To understand the formation process of vacuum gap in coaxial microsecond conduction time plasma opening switch(POS),we have made measurements of the line-integrated plasma density during switch operation using a time-resolved sensitive He-Ne interferometer.The conduction current and conduction time in experiments are about 120 kA and 1 s,respectively.As a result,more than 85%of conduction current has been transferred to an inductive load with rise time of 130 ns.The radial dependence of the density is measured by changing the radial location of the line-of-sight for shots with the same nominal POS parameters.During the conduction phase,the line-integrated plasma density in POS increases at all radial locations over the gun-only case by further ionization of material injected from the guns.The current conduction is observed to cause a radial redistribution of the switch plasma.A vacuum gap forms rapidly in the plasma at 5.5 mm from the center conductor,which is consistent with the location where magnetic pressure is the largest,allowing current to be transferred from the POS to the load.
