| In the laser-driven inertial confinement fusion(ICF),the core symmetry and areal density of the imploding capsule at the stagnation are critical to the ignition success.In experiments,X-ray backlighting as a kind of active diagnostic method are usually used to diagnose the capsule.The dense plasma in the core of the compressed capsule requires the diagnostic photons to have a strong penetrating ability;In addition,the self-emission from the core is extremely intense below 20 keV,even brighter than any other method produced X-ray source at present,and become a main source of background noise in backlighting.Multi-keV X-ray source generated by nanosecond lasers are usually softer than 10 keV,so cannot meets the needs of the diagnosis of imploding capsule.Advanced Radiography Capability(ARC)at National Ignition Facility(NIF)proposed that ultra-short laser-driven hard X-rays above 70 keV as diagnostic photons can penetrate the imploding capsule and achieve a good image contrast.As Compton scattering dominates the interaction between the high-energy photons and the fuel material,the diagnostic method is known as Compton radiography which is an essential diagnostic technique for the imploding capsule at nuclear reaction stage.The self-emission from the capsule decays sharply at high energy,and diagnostic photons above 70 keV are much brighter than the self-emission,hence able to suppress the noise from the self-emission.In the laser interactions with target,the energy conversion efficiency(CE)from laser to the broadband bremsstrahlung above 70 keV is higher than that to the line emissions.And,the cross-section of Compton scattering is insensitive to the photon energy from tens of keV to hundreds of keV.So,Compton radiography uses the broadband continuum emission above 70 keV as diagnostic photons.The spatial size of the imploding capsule at the stagnation is smaller than 150 ?m,and the duration of it is shorter than 150 ps.To detect it with high spatial and temporal resolution,the spatial and temporal scales of the probe photon source are required to reach the order of 10 ?m and 10 ps,respectively.The backlighter of Compton radiography is formed by a picosecond peta-watt laser beam irradiating a micro-wire.The picosecond pulse driving laser ensures the hard X-ray source with a duration on the order of 10 ps.The peta-watt laser beam is difficult to focus down to 10 ?m,but the demanding X-ray source size is possible to be achieved through the end face emission from a 10 um-diameter micro-wire.It is difficult to selfstand and aim at a micro-wire experimentally,so one usually attaches it to a CH foil substrate,to ease supporting and aiming.For the hard X-ray source produced by this kind of micro-wire with substrate,various parameters of the target and driving laser beam can significantly affect the X-ray emission.This thesis focus on the Compton radiography for implosion experiments suitable for SG? laser facility.We used Particle In Cell(PIC)codes and Monte Carlo(MC)codes,and conducted experiments,to study the variation of the hard X-ray emission with the atomic number of luminescent material and substrate material,the target thickness,the laser intensity,the laser pulse and other parameters.The effects of substrate of micro-wire target on the laser-driven hard X-ray source was studied by simulations for the first time.The results provided a scientific basis for the design of laser and target parameters of Compton radiography.Firstly,we studied the influence of target material on hard X-ray emission through Monte Carlo codes,and found that the emission intensity is proportional to the atomic number(Z),the material density(?),and the target thickness(L).It suggests that a quite bright X-ray source can be obtained by using high density,high-Z materials such as gold(Au)and uranium(U),and using substrate or target holder made of low-Z,low density materials such as CH can reduce noise signals from others than luminescent target.Secondly,PIC+MC codes combined with semi-analytical model were used to study the effect of laser intensity on hard X-ray emission,showing the effect of electron recirculation(Recir.)plays a key role:Without the electron Recir.,emission intensity increases with the laser intensity in the non-relativistic region and then decreases rapidly in the relativistic region;However,when considering the effect of electron Recir.,emission intensity reaches saturation at 1018W/cm2.It requires relativistic laser intensity for driving laser beam in Compton radiography.Thirdly,we studied the effect of substrate on the X-ray emission from wire using PIC+MC codes.The results predicted a suppression of electron Recir.motion by substrate,so the substrate should be made as thin as possible.A 10 ?n thick CH substrate significantly reduces emission above 1MeV,but has little effect on that near 100 keV.Therefore,a CH substrate can be used for Compton radiography,but a selfstanding micro-wire is needed in terms of y-ray sources.Finally,experiments were conducted on SG?-U laser facility,and the effects of substrate,target thickness,laser parameters and target holder material on hard X-ray emission intensity were investigated.The results included:i).The emission intensity drops to 1/4-1/2 by a thick substrates destroying the electron Recir.motion;ii).Electrons will escape in the process of recirculation.So the optimal thickness of a target is the balance of electron Recir.and escape,and the emission intensity can be enhanced by using a multi-layer target;iii).It was experimentally measured that a 10 ps non-relativistic laser pulse produced X-ray source below 350 keV is brighter than that produced by a lps relativistic laser pulse.It benefited from longer laser pulse and larger focal spot size,which will achieve a higher energy gain,a suitable hot electron temperature,and a lower electron escape probability;iv).Target holder made of high-Z material caused a lot of background noise,and thus reduced the SNR of imaging results.Hence,a CH holder should be used to minimize the background noise of imaging. |
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