Effects Of Atmospheric Extinction And Spherical Aberration On Ground-based Laser Space Debris Removal

Large amounts of space debris pose a serious threat to the security of the spacecraft.Using the ground-based laser to remove space debris is one of the effective methods.However,the ground-based laser space debris removal must face the problem of high-power laser beams propagating in the atmosphere.The self-focusing effect of high-power laser beams propagating in the atmosphere will seriously affect the beam quality.Therefore,it is important to study the effect of self-focusing on ground-based laser space debris removal.On the one hand,for ground-based laser space debris removal,the factors that reduce the target intensity are not only self-focusing effect,but also atmospheric extinction effect.The previous studies on ground-based laser space debris removal do not involve the effect of atmospheric extinction.The effect of atmospheric extinction on ground-based laser space debris removal is studied firstly in this thesis.On the other hand,the beams are often accompanied by spherical aberration due to thermal effects in generation process.Therefore,it is of great significance to study the effect of spherical aberration on the beam quality of high-power laser beams propagating upwards in the atmosphere.In this thesis,the effects of atmospheric extinction and spherical aberration on ground-based laser space debris removal are studied by numerical simulation.The specific research work includes:1.The optimal power of laser beams propagating from the ground through the atmosphere to space orbits is studied.It is shown that,when a high-power laser beam propagates from the ground through the atmosphere to space orbits,there is an optimal beam power that maximizes the target intensity due to self-focusing effect in the atmosphere,even if the atmospheric extinction exists.The B integral is an important parameter to describe self-focusing effect quantitatively.It is found that the value of B integral corresponding to optimal beam power is a constant.Based on this characteristic of the optimal B integral and the concept of the equivalent atmospheric extinction coefficient proposed in this thesis,we derive the analytical formula of the optimal beam power,which presents an effective design rule to optimize the target intensity.In addition,it is shown that the optimal beam power increases as the initial beam radius,the wavelength,the atmospheric extinction coefficient,and the aerosol scale height increase.2.The influence of spherical aberration on the beam quality of high-power laser beams propagating upwards in the atmosphere is studied by using numerical simulation.It is shown that for the large beam size case,the target intensity may be improved by applying the positive spherical aberration.However,for the small beam size case,the target intensity may be improved by using the negative spherical aberration.Furthermore,a laser beam with a large size is more suitable for laser ablation propulsion applications in space than that with a small size.Owing to the linear diffraction effect and the nonlinear self-focusing effect,there exists optimal beam power to maximize the target intensity.The formula of the optimal beam power is fitted for the large beam size case in this thesis.On the other hand,the focal shift appears due to diffraction,self-focusing and spherical aberration,which results in a degradation of the beam quality on the target.For the large beam size case,to move the actual focus to the target and improve the beam quality on the target,the formula of the modified focal length is also derived in this thesis.