Wire array load design and optimization for short (100 ns) and long (220 ns) current rise time on the 1 MA cobra generator

Summary form only given. Wire array implosions with different current pulse rise times have been studied on the 1 MA COBRA facility at Cornell University. The flexibility of the COBRA generator allowed switching between the short (100 ns) and long (220 ns) current pulse rise time to study the implosion of nested wire array loads in these two regimes. The load design and optimization was performed by simulations with the novel wire ablation dynamics model (WADM) code, which extended the original wire dynamics model by including the dynamics of wire ablation and precursor plasma formation on the array axis. As compared to the short current pulse, the longer current rise time allows increasing the array mass. By switching between the different pulse shapes the implosion dynamics of nested arrays made from the same wires, but with different wire numbers, have been compared. While the time of the wire core ablation is directly proportional of the array mass, the implosion time is proportional to the square root of the array mass. WADM simulations demonstrate that the implosion dynamics of the arrays with higher mass is more ablation dominated. If the process of core ablation is long enough, it causes the delay of the array implosion versus the predictions by a 0D model that does not account for ablation. Another factor that affects the result of the trade between the ablation and implosion time scales is the form of the current pulse, which can be very different from the classical sin2 shape. This factor was self-consistently taken into account in the WADM simulations. The prediction of the array implosion times by the WADM are in a very good agreement with the recent experiments on the COBRA generator.