Initial Plasma Characteristics And Optimization Design Of Laser-triggered Vacuum Switch

Laser-triggered vacuum switch is a high-voltage high-current pulse switch.It will have the advantages of large current capacity,re-triggerable triggering,isolation of the trigger system from the main circuit,and short trigger delay time.The initial plasma characteristics generated by laser triggering are significantly different from those of conventional electric pulse triggering.The introduction of laser will produce a unique microscopic process,and there are a large number of unknown problems waiting to be explored.Studying the initial plasma characteristics and its influencing factors of the switch triggering is helpful to expose the action mechanism of the laser and the target electrode,guide the setting of the laser parameters,reduce the energy threshold of the switch trigger,reduce the volume of the laser system,and facilitate the switch.Miniaturization and generalization drive the practical application process of the switch.Firstly,the physical process of laser-object interaction and the mechanism of plasma generation is analyzed.Under the action of high-power laser,the main mechanisms for the transformation of gaseous,liquid and solid materials into plasma state are photoionization,thermal ionization and impact ionization.Through electron microscopy,X-ray energy spectrum,target spectral transmission and other experiments,it is judged that during the interaction of laser and the target,titanium in the target electrode absorbs the laser energy,the temperature of the target electrode rises,and the potassium chloride vaporizes and partially ionizes.An initial plasma mainly composed of free electrons and positive and negative ions generated by potassium chloride crystals is formed.In the environment of high temperature,high field strength and strong laser,the initial plasma vapor enhances the ionization through the thermal mechanism and the impact ionization mechanism,and collides with the main electrode to generate more free electrons and positive and negative ions,which eventually leads to laser triggered vacuum switch to conduct.Secondly,the physical process of laser and trigger is studied,including analyzing the reflection and absorption of the laser by the target electrode,establishing a threedimensional heat conduction model of the target electrode,and analyzing the vaporization and ablation caused by the laser.The laser trigger energy threshold of each material was measured under different wavelengths of different metals.A trigger material such as an alkaline halide was used as a switching target electrode,and the delay time of the switch was measured to determine the influence of the target electrode on the switching performance.It is concluded that the combination of potassium chloride and titanium can make the laser trigger vacuum switch performance the best.Then,through the combination of theoretical analysis and experimental research,the effects of laser wavelength,energy and pulse waveform on the initial plasma characteristics were studied.The laser absorption rate model of the target electrode was established,and the influence of the wavelength on the initial plasma generation process was analyzed.It is found through experiments that the shorter the wavelength,the higher the laser absorption rate of the target electrode,and the more initial plasma is generated.A heat conduction equation describing the absorption of laser energy by the target electrode was established,and the influence of the laser energy on the initial plasma quantity and the moving speed was analyzed,and the ablation area of the target electrode was calculated.It is proved by experiments that the larger the laser energy,the more the initial plasma formed by ablation to remove the target electrode,and the faster the initial motion speed.From the energy density distribution of single-peak and bimodal pulses and the plasma formation process,combined with the plasma shielding effect,the influence of pulse waveform on the degree of target removal was analyzed.By using different waveforms of laser pulses,the delay time of the switch was measured,and the influence of the pulse waveform on the conduction performance of the switch was compared.It can be seen that the unimodal pulse reduces the effects of plasma shielding and removes more of the target material to form more initial plasma.Finally,the relevant experimental rules obtained in this paper are applied to optimize the triggering performance of the switch.The selection of the target electrode material and structure and the setting of the laser parameters are optimized,and the optimization direction and suggestions are proposed.The optimization includes target electrode particle size,material ratio,fabrication process,laser wavelength,energy,trigger position,and etc.Increasing the conduction performance of the laser-triggered vacuum switch and reducing the volume of the switching laser system are closely related to the initial plasma characteristics during the switch triggering process.In this paper,through the establishment of the target electrode heat conduction model and the laser absorption rate model,the physical process of the target electrode and laser action was theoretically analyzed,and a series of experiments were carried out to explore the basic rules of switch triggering.The research work done in this paper makes the research on the triggering process of the switch clearer,lays a foundation for the optimal design of the switch and the miniaturization of the laser,and advances the application process of the switch.