Validation and uncertainty quantification of ICEPIC/emphasis codes for a series of gas cell experiments at NRL

A series of gas-cell experiments were performed at NRL to be able to validate the dry air (as well as pure N₂) chemistry/collision models available in Particle-in-Cell (PIC) codes. These experiments were constructed and performed with many diagnostics enabling comprehensive comparison between simulation and experiment. This multi-physics/stage problem (i.e. the pulse power circuit, cathode, foil, gas region and collection/anode) has diagnostics that can validate the simulation before and after each component. Because of the experimental access at each individual stage, this experiment is ideal for investigating the many models incorporated in various PIC codes. These measurements include the nonlinear coupling between each stage affecting the results both upstream and downstream of each measurement location. Therefore, these measurements are a stringent test of the models. In addition to numerical error, there is uncertainty of the parameters (e.g. the multi-group cross section used in the foil and the collision cross-sections used in the gas cell) in each component of the experiment. This study will examine the error propagated from one stage to the next; by accounting for these uncertainties we will be able to definitively validate the models. The experimental set up is a co-axial power feed which transitions to a disk-shaped cathode. The electron beam then passes through a thin anode foil which separates the vacuum diode from the gas cell; scattering and propagation in the foil is energy and angle dependent as calculated from ITS and SCEPTRE. The energy of the electrons going into the gas peaks at about 100 keV and the beam last for about 200ns. In addition to voltage and current measurements at various locations, the novel feature of these experiments is laser interferometry which measures the line-integrated electron density at various locations in the gas cell for a gas pressures ranging from 50 mTorr to 300 Torr.