Investigation Into The Characteristics Of Electromagnetic Pulses Generation Due To Hybrid Laser Pulses Interacting With Solid Targets

Intense electromagnetic pulses(EMPs)can be generated by high-power laser interacting with solid targets,which reach a maximum amplitude of MV/m with frequencies ranging from MHz to THz.The intensity and distribution of EMPs are directly related to laser energy,laser pulse width,target material,target configuration,target holder material and configuration,target chamber shape and internal arrangement.Many previous studies focused on the electromagnetic radiations generated by a single laser beam shooting planar targets,but the physical process of its generation was not thoroughly interpreted.In this study,the regulation and mechanism of EMPs induced by hybrid laser pulse-target interactions are experimentally and theoretically studied.All EMPs are measured in SG-? series and XG-? high-power laser facilities using ultra-wideband microstrip electric field antennas and B-dot magnetic field antennas.We systematically investigate the temporal and spatial features of EMP inside and outside the high-power laser target chamber and the characteristics of the EMPs induced by the hybrid laser pulses-target interaction.The main contents and conclusions are presented as follows:(1)The spatial and temporal evolution of electromagnetic radiation generated by SG-II series laser-target interaction is studied and analyzed.The experimental results show that the EMP amplitude outside the SG-? target chamber exhibits exponential attenuation distribution with the increase of the distance between the antenna and the target chamber center(TCC),which is mainly related to the power loss of electromagnetic waves propagating outside the target chamber;(2)The distribution of electromagnetic radiation inside the XG-III target chamber is studied and analyzed.The experimental results indicate that as the distance increases between the antenna and TCC,EMPs inside the target chamber decrease first and then increase.However,the rise time and full width at a half maximum of the time-domain waveform increase first and then decrease.It can be concluded that the spatial distribution of electromagnetic pulse is affected by the echo oscillation of electromagnetic waves inside the target chamber through the electromagnetic simulation;(3)The characteristics of EMPs induced by hybrid laser pulse-targets interaction are investigated.The experimental results indicate that the picosecond laser-target interaction can produce the most intense EMP.EMPs generated by hybrid laser pulses-targets interaction can be significantly suppressed.Nevertheless,an aluminum foil added behind the picosecond and femtosecond target can reduce this reducing effect.The diagnostic results of electron and proton confirm that the variation of emitting electron and proton energy is consistent with that of electromagnetic radiation.Further research shows that the EMP amplitude generated by hybrid laser pulses target interaction is closely related to the time delay from nanosecond laser to picosecond laser.The reducing effect tends to decay when the time delay increases.According to the result of the FLASH simulation,we speculate that it is mainly due to the pre-ablation of picosecond and femtosecond target by X-rays generated by nanosecond laser-target coupling,which generates pre-plasma at the rear surface of picosecond and femtosecond target.The pre-plasma hinders the generation of electrons and protons behind the picosecond and femtosecond target,and indirectly decreases the EMP amplitudes.These intense electromagnetic pulses contain a lot of information about laser-plasma interaction and can be regulated by adjusting the pulse width and target parameters of the hybrid pulse laser.Moreover,EMPs can be widely used in signal transmission,ion acceleration and pulse weapons.The study of the law and its key influencing factors of EMPs is helpful to understand the physical process of laser-plasma interaction,by which the generated mechanism of it is revealed.The resulted conclusions provide theoretical support and an experimental basis for electromagnetic shielding and lay the foundation for the safe operation of high-power laser facilities in China.