Quantifying data constraints for velocimetry-based load current determination for inertial confinement fusion targets

Velocimetry-based load current inferences provide the most accurate method of determining delivered drive current on multi-mega-amp cylindrically imploding inertial confinement fusion platforms, allowing for drive uncertainties of < 2% for 20 MA targets. Low uncertainty current inferences are necessary inputs – along with high quality target metrology – for accurate multiphysics modeling postdictions. Velocimetry information is now routinely returned from cylindrical anode “return cans” concentric to the imploding gas filled cathode target across a wide variety of campaigns on the Z Machine. The standard 100 ns current rise results in shock formation in the return can, and current information is lost as wave characteristics coalesce. Understanding the uncertainty of the velocimetry-derived drive current in the region of the current constrained by the data lost to the shock formation requires advanced computational or mathematical methods. Here we discuss the application of Sobol indices – also referred to as variance-based sensitivity analysis – to identify the level of constraint of various portions of the derived current pulse. The Sobol indices approach is one of several methods under investigation at Sandia National Laboratories for quantifying constraining data of velocimetry-based load current inferences.