Microwave Radiation Generated From Intense Laser Plasma Interactions

With the rapid development of laser technology and the increase of laser intensity,intense laser plasma interactions can produce extremely high energy density in unit time and space in the laboratory,and then create extreme physical conditions which exist only in the nuclear explosions or the celestial objects.This makes it feasible to study laser fusion simulation,laboratory astrophysics,and related research in laboratory.Very broad electromagnetic spectra,including highlighted gamma rays and x–rays,ultra–strong terahertz radiation,microwave radiation,can be excited in laser plasma interactions.As one of the most common products of laser-plasma interactions,microwave radiation needs more in-depth and comprehensive research.This thesis mainly studies the dynamics of microwave radiation in laser plasma interactions.The mechanisms and radiation characteristics of the microwave radiation are studied by using different high-power laser facilities when interacting with solid or gas targets.The thesis mainly includes the following parts.The first part is the introduction,which mainly introduces the main mechanisms of the microwave radiation generated in laser plasma interactions and the related research progresses.Previous studies have shown that microwave radiation can be generated by the mechanisms of the dipole radiation,the current return on target holder,the X-ray ionization of metal materials,and the current oscillation in the gas channel during laserplasma interaction.Based on the different radiation mechanisms,we analyze the diagnosing methods of the electron temperature,electron density,and escape electron behavior by the time and frequency domain characteristics of microwave radiation.In the experimental study in many large laser facilities,the microwave radiation intensity can be as high as MV/m.Taking advantage of its strength advantage,we discuss the significance of microwave radiation in producing pulsed strong magnetic field and being a high-power microwave source.In laser plasma experiments,it is usually necessary to weaken the electromagnetic interference caused by microwave radiation to the greatest extent.The methods of reducing the electromagnetic interference are discussed.In the second part,the microwave radiation generated by the current oscillation mechanism in femtosecond laser–solid target interaction is studied.In laser-metal target interactions,the escape electrons can form the equivalent positive potential on the target surface,which will cause the return current on the target structure and finally produce microwave radiation.Basing on this mechanism,the effect of the target size on the characteristics of the return current and the microwave radiation is studied in time and frequency domains.It is found that metal targets can be used as radiation antennas to generate microwave radiation,and the frequency of microwave radiation can be accurately controlled by the structure and size of metal targets.By studying the influence of laser pulse duration and energy on the characteristics of microwave radiation,it is found that the intensity of microwave radiation generated by antenna structure is almost linearly correlated with laser energy,but weakly correlated with laser pulse width.This work indicates that we are expected to generate more intense and frequency-controllable microwave sources by the interaction of long pulse and high energy lasers with the antenna-like target structures.In the third part,the effect of terahertz pulses on the electromagnetic disturbances in microwave band is studied in intense picosecond laser-solid target interaction.The quasi-isotropic and GW-class microwave radiation is obtained experimentally.The two-dimensional electromagnetic simulation results show that the terahertz pulses with picosecond timescale may form strong electromagnetic interference in a closed metal chamber.In addition,we find experimentally that nanosecond laser plasma interaction,which occur earlier that the picosecond laser plasma interaction,can effectively inhibit the electromagnetic interference which is generated from the picosecond laser plasma interaction.In the fourth part,the microwave radiation mechanisms and characteristics in high energy nanosecond laser plasma interactions are studied.The time scales of electron escape and sheath field oscillation generated in nanosecond laser plasma interactions are generally in nanosecond order,which means that microwave radiation from hundred MHz to several GHz can be generated directly from the plasma region.Therefore,the study of the microwave radiation characteristics generated from nanosecond laser plasma interactions can help to diagnose the electron escape behavior and other processes.In Shenguang-II nanosecond laser facility,we investigate the influence of the laser energy on the microwave radiation intensity,waveform,and spectrum in GHz band during the interaction between high energy nanosecond laser and large size planar target.Using the dipole radiation model and the electron escape model,the transformation process of microwave radiation mechanism under different laser energy is studied.The effect of target thickness increasing on the attenuation of microwave radiation in the backside of target is studied in the interactions between large energy nanosecond laser and small size planar target.In addition,aiming at the process of microwave radiation generated only by electron escape,the characteristics of microwave radiation are diagnosed in the experiment at multiple angles and distances in space.Combining with numerical simulation,the distribution characteristics of microwave radiation field generated by electron escape in the whole 4? space are analyzed,which provides a reference for analyzing the spatial distribution characteristics of microwave radiation field in the experiment.In the fifth part,the strong microwave radiation generated in the interaction between femtosecond laser pulses and near-critical-density(NCD)gas targets is studied.The time domain and frequency domain characteristics of microwave radiation and the spatial angular distribution of microwave radiation are characterized.It is found that the intensity of microwave radiation produced by the laser-NCD gas target interaction can be equivalent to that of many high power laser facilities.This work may provide a new opportunity to use strong laser plasma to drive high power microwave in the future.