| The unique characteristics of terahertz,such as security,penetration,and feature recognition,make it suitable for a wide range of applications,such as biomedical imaging,communications,remote sensing,material detection,astrophysics,and other fields.Extremely intense terahertz radiation can open up several new fields of research,including nonlinear spin control,electron acceleration,and femtosecond science.How to obtain high-efficiency and high-quality terahertz radiation sources is a key issue restricting its development.Compared with schemes such as photoconductive antenna,optical rectification,and bichromatic fields that can generate micro-joulelevel terahertz,the coherent transit radiation scheme can generate 50 mJ terahertz,but it has the disadvantages of low energy conversion efficiency and large divergence angle.This thesis develops the theory of the transit radiation model and proposes a new method for manipulating the divergence angle of terahertz sources using a cone target.The simulation results show that by using a femtosecond laser with a pulse energy of 1.9 J to act on a cone target with an opening angle of 10 degrees,an energy of 40 mJ,a high-quality strong terahertz source with the peak power of 100 GW and a divergence angle of～20°can be obtained.This thesis systematically studies how to generate highly efficient collimated terahertz sources by means of theoretical analysis and numerical simulation.Firstly,the model of single-electron transit radiation is studied and discussed through theoretical analysis,and it is found that the incident angle of electrons has a great influence on the final terahertz radiation divergence angle.Based on this,a cone target scheme is designed,and the numerical simulation method is used to verify it.The numerical simulation method used in this thesis is the PIC particle simulation method,using the open-source programs EPOCH and Smilei,and relying on the supercomputing platform of the Institute of Physics and Electricity of Hunan University for calculation.By scanning the cone targets at different angles,it is found that the terahertz radiation results are consistent with the theoretical model,and the small-angle cone target can also effectively improve the energy conversion efficiency of the cone target.It is finally determined that the 10° cone target scheme is used as the terahertz source,and the terahertz radiation with an energy conversion efficiency of 2.11%can be obtained,which is an order of magnitude higher than the traditional transit radiation.After that,this thesis mainly verifies the robustness of the scheme by changing the laser and plasma parameters,and comparing the results of the inclined plane target and the cone target,it is determined that the cone target has a greater contribution,and the 3D simulation show that the source has a polarization direction of θ,and finally the far-field radiation ratio is determined to be 81.6%by the angular spectrum representation.Compared with the radiation angle of nearly 90 degrees of the traditional transit radiation,the terahertz source scheme proposed in this thesis has greatly improved collimation,and the energy conversion efficiency reaches 2.1%,compared with the traditional coherent transit radiation～0.1%The efficiency is improved by nearly an order of magnitude,which can greatly reduce the dependence on laser power.Based on this scheme,it is expected to obtain an ultra-intense terahertz radiation source on a～100 TW laser. |
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