Megagauss-Level Magnetic Field and Dielectric Breakdown Measured in Auto-Magnetizing Liner Experiments

Auto-magnetizing liners (AutoMag [1], [2]) are cylindrical tubes made of metallic helical conductors separated by electrically insulating material. In the first stage of AutoMag, helical current flows in the AutoMag liner and produces a strong internal axial field (20 to 100 T) during a 5 kA/ns current prepulse that lasts 100 to 300 ns. In the second stage, the rapidly-rising main current pulse (200 kA/ns) induces a strong electric field in the liner that causes the insulating material to undergo dielectric breakdown. Lastly, after breakdown, the liner current reorients to be primarily axial and the liner implodes radially via the Lorentz force. AutoMag is designed to eliminate the need for the external coil system [3] that is used to premagnetize fusion fuel in Magnetized Liner Inertial Fusion (MagLIF [4]) which impedes diagnostic access and necessitates use of a high inductance power feed that reduces current delivery to the liner. Experiments have been successfully executed on the Mykonos accelerator [5] to evaluate the magnetization and breakdown stages of AutoMag. Microscopic magnetic field probes (microBdots) measured magnetic field inside of the liner and an iCCD imager and fast photodiodes made spatially-resolved and temporally-resolved measurements, respectively, of optical plasma emission in the load region. Fields near 1 MG were measured in multiple experiments and plasma emission indicative of breakdown was observed for select liner designs. These experiments, the first of their kind, represent an advance in magneto-inertial fusion that will enable the design of a new class of MagLIF targets capable of reaching higher current delivered to the liner (better fuel compression) and higher magnetization fields inside of the fusion fuel (better thermal insulation). Success of the experiments on Mykonos has resulted in the design of Z Machine experiments (planned Spring 2018) to study the implosion stage of AutoMag.