A Dispersion Interferometer Diagnostic used for Low Electron Density Measurements in Magnetically Insulated Transmission Lines on Sandia's Z-Machine

A needed expansion in the understanding of current loss mechanisms in presently operating pulsed power machines, such as Sandia's Z-machine, will improve reliable delivery of current to magnetically driven loads. This is an important issue for the design of next generation large-scale pulsed power drivers currently being developed. Electron sheath formation on magnetically insulated transmission lines (MITLs) is a fundamental characteristic of power delivery in such pulsed power systems, as increased electron flow reduces coupling efficiency and electron flow can generate electrode plasmas. A fiber-based dispersion interferometer (DI) will enable the first direct measurements of electron sheath flow on Z and will reduce the current lower limit for electrode plasma density measurements available. This DI design will operate at 1550 nm CW, with frequency-doubling to 775 nm. Prior to deployment on Z, the interferometer will be characterized on two smaller devices. First, the HelCat (Helicon-Cathode) basic plasma device at the University of New Mexico while being compared against well-known density profiles measured by a 40GHz microwave interferometer and scanning double Langmuir probe. Next, it will be fielded in a moderate pulsed power environment on the 1 MA Mykonos driver at Sandia National Labs. Finally, it will be deployed to directly measure the electron densities in magnetically insulated flows on the Z-Machine, as well as to characterize plasmas that form on the electrodes.