Research On The EMP Transmission Performance Along The Plasma Channel Induced By The Femtosecond Laser

As a kind of special electromagnetic medium,plasma has the ability of guiding the electromagnetic pulse(EMP).Therefore,EMP transmission technology based on the plasma channel induced by the intense femtosecond laser pulse is of pratical significance.The EMP transmission performace of the laser plasma channel has been discussed theoretically,emulationally and experimentally.The main research contents and conclusions are as follows:Firstly,the generation and recombination of the femtosecond laser plasma channel are studied.According to the nonlinear physical propagation process of the femtosecond laser in the air,the principle and process in the generation of the plasma filament are discussed.Meanwhile,the igniting positions of plasma filament with and without a lens are investigated by using the moving-focus model.In addition,the recombination mechanism of laser-induced plasma is analyzed.The results show that,based on the third-order nonlinear effect in the air,the focus position of self-focusing is related to the power and radius of the laser pulse beam in free propagation,while the final focus position is related to the focal length of lens,the power and radius of the laser pulse after the lens is added.During the plasma relaxation,the recombination of electrons with the ions is dominant at a high electron density,while later,when the density is relatively low,electron attachment to molecules becomes the main decay mechanism.Secondly,the EMP transmission performance of the the single channel plasma and parallel double channel plasma induced by the femtosecond laser is discussed theoretically and emulationally.Based on the Maxwell's equations and the boundary conditions of the EM field,the interaction between the EM wave and planar femtosecond laser plasma,and the propagation characteristics of the surface wave are investigated.Then,the physical model of the interaction between EMP and single channel plasma is established.Based on the FDTD method,the spatial EM field distribution around the plasma channel and the propagation characteristics of EMP along the single channel plasma are studied.Finally,the physical model of the interaction between high-power microwave(HPM)and double channel plasma is established,and the transmission peroformance is carried out with the FIT method.The results show that the EM wave can propagate along the single channel plasma as a surface wave rather than in the plasma,with its wave velocity being lower than the speed of light in vacuum,while the wavelength is slightly less than that being in free space.Besides,the EM wave intensity enhancement more than three times of that in free space is realized when the plasma electron density is 10¹⁶cm^-3.We can also conclude that for the TE polarization wave,as well as the low frequency EM wave,the guiding performance of the single channel plasma can be better.The EM wave can propagate along the double channel plasma as a TEM wave.The electric field,the magnetic field and the induced current of HPM propagate along the double channel plasma at the speed of light,and their amplitudes are inversely proportional to the propagation length and the channel spacing.In addition,the influences of the plasma parameters,the EM wave parameters and the transmission line structure parameters on the transmission performance of the double channel plasma are also researched.Thirdly,the investigation of the HPM transmission and radiation performance of the V-shaped double channel and multi-channel plasma antennas are carried out.The physical models of their interactions are established.The HPM transmission and radiation performance of the antennas are studied by analyzing the electric field spatial distribution and radiation pattern.The results show that the radiation performance of the plasma antennas for waves of different frequencies is different.The smaller the plasma collision frequency is,the better the HPM radiation performance of the plasma antenna is.By adjusting the angle of V-shaped double channel plasma,the main lobe of the antenna can exactly point along the angular bisection line,that is,the HPM radiation is the largest in the target direction.With the increase of the length of the laser plasma channel,the optimal angle of the antenna becomes smaller.With the increase of the channel angle,the maximum directivity coefficient of the antenna increases at first and then decreases.It can also be found that the directional radiation performance of the open horn multi-channel plasma antenna is the best with the same parameters.Fourthly,the EMP guiding performance of the the single channel plasma and parallel double channel plasma induced by the femtosecond laser is verified experimentally.The results show that the duration of the plasma channel is about ten nanoseconds.The high conductivity and the large plasma size can enhance the EM wave guiding performance of the plasma channel.We can also conclude that the TE polarization wave,the short focal length of the lens,as well as the large laser pulse energy can increase the guiding performance of the plasma channel.The longitudinal lengths of the femtosecond laser plasma channel are approximately 4.7cm,7cm and23cm,respectively with focal lengths of 50cm,100cm,and 200cm.The peak intensity position along the channel is located around the focus of the lens.The enhancement factor of the EM wave with the double plasma channel is about 30.The research results imply that the femtosecond laser plasma channel can provide efficient transmission of the EMP over a long distance,which greatly reduces the diffusion attenuation in the propagation.