Post Author: Thomas J. Awe
Images taken from Phys. Plasmas 32, 082109 (2025), https://doi.org/10.1063/5.0279628.
Recent developments at the 1 MA/100 ns Mykonos Facility at Sandia National Laboratories allow experiments to add an applied axial magnetic field to z pinch targets. Inclusion of axial field, Bz, is known to have transformative impact on the implosion dynamics of magnetically-accelerated metallic liners. Whether the field is applied by slowly rising Helmholtz coils (static Bz) or pulsed using a helical return can or HRC (dynamic Bz), helically-oriented magneto-Rayleigh Taylor (MRT) modes grow on the liner, persist throughout the implosion, and alter how the liner compresses the fusion fuel upon stagnation. Fundamental questions concerning the observed helical instabilities remain, including: (1) what is the fundamental seeding mechanism of the helical perturbation and when is it initiated? And (2), under what conditions does the helical perturbation grow/persist throughout the implosion? To evaluate one potential helical-perturbation-seeding mechanism, recent experiments observed the topography of ETI-driven overheating structure on 1.00-mm-diameter, 10-nm-surface roughness, 99.999%-pure-aluminum rods which are pulsed with intense helically-polarized surface magnetic field using an HRC. Rods were machined to include pairs of 10-micron-scale quasi-hemispherical voids or “engineered defects (ED)” which provided the dominant current density perturbation from which ETI grew most rapidly. Most experiments generated surface magnetic field at a 15-degree field polarization angle defined as ɸB=arctan(Bz/Bɵ). Emission patterns from individual ED were observed to rotate along ɸB, while emission patterns from neighboring ED pairs were shown to elongate and preferentially merge along ɸB, in qualitative agreement with 3D-magnetohydrodynamic simulations. Reasonable extension of these data suggests that for a random distribution of local current density perturbations (pits, resistive inclusions), neighboring perturbations will favorably merge along ɸB, with the degree of merging increasing with current. In this way, elongated helically-oriented perturbations may be generated. Such observations offer fundamental new understanding of the seeding mechanisms of the helical MRT instabilities observed from axially-magnetized magnetically-driven imploding liners.
Results have recent been published in Physics of Plasmas (https://doi.org/10.1063/5.0279628): T.J. Awe, E.P. Yu, G.A. Shipley, K.C. Yates, K. Tomlinson, and M. Hatch., “Rotation of electrothermal-instability-driven overheating structure due to helically oriented surface magnetic field on a high-current-density aluminum rod“, Phys. Plasmas 32, 082109 (2025).