Mitigation of Rayleigh-Taylor instability in high-energy-density plasmas

The Rayleigh-Taylor (RT) instability and related hydrodynamic mechanisms of interfacial mixing are ubiquitous in nature, occurring on a large variety of scales, from atmospheric and oceanic to supernovae. They are of particular importance for many applications that involve high energy density plasmas, such as Z-pinch plasma sources of x-ray and neutron radiation or inertial confinement fusion (ICF). Most of these applications rely upon a dense plasma that is accelerated by the pressure of a lighter fluid, such as magnetic field in imploded Z-pinch or Magnetized Liner Inertial Fusion (MagLIF), a low-density ablated plasma imploding a laser fusion capsule, or decelerated by the pressure of hot-spot ICF plasma at stagnation. To improve performance of such systems and achieve the practical goals that range from increasing x-ray and neutron yields to the most ambitious goal of all, the ICF ignition, it is critically important to develop effective methods of mitigating the RT instability, making the loads and targets more resistant to the distortion. I will review the approaches to this problem that have been advanced over the last two decades. Most of them employ elaborate structuring of the density and/or material profiles in the loads and targets, which is achieved by use of nested wire arrays, multi-shell nozzle design for gas-puff Z-pinch injection, laser capsule manufacturing to control the interaction of laser targets with laser and x-ray radiation pulses. Application of an external axial magnetic field has been demonstrated to be an effective mechanism for mitigating the RT instability of electromagnetically driven implosions of cylindrical loads, from gas-puff Z pinches to MagLIF liners. We discuss the physics behind the most effective RT mitigation mechanisms, the status and prospects of the most promising methods of mitigation, and the unresolved physics issues, including the new opportunities that deserve to be studied.