Numerical Simulation On The Initial Plasma Dynamics In A Laser-Triggered Vacuum Switch

Laser-Triggered Vacuum Switch(LTVS)can be widely used in pulse power technology as a high voltage and high current closing switch.Because its trigger source has the advantages of photoelectric isolation,high stability,easy to achieve refrequency trigger,short delay time,long life time and so on.However,the high cost and large volume of the laser have also become the main obstacles to the real practical application of LTVS.LTVS requires both sufficient laser-induced initial plasma and a compact and low-cost laser.This paper is based on the problem of how to optimize the laser trigger source of LTVS and focus on the laser-induced initial plasma process.Numerical simulation studies were carried out for this process.The generation and development process of laser-induced initial plasma were theoretically analyzed and compared by experiments,which provided theoretical and experimental support for the optimal design of LTVS.First,the physical model of the initial plasma generated by the laser-induced target in LTVS is analyzed and established.In this model,the non-Fourier heat transfer of the target,the laser ablation model of ordinary evaporation and phase explosion,the plasma flow model,the plasma shielding effect and the radiation transmission of the plasma’s own radiation are taken into account,and the physical model of laser induced initial plasma generation is established.The coupling calculation of these parts is carried out by the finite volume method and the discrete coordinate method.The error of the numerical calculation is analyzed,and the simulation results are compared with experimental results to verify the accuracy of the model.Secondly,based on the established numerical model of the laser-induced initial plasma,the effects of different laser parameters(laser intensity,wavelength,beam diameter,radial light intensity distribution,etc.)on the dynamic characteristics of the initial plasma were studied and analyzed.The results show that the absorption of laser energy by the initial plasma through the shielding effect is the main reason for the substantial increase in its temperature and speed.Increasing the laser intensity,shortening the laser wavelength,and using a ring-shaped Gaussian beam helped increase the temperature and velocity of the initial plasma.At the same time,the two-dimensional model is compared with the one-dimensional model,and the limitations of the one-dimensional model in describing the dynamics of laser-induced plasma are pointed out.Then,according to the conclusion that the plasma shielding effect is the main reason for the large increase in the initial plasma temperature and velocity,a double-pulse scheme is designed to improve the absorption ratio of the plasma to the laser energy.The results show that when the total pulse width,pulse energy and peak power remain unchanged,the double-pulse laser has stronger shielding effect,higher plasma temperature and faster speed than the single-pulse laser.At the same time,the effects of pulse interval and pulse width ratio are also analyzed.Finally,based on the established numerical model of the laser-induced initial plasma,the effects of different target electrode parameters are studied and analyzed.The results show that different incident angles have little effect on the kinetic characteristics of the initial plasma,while the composition ratio of the mixed target can have a significant effect on the initial plasma.At the same time,the ablation process of target electrode was studied experimentally.The experimental results show that the mass fraction of KCl in the steam produced by ablation is always higher than that in the solid target,which means that for the same laser irradiation point,when the laser ablation times are enough,KCl will always be consumed before Ti.The calculated results agree with the experimental results.By numerical simulation and experimental study of LTVS laser-induced initial plasma,the generation and development of laser-induced initial plasma and the influence law of related parameters are revealed.The results and data will be helpful to the structural optimization of LTVS and the miniaturization design of lasers.The numerical simulation and theoretical analysis methods are of great significance for studying the mechanism of laser interaction with the target electrode,optimizing the design of the switch,and saving the time and economic cost caused by the experiment.