Kurt Tomlinson

Measurements of magneto-Rayleigh–Taylor instability growth during the implosion of initially solid metal liners a)

A recent publication [D. B. Sinars et al., Phys. Rev. Lett. 105, 185001 (2010)] describes the first controlled experiments measuring the growth of the magneto-Rayleigh–Taylor instability in fast (∼100 ns) Z-pinch plasmas formed from initially solid aluminum tubes (liners). Sinusoidal perturbations on the surface of these liners with wavelengths of 25–400 μm were used to seed single-mode instabilities. The evolution of the outer liner surface was captured using multiframe 6.151 keV radiography. The initial paper shows that there is good agreement between the data and 2-D radiation magneto-hydrodynamic simulations down to 50 μm wavelengths. This paper extends the previous one by providing more detailed radiography images, detailed target characterization data, a more accurate comparison to analytic models for the amplitude growth, the first data from a beryllium liner, and comparisons between the data and 3D simulations.

Beryllium liner implosion experiments on the Z accelerator in preparation for magnetized liner inertial fusion

Multiple experimental campaigns have been executed to study the implosions of initially solid beryllium (Be) liners (tubes) on the Z pulsed-power accelerator. The implosions were driven by current pulses that rose from 0 to 20 MA in either 100 or 200 ns (200 ns for pulse shaping experiments). These studies were conducted in support of the recently proposed Magnetized Liner Inertial Fusion concept [Slutz et al., Phys. Plasmas 17, 056303 (2010)], as well as for exploring novel equation-of-state measurement techniques. The experiments used thick-walled liners that had an aspect ratio (initial outer radius divided by initial wall thickness) of either 3.2, 4, or 6. From these studies, we present three new primary results. First, we present radiographic images of imploding Be liners, where each liner contained a thin aluminum sleeve for enhancing the contrast and visibility of the liners inner surface in the images. These images allow us to assess the stability of the liners inner surface more accurately and more directly than was previously possible. Second, we present radiographic images taken early in the implosion (prior to any motion of the liners inner surface) of a shockwave propagating radially inward through the liner wall. Radial mass density profiles from these shock compression experiments are contrasted with profiles from experiments where the Z accelerators pulse shaping capabilities were used to achieve shockless (“quasi-isentropic”) liner compression. Third, we present “micro-B” measurements of azimuthal magnetic field penetration into the initially vacuum-filled interior of a shocked liner. Our measurements and simulations reveal that the penetration commences shortly after the shockwave breaks out from the liners inner surface. The field then accelerates this low-density “precursor” plasma to the axis of symmetry.

Modified helix-like instability structure on imploding z-pinch liners that are pre-imposed with a uniform axial magnetic field.

Recent experiments at the Sandia National Laboratories Z Facility have, for the first time, studied the implosion dynamics of magnetized liner inertial fusion (MagLIF) style liners that were pre-imposed with a uniform axial magnetic field. As reported [T. J. Awe et al., Phys. Rev. Lett. 111, 235005 (2013)] when premagnetized with a 7 or 10 T axial field, these liners developed 3D-helix-like hydrodynamic instabilities; such instabilities starkly contrast with the azimuthally correlated magneto-Rayleigh-Taylor (MRT) instabilities that have been consistently observed in many earlier non-premagnetized experiments. The helical structure persisted throughout the implosion, even though the azimuthal drive field greatly exceeded the expected axial field at the liners outer wall for all but the earliest stages of the experiment. Whether this modified instability structure has practical importance for magneto-inertial fusion concepts depends primarily on whether the modified instability structure is more stable th...

Experimental Demonstration of the Stabilizing Effect of Dielectric Coatings on Magnetically Accelerated Imploding Metallic Liners.

Enhanced implosion stability has been experimentally demonstrated for magnetically accelerated liners that are coated with 70  μm of dielectric. The dielectric tamps liner-mass redistribution from electrothermal instabilities and also buffers coupling of the drive magnetic field to the magneto-Rayleigh-Taylor instability. A dielectric-coated and axially premagnetized beryllium liner was radiographed at a convergence ratio [CR=Rin,0/Rin(z,t)] of 20, which is the highest CR ever directly observed for a strengthless magnetically driven liner. The inner-wall radius Rin(z,t) displayed unprecedented uniformity, varying from 95 to 130  μm over the 4.0 mm axial height captured by the radiograph.

Diagnosing magnetized liner inertial fusion experiments on Za)

Magnetized Liner Inertial Fusion experiments performed at Sandias Z facility have demonstrated significant thermonuclear fusion neutron yields (∼1012 DD neutrons) from multi-keV deuterium plasmas inertially confined by slow (∼10 cm/μs), stable, cylindrical implosions. Effective magnetic confinement of charged fusion reactants and products is signaled by high secondary DT neutron yields above 1010. Analysis of extensive power, imaging, and spectroscopic x-ray measurements provides a detailed picture of ∼3 keV temperatures, 0.3 g/cm3 densities, gradients, and mix in the fuel and liner over the 1–2 ns stagnation duration.

Evolution of Gas Cell Targets for Magnetized Liner Inertial Fusion Experiments at the Sandia National Laboratories PECOS Test Facility

Abstract Z-beamlet experiments conducted at the PECOS test facility at Sandia National Laboratories (SNL) investigated the nonlinear processes in laser plasma interaction (or laser-plasma instabilities) that complicate the deposition of laser energy by enhanced absorption, backscatter, filamentation, and beam-spray that can occur in large-scale laser-heated gas cell targets. These targets and experiments were designed to provide better insight into the physics of the laser preheat stage of the Magnetized Liner Inertial Fusion scheme being tested on the SNL Z-machine. The experiments aim to understand the trade-offs between laser spot size, laser pulse shape, laser entrance hole window thickness, and fuel density for laser preheat. Gas cell target design evolution and fabrication adaptations to accommodate the evolving experiment and scientific requirements are described in this paper.

Enhanced Dual Confocal Measurement System

Abstract A measurement instrument utilizing dual, chromatic, confocal, distance sensors has been jointly developed by General Atomics and Sandia National Laboratories (SNL) for thickness and flatness measurement of target components used in dynamic materials properties (DMP) experiments on the SNL Z-Machine (Z). Compared to previous methods used in production of these types of targets, the tool saves time and yields a 4× reduction in thickness uncertainty which is one of the largest sources of error in equation of state measurements critical to supporting the National Nuclear Security Administration Stockpile Stewardship program and computer modeling of high energy density experiments. It has numerous differences from earlier instruments operating on the dual confocal sensor principle to accommodate DMP components including larger lateral travel, longer working distance, ability to measure flatness in addition to thickness, built-in thickness calibration standards for quickly checking calibration before and after each measurement, and streamlined operation. Thickness and flatness of 0.2- to 3.3-mm-thick sections of diamond-machined copper and aluminum can be measured to submicron accuracy. Sections up to 6 mm thick can be measured with as-yet undetermined accuracy. Samples must have one surface which is flat to within 300 µm, lateral dimensions of no more than 50 ×50 mm, and height less than 40 mm.

Development of a Multi-Press Assembly Device for Planar Dynamic Material Property Targets

Abstract A class of dynamic material property (DMP) experiments on the Sandia National Laboratories pulse power Z-Machine requires planar samples to be held in a panel assembly. A custom press device to fabricate the assemblies has the ability to assemble one sample, window, or stack at a time, resulting in a 1-week lead time for a typical three-pocket panel assembly. Fabrication of targets with more than three pockets can take longer. In late 2015, General Atomics conceptualized a new multi-press device to enable several samples, windows, or stacks to be assembled simultaneously, and a prototype was designed, procured, and outfitted in 6 months. Since June 2016, this multi-press design has successfully assembled 60 planar DMP targets. The development considerations for this new device and the resulting benefits for the fabrication of targets are discussed.

Dramatic reduction of Magneto-Rayleigh Taylor instability growth in magnetically driven Z-pinch liners

Summary form only given. In this paper, we will present new Z-pinch liner experiments on Sandias Z facility (20 MA, 100ns current pulse) that demonstrate the integral Magneto-Rayleigh-Taylor (MRT) instability growth can be dramatically limited by controlling the growth of electrothermal instabilities that form early in the electrical current pulse as Joule heating melts and vaporizes the liner surface[1]. In these experiments, we implode Al and Be solid liners to inertial confinement fusion relevant velocities [2] and large convergence ratios and show that thick (~70 μm) dielectric coatings are very effective in controlling the deleterious effects of the electrothermal instability, limiting the seed for subsequent MRT growth, and ultimately lead to a more stable implosion. These experiments extend the previously reported work on the Z facility which also showed dramatic reduction of instability growth in non-imploding solid metallic rods[3].

The effect of surface roughness and structure on subsequent magneto-Rayleigh-Taylor instability growth in beryllium liner implosions on Z

Summary form only given. Sandia is investigating a new magnetized liner inertial fusion (MagLIF) concept that uses cylindrical Be or Al liners to compress magnetized and preheated fusion fuel. As part of this work, we have been studying the growth of instabilities in initially solid liners driven with 20-24 MA, 100-ns current pulses on the Z pulsed power facility. The magneto-Rayleigh-Taylor instability in particular can disrupt the plasma liner during its implosion. A remarkable degree of azimuthal symmetry is observed near stagnation in beryllium liner implosions. This symmetry is captured in 3D calculations only when some azimuthally correlated perturbations are seeded initially. One possibility is that the MRT instability is directly seeded by azimuthally correlated, fine-scale structures on the surface that result from diamond turning the liner on a lathe. A second possibility is that the electro-thermal instability is seeding the MRT instability. Simulations suggest that the level of instability growth seeded by the ET instability is not strongly dependent on the surface roughness. Understanding the surface finish requirements for liner implosions is an important practical question. We will discuss the results of experiments in which the surface of Be liners was altered by polishing the liners along the axial direction after machining. This removes the azimuthally correlated structure, leaving only axially correlated grooves. The latter should noticeably affect the growth of MRT if the surface structure is directly seeding the instability, but may not have any impact if the ET instability is the dominant seed.