Experimentally assessing the origins and scaling of the helical instability in axially magnetized magnetically driven implosions

In magnetically driven cylindrical liner implosions, an axial current flows through the target generating an azimuthal magnetic field. The magnetic pressure causes the liner to implode, and azimuthally correlated instability structures grow on the outside of the liner due to the magneto-Rayleigh-Taylor instability. When a static axial magnetic field (~10 T) is applied to the system, helical structures form, despite the azimuthal magnetic field being orders of magnitude larger (~1000 T) than the applied axial field [T.J. Awe et al., Phys. Rev. Lett. 111, 235005 (2013)].

Magnetized liner inertial fusion (MagLIF) requires an axial magnetic field to provide thermal insulation between preheated fusion fuel and the imploding liner. The stability of the liner implosion can significantly impact stagnation morphology and performance. Understanding the formation of the helical structures and predicting the growth of the instability is a challenging but important aspect of scaling MagLIF to high yield.

We conducted a series of imploding-liner experiments with axial magnetic fields spanning 0-17 T to obtain data against which our computational tools can be tested. X-ray radiography was used to diagnose instability structures as a function of target convergence and applied magnetic field. The angle of the helices increased with both applied magnetic field and target convergence but saturated at high field; at 17 T the helical structures were indistinguishable from the structures at 10 T at the same convergence. X-ray self-emission images show lower density, hot plasma surrounds the imploding liner, and optical diagnostics indicate even lower density plasma at larger radius. Additionally, B-dot monitors detected flux compression of the axial magnetic field just outside the target region. These experimental results were compared with synthetic diagnostics from computational tools used to model the formation of helical structures in axially magnetized liner implosions to assess the predictability of the modeling approaches.