Mitigation of Magneto Rayleigh Taylor Instability
Reported herein is a comprehensive study of magneto-Rayleigh-Taylor instability conducted at the University of Michigan, using a one mega-ampere linear transformer driver on a cylindrical thin foil. It is a combined theoretical and experimental effort, on both magnetized and nonmagnetized liners, unseeded and seeded with a helical perturbation, and for a thin foil that is stationary, imploding or exploding. Also studied is the electrothermal instability, thought to be the seed for magneto-Rayleigh-Taylor instability. These subjects are important to magnetized liner inertial fusion (MagLIF). We interpret the helical features usually observed in a magnetized cylindrical liner as a manifestation of a discrete eigenmode, from implosion to stagnation. We discover that the observed pitch angle (phi) of the helix follows the simple relation, phi = m/kR, for both MagLIF experiments at the Sandia National Laboratories, and for our experiments, where m is the azimuthal mode number, k is the axial mode number, and R is the radius of the helical feature. This discrete mode persists from implosion to explosion, even through the highly nonlinear stage where the axial perturbations clump together. When the latter occurs, we propose a simultaneous decrease of mode numbers, from (m, k) to (m/2, k/2), kinematically. We show, both theoretically and experimentally, that higher m modes are excited with higher axial magnetic field. We find that seeding is far more important than the intrinsic instability of a magnetized liner. On the electro-thermal instability, we discover that refractory metals with a low ratio of critical temperature to melting temperature (i.e. tantalum) are very robust against electrothermal instability. We perform the first experiments that show the transition of electrothermal instability from the striation to filamentation mode. We experimentally confirm the importance of surface defects in the development of the electrothermal instability. We develop and publish a fabrication method for ultra-thin metallic liners. Three (3) graduate students completed their PhD theses with the support of this grant.