Experimental Study On Electromagnetic Radiation Of LY12-Al Produced By Hypervelocity Impact

Electromagnetic radiation produced in hypervelocity impact is an important physical response of solid material under strong impact loading.Researching on electromagnetic radiation produced in hypervelocity impact relates to a lot of disciplines,such as impact dynamics,solid state physics,electromagnetism,optics,plasma physics,high pressure physics.It is of important theoretical significance for understanding the formation mechanism of electromagnetic radiation,the relationship between plasma and electromagnetic radiation,the relationship between collision conditions and the frequency of electromagnetic radiation.At the same time,the research on electromagnetic radiation is of important applied meaning for assessing the collision between space debris and spacecraft,identifying the material properties of planet's surface,evaluating the damage of weapon system.In this article,we conducted a series of studies on a spherical LY12 aluminum projectile impacting on the same material target through the experimental measurement and theoretical analysis.Electromagnetic radiation produced in hypervelocity impact have some characteristics of wideband,short duration,nonlinear-nonstationary electromagnetic signal.Considering the above features,we designed and set up reasonable experimental system for measuring the electromagnetic signal,especially in the visible and microwave frequencies.A LY12-AL spherical projectile which is loaded by two stage light gas gun impacts LY12-AL thick target.In the process of hypervelocity impact,frequency-resolved and time-resolved signal of impact flash is measured by spectrograph and pyrometer(composed of photomultiplier tubes),and impact flash photos were obtained by ultra-high speed camera whose shortest exposure time is 5ns and shortest scanning time is less than 10 ps.Frequency-resolved and timeresolved signal of microwave is measured by real-time spectrum analyzer and superheterodyne receiver.Due to electromagnetic radiation is closely related to plasma,Langmuir triple probe and charge sensor were adopted for diagnosing plasma characteristics in hypervelocity impact.Flash spectrum measured by spectrograph is mainly composed of line spectrum and continuous spectrum.Flash intensity has an exponential relationship with impact velocity,and flash intensity is directly proportional to the sine of impact angle.With the speed decreases,the intensity of line spectrum is decreased in spectrum and the characteristics of thermal radiation are more obvious.The relationships between flash temperature and impact velocity?impact Angle were obtained by pyrometer.The peak of flash temperature exceeds the vaporization temperature of LY12-AL remarkably.The materials near the collision point are vaporized and plasma is generated.By Langmuir triple probe,the relationship between impact velocity,impact Angle and plasma electron temperature are obtained.Through real-time spectrum analyzer,microwave spectra are measured in different impact velocity and impact angle.Along with the increase of impact velocity,the radiation power of high frequency microwave increases significantly.The radiation power of high frequency microwave increases with the increase of impact angle,but this trend is not obvious in low frequency.Through the super-heterodyne receiver and horn antenna,Microwave signals in time domain were measured in the frequency range of 22.5-23 GHz.The signal is composed of a lot of electromagnetic pulse.The intensity of electromagnetic pulse is more intense in higher impact velocity and impact angle.The time-frequency spectrum of high frequency microwave was acquired by Hilbert-Huang Transform.On the time,electromagnetic pulse is concentrated in the initial stage of collision,and the intensity of electromagnetic pulse gradually decreases over time.On the frequency,the intensity of electromagnetic pulse increases with the increase of frequency in collision initial phase.Through broadband oscilloscope and yagi antenna,microwave signals in time domain were measured in less than 1GHz.By analyzing the results of experiments,the mechanism of electromagnetic radiation produced in hypervelocity impact is analyzed.They are the plasma produced in hypervelocity impact,material damage in the process of collision and thermal radiation.The plasma produced in hypervelocity impact can generates flash and microwave.The theoretical results of spectrum are consistent with the experimental results.The model of microwave radiation in plasma is put forward.Materials damage in hypervelocity impact can cause charge separation.So,an electric dipole will be generated.Electromagnetic pulse is produced due to discharge between dipole.Interface temperature rise sharply in the process of collision,which can cause thermionic emission.The model of thermionic emission is put forward,and the radiant power of thermal radiation are calculated in the experiment.Finally,the mechanism of electromagnetic radiation under different impact velocities was determine.