Atomic Scale Simulation Of Vacuum Breakdown Mechanism In High Power Microwave System

High power microwave(HPM)weapon is a new type of equipment which damages the enemy’s communication system and equipment by directional irradiation of highenergy electromagnetic wave.Its core device is high-power microwave source.The breakdown of high-power microwave devices in strong vacuum field seriously limits the single pulse energy of high-power microwave source,which has become a technical bottleneck and research focus.Due to the complexity of the micro process of strong field breakdown,there is a lack of accurate physical model.Material calculation and simulation methods can be used to study the intrinsic properties and phase transition process of materials at atomic scale,or will become an effective means to study the breakdown of high power microwave devices.First of all,the first principles calculation method was used to establish the models of different surfaces of Cu,Ti and Austenitic 304 stainless steel,and the work functions were calculated under the conditions of no electric field and external electric field respectively.Without external electric field,the surface work function and surface energy are completely opposite,that is,the more stable the surface is,the more difficult it is to emit electrons.The work function of each surface decreases approximately linearly when the external electric field perpendicular to the surface is applied.The breakdown threshold of Cu(211)surface and stainless steel is the largest,and that of Ti(11(?)2)surface is the largest.Among the three materials,Ti has the strongest electron emission ability,followed by stainless steel,and Cu has the weakest electron emission ability.The effects of vacancies and alloying elements on the work function and breakdown threshold of different surfaces of Cu,Ti and stainless steel are different.Vacancies in Cu decrease the maximum breakdown threshold,while vacancies in Ti and stainless steel increase the maximum breakdown threshold.When the mass fraction of Ni is 2.89% and the electric field direction is perpendicular to the Cu(211)plane,the maximum breakdown threshold can be obtained.When the mass fraction of Al is 7.32% and the electric field direction is perpendicular to the Ti(112(?)2)surface,the maximum breakdown threshold can be obtained.Based on the first principles calculation results,in order to further study the detailed physical process of breakdown,molecular dynamics models of Cu,Ti and Austenitic 304 stainless steel were established to simulate single electron bombardment.The whole process can be divided into ballistic phase,hot peak phase and quenching phase.The number of defect pairs increases sharply at first,and then decreases rapidly due to the recombination of vacancy and interstitial atom.Changing the incident direction of the primary atoms,the number of Frenkel defect pairs changes with time.A Cu grain boundary model was established to simulate the single electron bombardment.It was found that the grain boundary had a tendency of self healing,which could reduce the damage of electron bombardment and accelerate the recovery of vacancy and interstitial atoms.When the angle between the incident direction of the primary dislocated atom and the grain boundary is 30° At the same time,the damage to the material is the smallest.The results of single electron bombardment simulation show that the peak defect number and stable defect number increase linearly with the increase of primary atom energy,and the relationship between the peak defect number and stable defect number of the three materials is basically the same.Therefore,stainless steel should be selected as anode material when the electron energy is low,and Ti should be selected as anode material when the electron energy is high.A certain concentration of vacancy can improve the resistance of Cu to electron breakdown,but has little effect on the number of defects in Ti model,and has a complex effect on the electron bombardment process of stainless steel,which is related to the energy of primary atoms.Because the actual bombardment process is continuous electron beam bombardment,the thermal spike models of Cu,Ti and Austenitic 304 stainless steel are established to simulate the electron beam bombardment process.It can be observed that there are three energy consumption modes of sputtering,heat conduction and shock wave in the thermal spike region.In the process of bombardment,the lattice melts and re solidifies,which makes the irradiated surface no longer smooth.When the energy of the hot peak reaches a certain value,the atoms vaporize.After vaporization stops,the melted atoms on the surface are concentrated by the surface tension and the surface shrinks,that is,the surface roughness decreases.According to the degree of surface atomic vaporization and ablation,the resistance to electron breakdown of Cu is the worst,while that of Ti and stainless steel is the best.