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Mitigation of Magneto Rayleigh Taylor Instability

Technical Report ·
DOI:https://doi.org/10.2172/1436495· OSTI ID:1436495

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.

Research Organization:
Univ. of Michigan, Ann Arbor, MI (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Fusion Energy Sciences (FES)
DOE Contract Number:
SC0012328
OSTI ID:
1436495
Report Number(s):
DOE-UM-12328-1
Country of Publication:
United States
Language:
English

References (9)

Seeded and unseeded helical modes in magnetized, non-imploding cylindrical liner-plasmas October 2016
Determination of plasma pinch time and effective current radius of double planar wire array implosions from current measurements on a 1-MA linear transformer driver October 2016
Discrete helical modes in imploding and exploding cylindrical, magnetized liners December 2016
Coupling of sausage, kink, and magneto-Rayleigh-Taylor instabilities in a cylindrical liner March 2015
Double and Single Planar Wire Arrays on University-Scale Low-Impedance LTD Generator April 2016
Evolution of sausage and helical modes in magnetized thin-foil cylindrical liners driven by a Z-pinch May 2018
Technique for fabrication of ultrathin foils in cylindrical geometry for liner-plasma implosion experiments with sub-megaampere currents November 2015
The electro-thermal stability of tantalum relative to aluminum and titanium in cylindrical liner ablation experiments at 550 kA March 2018
Temporal evolution of surface ripples on a finite plasma slab subject to the magneto-Rayleigh-Taylor instability December 2014