Influence of Magnetic Field on Space Charge Effect in Pulse-Dilation Framing Camera
Objective In the inertial confinement fusion(ICF)experiment,the microchannel plate(MCP)framing camera is an important two-dimensional ultrafast diagnostic device that is used to acquire the duration and dynamic image of plasma at the stage of implosion compression.However,due to limitations of the electronic transmission time and its dispersion in the channel of the MCP,the temporal resolution is restricted to 60-100 ps for a long time.In order to further improve temporal resolution,a pulse-dilation framing camera(PDFC)is developed,which couples the MCP framing camera with the temporal dilation technique of the electron beam.With an ultrafast temporal resolution of better than 10 ps,it is easier to meet the detection requirements of the shorter duration states in the ICF.Therefore,the relevant ways and techniques of the PDFC are gradually focused on the field of ultrafast diagnosis.The PDFC is a new kind of framing camera with a long drift region using the imaging of magnetic focusing technique.Due to its similar structure to the streak camera,the improvement of its spatio-temporal performance to a higher order is restricted by the space charge effect(SCE).Moreover,the temporal width and radius of the electronic pulses(EPs)are dynamically changed by the dilating pulse and magnetic focusing in the PDFC.Therefore,building the model of the SCE that meets the dynamic parameters of the EPs and analyzing the influence of the dynamic radius caused by magnetic focusing on the spatio-temporal dispersions will be an important theoretical significance for systemically studying the SCE of the PDFC.Methods In the research,to analyze the influence of magnetic field on the spatio-temporal dispersions of the SCE,first of all,the model of the PDFC using magnetic focusing is built,and the dynamic characteristics of EPs during transmission are analyzed by the working principle of the PDFC.Then,the spatio-temporal dispersion model of the SCE is deduced by solving the equation of the electric field force based on the two-dimensional potential distribution of the EPs.To build a relationship between magnetic field and imaging area,while ensuring consistent imaging magnification,the different imaging magnetic fields are reasonably calibrated through the analysis procedure of optimal spatial performance of the PDFC based on the method of regional imaging.Finally,the dynamic temporal width and radius of the EPs are applied to the model of the SCE in different magnetic fields,and the spatio-temporal dispersions of different off-axis positions are analyzed.Results and Discussions The innovative and significant research results are mainly summarized in three aspects.First,the dynamic variation of the electronic density during transmission is analyzed in the PDFC,on which the dynamic temporal width and radius of the EPs are based[Fig.2(d)].Second,under consistent imaging magnification,the optimal spatial performance of the PDFC is analyzed,and different imaging magnetic fields are reasonably calibrated by regional imaging(Fig.3).Third,the dynamic temporal width and radius of the EPs are applied to the model of the SCE.When the radius of the imaging region is 1 mm,as the off-axis position increases from 0 mm to 15 mm,the magnetic field intensity is enlarged from 4.585 ×10-3 T to 4.763 ×10-3 T.The defocusing and dynamic radius of EPs of the off-axis are much larger than those of the on-axis.Therefore,as the electronic density of the EPs reduces,the temporal dispersion of the SCE is reduced from 2.94 ps to 483 fs,and the spatial dispersion is reduced from 668 μm to 22 μm(Fig.4).When the radius of imaging region is gradually enlarged to 20 mm,the magnetic field intensity of the on-axis is reduced from 4.585× 103 T to 3.359× 10-3 T,and the spatio-temporal dispersions of the SCE are optimum value in the range of 3.4× 10-3-3.5× 10-3 T.The range of temporal dispersions of different positions is 256-392 fs,and spatial dispersions is 3.1-15.4 μm(Fig.5).Conclusions In the PDFC,the temporal width,radius,and electronic density of the EPs during the transmission process are dynamically changed by the effect of the dilating pulse and the imaging system of magnetic focusing.Moreover,the spatio-temporal dispersions of the SCE are significantly affected by the defocusing of the EPs and the fluctuation of radius caused by a magnetic field.According to the research methods and results,on the one hand,the different magnetic field is reasonably calibrated through the analysis of the optimal spatial performance of the PDFC;on the other hand,it also provides a theoretical basis for analyzing the relationship between the magnetic field and the SCE.In the next stage,to provide a theoretical basis for achieving faster temporal resolution of the PDFC,the spatio-temporal dispersions of the SCE are systematically studied from different types of magnetic focusing imaging systems and the long drift regions.
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