Session QI2: Rosenbluth Award, ICF Instabilities and Astrophysics

Spinning Unmagnetized Plasma for Laboratory Studies of Astrophysical Accretion Disks {\&} Dynamos

A technique for creating a large, fast-flowing, unmagnetized plasma has been demonstrated experimentally. This marks an important first step towards laboratory studies of phenomenon such as magnetic field generation through self-excited dynamos, or the magnetorotational instability (MRI), the mechanism of interest for its role in the efficient outward transport of angular momentum in accretion disks. In the Plasma Couette Experiment (PCX), a sufficiently hot, steady-state plasma is confined in a cylindrical, axisymmetric multicusp magnetic field, with Te\textless 10 eV, Ti\textless 1 eV, and n\textless 10$^{11}$ cm$^{-3}$. Azimuthal flows are driven by JxB torque using toroidally localized, biased hot cathodes in the magnetized edge region. Measurements show that momentum couples viscously from the magnetized edge to the unmagnetized core, and the core rotates when collisional ion viscosity overcomes the drag due to ion-neutral collisions. Torque can be applied at the inner or outer boundaries, resulting in controlled, differential rotation. Maximum speeds are observed (He $\sim $ 12 km/s, Ne $\sim $ 4 km/s, Ar $\sim $ 3.2 km/s, Xe $\sim $ 1.4 km/s), consistent with a critical ionization velocity limit reported to occur in partially ionized plasmas. PCX has achieved magnetic Reynolds numbers of Rm $\sim $ 65 and magnetic Prandtl numbers of Pm $\sim $ 0.2-10, which are approaching regimes shown to excite the MRI in a global Hall-MHD stability analysis. Ion-neutral collisions effectively add a body force that undesirably changes the flow profile shape. Recent upgrades have increased the ionization fraction with an additional 6 kW of microwave heating power and stronger magnets that reduce loss area and increase plasma volume by 150{\%}. In addition, an alternative scheme using volume-applied JxB force will maintain the shear profile and destabilize the MRI at more easily achievable plasma parameters.   

Experimental Observation of Nonlinear Mode Coupling In the Ablative Rayleigh-Taylor Instability on the NIF

We investigate on the National Ignition Facility (NIF) the ablative Rayleigh-Taylor (RT) instability in the transition from linear to highly nonlinear regimes. This work is part of the Discovery Science Program on NIF and of particular importance to indirect-drive inertial confinement fusion (ICF) where careful attention to the form of the rise to final peak drive is calculated to prevent the RT instability from shredding the ablator in-flight and leading to ablator mixing into the cold fuel [1,2]. The growth of the ablative RT instability was investigated using a planar plastic foil with pre-imposed two-dimensional broadband modulations and diagnosed using x-ray radiography [3]. The foil was accelerated for 12ns by the x-ray drive created in a gas-filled Au radiation cavity with a radiative temperature plateau at 175 eV [4]. The dependence on initial conditions was investigated by systematically changing the modulation amplitude [5], ablator material and the modulation pattern. For each of these cases bubble mergers were observed and the nonlinear evolution of the RT instability showed insensitivity to the initial conditions. This experiment provides critical data needed to validate current theories on the ablative RT instability for indirect drive that relies on the ablative stabilization of short-scale modulations for ICF ignition. This paper will compare the experimental data to the current nonlinear theories. \\[4pt] This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC. \\[4pt] [1] J.D. Lindl \textit{et al}., Phys Rev. Lett 1975 \\[0pt] [2] T. Ma \textit{et al.,} Phys. Rev. Lett. 2015\\[0pt] [3] A. Casner et al., Physics of Plasmas 22, 056302 (2015).\\[0pt] [4] A. Casner \textit{et al.,} HEDP 2014; \\[0pt] [5] D. A. Martinez \textit{et al.,} Phys. Rev. Lett. 2015.   

Modeling, measuring, and mitigating instability growth in liner implosions on Z

Electro-thermal instabilities result from non-uniform heating due to temperature dependence in the conductivity of a material. In this talk, we will discuss the role of electro-thermal instabilities [1] on the dynamics of magnetically accelerated implosion systems. We present simulations that show electro-thermal instabilities form immediately after the surface material of a conductor melts and can act as a significant seed to subsequent magneto-Rayleigh-Taylor (MRT) instability growth. We discuss measurement results from experiments performed on Sandia National Laboratories Z accelerator to investigate signatures of electro-thermal instability growth on well-characterized initially solid aluminum or beryllium rods driven with a 20 MA, 100 ns risetime current pulse. These measurements show good agreement with electro-thermal instability simulations and exhibit larger instability growth than can be explained by MRT theory alone. Recent experiments have confirmed simulation predictions of dramatically reduced instability growth in solid metallic rods when thick dielectric coatings are used to mitigate density perturbations arising from the electro-thermal instability [2]. These results provide further evidence that the inherent surface roughness of the target is not the dominant seed for the MRT instability, in contrast with most inertial confinement fusion approaches. These results suggest a new technique for substantially reducing the integral MRT growth in magnetically driven implosions. Indeed, recent results on the Z facility with 100 km/s Al and Be liner implosions show substantially reduced growth. These new results include axially magnetized, CH-coated beryllium liner radiographs in which the inner liner surface is observed to be remarkably straight and uniform at a radius of about 120 microns (convergence ratio $\sim$20).\\[4pt] [1] K.J. Peterson, D. B. Sinars, et al., Phys. Plasmas 20, 056305 (2012)\\[0pt] [2] K.J. Peterson, T. J. Awe, et al., PRL 112, 135002 (2014)   

The generation of Biermann battery fields in laser-plasma interactions and the interplay with the Weibel instability

Recent experiments with intense lasers are probing the dynamics of self-generated large scale magnetic fields with unprecedented detail. In these scenarios the Biermann battery effect is critical to understand the field dynamics but a multi-dimensional detailed study of this mechanism was not present yet in the literature. Moreover, the interplay between the Biermann battery effect and plasma micro instabilities and the evolution of plasma turbulence is still unknown. In this work, particle-in-cell simulations are used to investigate the formation of magnetic fields, $B$, in plasmas with perpendicular electron density and temperature gradients. For system sizes, $L$, comparable to the ion skin depth, $d_i$, it is shown that $\beta\sim d_i/L$, consistent with the Biermann battery effect. However, for large $L/d_i$ , it is found that the Weibel instability (due to electron temperature anisotropy) supersedes the Biermann battery as the main producer of $B$. The Weibel-produced fields saturate at a finite amplitude (plasma $\beta \sim 1$), independent of L. The magnetic energy spectra below the electron Larmor radius scale are well fitted by power law with slope $-16/3$, as predicted in Schekochihin et al., Astrophys. J. Suppl. Ser. 182, 310 (2009).The relevance of these results for several ongoing experiments is also discussed.