Observation of Rayleigh--Taylor-instability growth in a plasma regime with magnetic and viscous stabilization*

Rayleigh--Taylor-instability (RTI) growth during the interaction between a high-Mach-number, unmagnetized plasma jet [1] and a stagnated, magnetized plasma is observed in a regime where the growth of short-wavelength modes is influenced by plasma viscosity and magnetic fields [2]. The time evolution of mode growth at the mostly planar interface is captured by a multi-frame fast camera. Interferometry, spectroscopy, photodiode, and magnetic probe diagnostics are employed to experimentally infer $n_i$, $T_e$, $\bar{Z}$, acceleration, $\vec{B}$, and ion viscosity in the vicinity of the evolving interface. As the instability grows, an evolution from mode wavelengths of $\approx1.7$~$\mathrm{cm}$ to $\approx2.8$~$\mathrm{cm}$ is observed. The growth time ($\sim 10$~$\mathrm{\mu s}$) and wavelength ($\sim 1$~$\mathrm{cm}$) of the observed modes agree with theoretical predictions computed from the experimentally inferred density ($\sim 10^{14}$~$\mathrm{cm^{-3}}$), deceleration ($\sim 10^{9}$~$\mathrm{m/s^2}$), and magnetic field ($\approx 15$~$\mathrm{G}$ in direction of wavevector). Furthermore, comparisons of experimental data with idealized magnetohydrodynamic simulations (which include a physical viscosity model) suggest that both magnetic and viscous stabilization contribute to the observed mode evolution. These data are relevant for benchmarking astrophysical and magneto-inertial-fusion-relevant computations of RTI.\\[4pt] [1] S. C. Hsu et al., Phys.\ Plasmas {\bf 19}, 123514 (2012).\\[0pt] [2] C. S. Adams, A. L. Moser, and S. C. Hsu, submitted (2015); http://arxiv.org/abs/1412.6033.