Simultaneous Diagnosis of Refractive Index and Roughness of Ice Layer in Laser Fusion Targets
Objective Inertial confinement fusion(ICF)is a promising fusion energy research technique that involves subjecting a tiny fuel target,typically a pellet containing a mixture of deuterium and tritium,to extreme temperatures and pressures,causing the fuel to undergo nuclear fusion.Achieving uniformity in the fuel layer within the target is critical for a successful fusion reaction,necessitating precise measurement and control of various parameters such as refractive index,thickness,and surface roughness.Any deviation from the ideal uniformity can lead to instability during the implosion process,significantly reducing fusion reaction efficiency.Recent advancements in ICF research have introduced various methods for characterizing the fuel layer,including X-ray imaging for density measurements,liquid refractive index matching for optical profiling,and confocal microscopy for surface analysis.Despite these innovations,challenges remain,particularly in adapting these methods in a vacuum environment and achieving the precision necessary for accurate characterization.To address these challenges and provide comprehensive guidance for the ICF target fabrication process,there is an urgent need to develop a characterization system capable of in situ synchronous detection of the roughness and refractive index of the target's ice layer within a vacuum chamber.Methods We establish a characterization model for optical path difference and light deflection based on Mach-Zehnder(M-Z)interferometry and backlight shadow imaging techniques.By integrating interferometric detection and backlight shadow technology,a synchronous measurement method for refractive index and thickness is applied,allowing precise iteration to obtain the average thickness and refractive index of the ICF target's ice layer.Utilizing an optical projection tomography approach,a mirror-scanning synchronous characterization system for the target is constructed,which synchronously measures the average thickness and refractive index of the target's ice layer under a single angle.Edge recognition,polar coordinate transformation,and feature extraction algorithms are employed to extract the fringe radius from interferometric detection images,facilitating inference of the average refractive index of the ice layer.During multi-angle scanning,piezoelectric ceramics control the mechanical phase shift of the reflecting mirror in the M-Z interferometric optical path,enabling the determination of the two-dimensional refractive index distribution of each projected plane.Subsequently,an algebraic reconstruction algorithm is used to perform a three-dimensional refractive index reconstruction of the target's ice layer.Additionally,edge extraction,Hough transformation,and least square fitting are used to analyze backlight shadow images,enabling extraction of the ice layer's contour for accurate thickness correction when combined with the refractive index distribution obtained through interferometry.The result is a detailed map of the target ice layer's contour roughness and power spectral density derived from average thickness measurements.Results and Discussions A significant contribution of our work is the innovative reconstruction of the target ice layer's contour roughness and three-dimensional refractive index distribution.The experimental outcomes demonstrate that the relative error in characterizing the contour roughness is less than 2.2％,as shown in Fig.8 and Fig.9.Using algebraic reconstruction and light deflection algorithms,in conjunction with a light tracing model,we reconstruct the three-dimensional refractive index distribution of the target shell and ice layer within the mirror scanning area.Simulation results indicate that the average relative error of the three-dimensional refractive index reconstruction results for single-layer shells and double-layer targets is less than 0.16％,with peak-to-valley value and root mean square value relative deviations of less than 2.88％and 0.21％,respectively.The algorithm reconstruction time is better than 86 seconds.The results are shown in Fig.6,Fig.7,Table 2,and Table 3.Conclusions We comprehensively examine the ICF target uniformity characterization technology,covering fundamental principles,detailed algorithmic procedures,extensive simulations,system design,and experimental validation.The thorough analysis confirms the reliability of the proposed technology and its potential to bolster the fabrication process of fusion targets significantly.By addressing the need for high-precision measurements and offering a solution adaptable to vacuum environments,our study contributes to the ongoing efforts to achieve the goal of efficient and controlled fusion energy production.
laser fusion targetgalvanometric scanninginterferometrybacklight shadowroughness diagnosisthree-dimensional refractive index distribution
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