXAFS technique to diagnose material in high pressure state:first,laser driven X-ray source is generally used as probe light,which is an X-ray pulse radiation of ns magnitude;second,material state is transient,the existence time is ns magnitude;third,sample homogeneity can not be controlled easily in compression process;and last,EXAFS signal is relatively weak,and easily affected by noise and temperature.Because of difficulties above,EXAFS technique for compressed material on laser device can not be realized easily.However,because of sensitivity to temperature,EXAFS can be used as an important method for in situ temperature measurement on laser device,which is one of the reasons people have research interest on it.At abroad,this technique is now under studying and some meaningful results have been achieved.In China,study of large laser devices is just at its beginning,and no public reports have been published yet.The work in this thesis is mainly on EXAFS parameter diagnostic technique researches for compressed material on the large laser devices,and in a progressive way,a series of researches about this problem were carried out,and good results have been achieved,and EXAFS parameter diagnostic technique for compressed material has been established on the SG? prototype laser device.This is the first systematic study of this problem on the large laser device in China,and the results are of great importance to the development of this research field.The main contents of this paper are summarized as follows:1.Through literature research and analysis,and theoretical derivation,history of EXAFS technique development on large laser devices was reviewed,research status at home and abroad was understood,the purpose and significance of the study were clarified,key points and difficulties of developing the technology were analyzed,and basic principle and the applicable scope of EXAFS technique were expounded;2.Studies on Debye Waller Factor by numerical calculation were carried out,and the relationship of DWF to compression rate and temperature has been obtained for metal Ti,V,Fe.DWF is an important parameter to correlate the EXAFS spectrum with the temperature and is the basis of temperature diagnosis by EXAFS technique.3.With variables control method,compression ratio and transient back light factors were excluded temporarily,relationship between EXAFS spectroscopy and temperature was focused on,and experimental researches on EXAFS of varied temperature on synchrotron radiation device were carried out.EXAFS trends as temperature changed were experimentally studied,and the relationship between DWF and temperature was derived from experiment data.Through EXAFS spectroscopy data fitting,parameter measurement of the sample by EXAFS technique was demonstrated principally,and data support was provided for the large laser device EXAFS technology research for the following researches.4.Based on the SG? prototype laser device,EXAFS target design was carried out,including the target structure design,back light design,EXAFS sample design,EXAFS spectroscopy crystal spectrometer design,quasi isentropic compression process design,etc.The EXAFS target is a dual target structure,and multiple diagnostic devices must be considered.The sample design should consider EXAFS spectrum measurement,quasi-isentropic compression and VISAR measurement simultaneously,and is relatively complex,requiring comprehensive full consideration to various factors.Experimental results show that the target structure is reasonable and can be used to measure multiple parameters effectively.5.Based on the SG? prototype laser device,with the EXAFS target design,a series of experiments were carried out,good spectrums were obtained,and EXAFS technique for parameter diagnosis of compressed materials has been established.In the experiment,we had observed the phase transition phenomenon of Ti in the quasi-isentropic compression process through EXAFS spectrum measurement and good EXAFS spectrum of ? phase were obtained.In the experiment,we also measured EXAFS spectrums of compressed Ti in a phase,and the comparison between them fully confirmed the structural phase transition of Ti.Through experimental data processing,we obtained compression ratio C and temperature T of compressed Ti and completed the parameter measurement process.6.In order to further improve and optimize EXAFS parameter diagnostic technique on the SG? prototype,experimental studies of backlight sources for EXAFS spectroscopy measurement were carried out.CH ball targets,glass ball targets and metal targets were studied in different spectral energy range,respectively.By means of doping Cl atoms at inside interface,the CH ball target backlight brightness was increased by more than 10 times.high luminance,smooth spectrum and small light spot were achieved simultaneously.The result is very beneficial to further EXAFS technology researches.Experiments for exploration of Ti,Fe,Al EXAFS were carried out on SG? prototype device,for Ti,Al,EXAFS spectrums with good signal-to-noise ratio were obtained,and provide strong data supports for further EXAFS experimental studies.