Proposed Laser-Based HED physics experiments for Stockpile Stewardship

Abstract

An analysis of the scientific areas in High Energy Density (HED) physics that underpin the enduring LANL mission in Stockpile Stewardship (SS) has identified important research needs that are not being met. That analysis has included the work done as part of defining the mission need for the High Intensity Laser Laboratory (HILL) LANL proposal to NNSA, LDRD DR proposal evaluations, and consideration of the Predictive Capability Framework and LANL NNSA milestones. From that evaluation, we have identified several specific and scientifically-exciting experimental concepts to address those needs. These experiments are particularly responsive to physics issues in Campaigns 1 and 10. These experiments are best done initially at the LANL Trident facility, often relying on the unique capabilities available there, although there are typically meritorious extensions envisioned at future facilities such as HILL, or the NIF once the ARC short-pulse laser is available at sufficient laser intensity. As the focus of the LANL HEDP effort broadens from ICF ignition of the point design at the conclusion of the National Ignition Campaign, into a more SS-centric effort, it is useful to consider these experiments, which address well-defined issues, with specific scientific hypothesis to test or models to validate or disprove, via unit-physics experiments. These experiments are in turn representative of a possible broad experimental portfolio to elucidate the physics of interest to these campaigns. These experiments, described below, include: (1) First direct measurement of the evolution of particulates in isochorically heated dense plasma; (2) Temperature relaxation measurements in a strongly-coupled plasma; (3) Viscosity measurements in a dense plasma; and (4) Ionic structure factors in a dense plasma. All these experiments address scientific topics of importance to our sponsors, involve excellent science at the boundaries of traditional fields, utilize unique capabilities at LANL, and contribute to the Campaign milestone in 2018. Given their interdisciplinary nature, it is not surprising that these research needs are not being addressed by the other excellent high-energy density physics (HEDP) facilities coming on line, facilities aimed squarely at more established fields and missions. Although energy rich, these facilities deliver radiation (e.g., particle beams for isochoric heating) over a timescale that is too slow in these unit physics experiments to eliminate hydrodynamic evolution of the target plasma during the time it is being created. A theme shared by all of these experiments is the need to quickly create a quasi-homogeneous 'initial state' whose properties and evolution we wish to study. Otherwise, we cannot create unit experiments to isolate the physics of interest and validate the models in our codes, something that cannot be done with the integrated experiments often done in HED. Moreover, these experiments in some cases involve combinations of solid and plasmas, or matter in the warm-dense matter state, where neither the theoretical approximations of solid state or of fully-ionized weakly-coupled plasmas can be used. In all cases, the capability of 'isochoric heating' ('flash' heating at constant density) is important. In some cases, the ability to selectively heat to different degrees different species within a target, whether mixed or adjacent to each other, is critical for the experiment. This capability requires the delivery of very high power densities, which require the conversion of the laser into very short and intense pulses of secondary radiation (electrons, ions, neutrons, x-rays). Otherwise, there is no possibility of a clean experiment to constrain the models, in the cases there are any, or inform the creation of one. Another typical requirement of these experiments is the ability to probe these exotic extreme conditions of matter with flexible and diverse sources of secondary radiation. Without a high-intensity high-power laser with some unique attributes available on Trident today (e.g., ultra-high laser-pulse contrast), there is no way to deliver such diverse probes, with the required flux and/or brilliance, exquisitely synchronized to the plasma formation source.