Measuring laser illumination asymmetry for OMEGA symmetric direct-drive implosions
Dhrumir Patel, PhD Qualifying Exam, Advised by Riccardo Betti
Thursday, January 28, 2021
Cryogenic direct-drive inertial confinement fusion experiments on Omega laser facility use a 60 beam 30kJ laser to implode hollow spherical shells of 1mm diameter with outer layer of plastic and an inner layer of solid density deuterium-tritium fuel. The primary objective of these experiments is to assess possibility of achieving a threshold condition known as “Ignition” which is when self-sustaining fusion reactions generate large amounts of fusion energy. The promise of inertial confinement fusion lies in the fact that the current understanding of 1-dimensional physics suggests that a perfectly symmetric spherical implosions (known as 1D implosions) would achieve ignition. Although, either because of inaccurate 1D physics or 3-dimensional effects (or both), achieving ignition has proven elusive.
However, high performance cryogenic campaign on Omega  has improved implosion performance by using a predictive statistical model to design implosions, with the highest performing implosions promising break-even – defined as – when scaled hydrodynamically to laser energies available at National Ignition facility. A salient feature of this model is that it allows inferring possible mechanisms degrading the performance relative to a 1D implosion. A major degradation mechanism suggested by this model is the laser drive asymmetry induced by 60 beam port geometry.
At present laser drive asymmetry has not been measured experimentally, instead 3D radiation hydrodynamic simulations are used to infer its effect on implosion performance. These simulations show negligible effect of laser drive asymmetry from port geometry on nominal implosions.
In this PhD proposal, we discuss analysis of observations from past experiments which exposes shortcoming of current physical models used in 3D hydrodynamic simulation. We also present design of experiments which image x-ray coronal self-emission of imploding multi-layer plastic shells differentially doped with Germanium, to infer laser drive asymmetry. Additionally, resonant absorption will be discussed as possible physical mechanism responsible for enhancing laser drive asymmetry.
1V. Gopalaswamy et. al , Nature565, 581–586 (2019)