Speaker
Description
Magnetically driven implosions on the Z Facility assemble high-energy-density plasmas for radiation effects and ICF experiments. MDIs are hampered by the Magneto-Rayleigh-Taylor (MRT) instability, which can limit achievable stagnation pressures and temperatures. The metallic liners used in Magnetized Liner Inertial Fusion (MagLIF) experiments include astonishingly small (~10 nm RMS) initial surface roughness perturbations; nevertheless, unexpectedly large MRT amplitudes are observed in experiments.
Early in the implosion, an electrothermal instability (ETI) may provide a perturbation which exceeds the liner’s initial surface roughness. For a condensed metal, resistivity increases with temperature. Locations of higher resistivity can undergo increased Ohmic heating, resulting in locally higher temperature, and thus still higher resistivity. This drives unstable temperature and pressure growth which produces density perturbations when the locally overheated metal changes phase. Such ETI-driven surface perturbations then seed MRT growth. For liners imploded on Z, ETI seeding of MRT is inferred by evaluating MRT amplitude late in the experiment. A direct observation of ETI is vital to ensure simulations accurately represent the seed of the MRT instability.
ETI growth was directly observed on the surface of 1.0-mm-diameter solid Al rods which were pulsed with 1 MA of current in 100 ns. Fine structures resulting from ETI-driven temperature variations were observed directly through high resolution gated optical imaging (2ns temporal and 3 micron spatial resolution). Data from two Aluminum alloys (6061 and 5N) with a variety of surface finishes enabled the determination of which types of imperfections most rapidly drive overheating. Surfaces coated with ~70 microns of mass-tamping dielectric evolve very differently than uncoated surfaces. Data are relevant to the early stages of MagLIF liner implosions, when the ETI seed of MRT may be initiated. We present a fundamentally new and highly 3-dimensional dataset which informs ETI physics, and provides a unique test for state-of-the-art simulation tools.
