Research On The Radiation Characteristics And Mechanism Of Electromagnetic Pulses Generated By Coupling High-power Lasers With Hohlraum Target

The development of laser technology and its related physical research,as well as optical diagnosis technology makes it possible to drive inertial confinement fusion（ICF）indirectly by laser.In each drive process of large laser facilities in various countries,the interaction of high-power laser energy on the target will produce intense transient electromagnetic pulses（EMPs）.A large number of experiments have confirmed that the main source of such EMPs is the positive charge near the laser focal spot on the target surface caused by escaping thermal electrons.For this primary source,researchers have proposed several physical models describing the mechanism of EMP production.However,the mechanism of EMP signal generation and the influence of many factors such as hohlraum configuration,laser parameters and hohlraum energy distribution on the signal of cylindrical hohlraum target that is widely accepted by many researchers to achieve indirect driving of ICF have yet to be studied in depth.Based on the 100 k J Laser Facility,using B-dot probes and a monopole antenna to detect the EMP signal inside and outside the spherical target chamber,this paper analyzes the electromagnetic pulse radiation regularity and mechanisms of traditional two-hole hohlraum targets and new multi-hole hohlraum targets profoundly,and explains the physical process corresponding to the characteristic frequency of the signal.The main research contents and conclusions are as follows:（1）The radiation characteristics of electromagnetic pulse generated by the two-hole hohlraum targets with different structures and configurations were analyzed and compared and it was confirmed that the thermoelectron source emitted by the hohlraum target was one of the main sources of EMP signal.The EMP signal intensity was strongly inhibited during the implosion process,and the peak value of the vertically polarized electric field at 1.5 m from the target are only 11.6 and 17.4 k V/m,mainly because most of the X-ray energy in the reradiation process was absorbed by the capsule;The duration of the main band of the EMP signal generated by the implosion target is up to 80-100 ns directly related to the irradiation time of the X-rays,which mainly depends on the laser pulse width.The results also show that the EMP signal strength is closely related to the hohlraum target parameters.（2）The electromagnetic pulse radiation characteristics generated by multi-hole hohlraum targets taking the inflatable six-cylinder-port（SCP）hohlraum target as an example were analyzed.The results show that the peak electric field intensity corresponding to the EMP signal generated by the SCP hohlraum is higher and the duration is shorter than that of the two-hole hohlraum targets;The difference in electric field intensity is attributed to the higher laser power density and the plasma filling density in the hohlraum when the multi-hole hohlraum target is driven.The analysis suggests that the shorter duration may be related to the active duration of the plasma plume.In addition,the resonance theoretical analysis gives the measured resonance modes in the Diagnostic Instrument Manipulator（DIM）and their corresponding eigenfrequencies:TE₂₁₁、TE₂₁₂and TE₀₁₁.（3）The characteristics of EMP radiation generated when a hybrid laser pulse bombards multiple targets were studied.The delay in the peak arrival time of the EMP voltage amplitude and the narrowing of the spectrum reveal the attenuation regularity of electromagnetic radiation propagating from the inside of the target chamber to the outside;Taking the X-ray streak camera（XSC）as an example,the electromagnetic simulation software CST was used to load the measured signals to analyze the electromagnetic compatibility problem of key diagnostic equipment near the target.The experimental and simulation results in this paper are conducive to deepening the understanding of the electromagnetic pulse radiation mechanism of hohlraum targets in large laser facilities,and provide a basis for solving the electromagnetic compatibility problem of diagnostic equipment near the target.