The Rayleigh-Taylor instability (RTI) is one of the primary hydrodynamic instabilities that acts as a disputer to achieving high yield inertial confinement fusion (ICF). The potential for RTI to grow on the interior surface of ICF capsules, caused by deceleration during the implosion, further emphasises the need to better understand the seed mechanisms for RTI and possible mitigation methods for damping the instability growth. Reducing the growth of RTI during deceleration could preserve the spherical symmetry of ICF implosions and reduce the amount of mix between the solid capsule liner and fuel hot-spot. Additionally, it has been shown that magnetic fields do damp RTI growth, and the presence of a magnetic field lowers the threshold for achieving fusion and increases the yield.
Understanding the seed mechanisms of the RTI, especially on the interior surface of ICF capsules, further allows for better understanding of the morphology of the RTI growth dur- ing deceleration. Classically RTI has been studied using single or multi-mode sinusoidal perturbations, which result in bubble and spike morphology. However in addition to si- nusoidal perturbations, single-feature perturbation, such as voids or divots, can seed RTI.
This form of RTI is considered the thin-layer RTI, where the perturbation's wavelength is longer than the dense layer's thickness. This specific RTI evolution results in a morphology consisting of a single central spike and arms that extend horizontally away from the spike and eventually fall back towards the interface. Thin-layer RTI is important to explore dur- ing deceleration due to the presence of the fill-tubes in ICF capsules causing holes in the shell.
Creating experimental platforms for current laser configurations on Omega and the Na- tional Ignition Facility (NIF) is necessary to study deceleration-stage RTI experimentally and validate computational modeling. A comprehensive exploration of potential experimen- tal designs on Omega, Omega-EP, and NIF are explored to identify a platform with which deceleration-stage RTI can be studied with and without the presence of an externally applied magnetic field. Additionally, the design of a novel experimental platform for Omega-EP to study thin-layer RTI during deceleration with and without an externally applied magnetic field is presented, along with data collected during the first experiments performed utilizing the platform. Lastly, a first of it's kind RTI platform for NIF is fielded and the results are presented, including an exploration of the possible impacts high-intensity-laser generated hot-electrons can have on experimental targets. The results of these experimental platforms are used to benchmark computational models, and demonstrate the potential for magnetized RTI to be studied comprehensively in future experiments. / Doctor of Philosophy / The potential of controlled sustained nuclear fusions as a viable energy source has rapidly become a reality in recent years. Monumental progress has been made in the pursuit of con- trolled fusion, including the repeated achievement of fusion ignition at the National Ignition Facility (NIF), meaning there was successful production of more energy from the fusion reac- tion than laser energy used to trigger the reaction. However, in order for fusion to become a truly viable energy source improvements in capsule design and the mitigation of disruptions, like hydrodynamic instabilities, must be explored to produce higher energy yields.
The Rayleigh-Taylor instability (RTI) is one of the most detrimental hydrodynamic insta- bilities in inertial confinement fusion (ICF). RTI occurs when a lighter fluid, like the fuel used in fusion reactions, supports a heavier fluid, the ICF capsule itself, under the influence of gravity. An ICF capsule is imploded, induced by the driving mechanism, such as a laser, but once the driver stops the capsule will begin to decelerate. During this deceleration stage, the interior surface of the ICF capsule in susceptible to RTI growth causing the cold capsule material to mix with the hot fusion fuel. This mixing reduces the fuel's ability to reach the necessary temperatures and densities need to achieve ignition and produce high energy yields. As a result, it is crucial to better understand the defects that cause RTI to grow and explore methods that could damp the RTI growth and preserve the integrity of the implosion and fusion fuel.
The work presented here focuses on exploring both the seed mechanisms for RTI and miti- gation strategies. Specifically, using an externally applied magnetic field has been shown to damp RTI growth and in know to lower the threshold of the conditions needed to achieve ignition. A study of possible experimental setups at both the Omega laser and NIF is ex- plored in order to identify a design with which the damping effects of an externally applied magnetic field on deceleration-stage RTI can be studied experimentally. From this design study platforms for the Omega-EP and NIF were conceptualized and ultimately fielded.
The results from these novel experiments are presented, along with an exploration of pos- sible effects on RTI unexpected preheating of the experimental targets. Additionally, an exploration of the seed mechanisms of RTI is presented with a look at the classic sinusoidal perturbation as well as using a divot to seeded thin-layer RTI, which evolves with a spike and arm morphology rather than the classical bubble and spike. The experimental results from Omega-EP using a divot as the perturbation are presented. Novel results of varying RTI platforms and their potential for further development provide crucial insight into the possible presence of deceleration-stage RTI in ICF capsules and can be iterated on in the future to further explore RTI evolution and damping methods.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/120577 |
| Date | 01 July 2024 |
| Creators | Samulski, Camille Clement |
| Contributors | Aerospace and Ocean Engineering, Srinivasan, Bhuvana, Adams, Colin, Scales, Wayne A., Brizzolara, Stefano |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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