Why Ignition? NIF Experiments and Stockpile Stewardship

Editor’s Note: The National Ignition Facility is one of three National Nuclear Security Administration (NNSA) facilities charged with helping maintain the safety, security, and effectiveness of the nation’s nuclear weapons stockpile. This article, reprinted from the March issue of NNSA’s Stockpile Stewardship Quarterly, describes some of the current NIF inertial confinement fusion (ICF) experiments designed to support the Stockpile Stewardship Program by achieving ignition.

Predicting the performance of an operating nuclear weapon requires understanding both how matter behaves in the high energy density (HED) regime and also the nature of key physical phenomena such as radiation transport, radiation-driven hydrodynamics, and thermonuclear burn.

The Inertial Confinement Fusion Program within the National Nuclear Security Administration uniquely provides experimental access to extreme HED environments through a set of complementary, specialized facilities. The National Ignition Facility at the Lawrence Livermore National Laboratory, the Z Pulse Power Facility at Sandia National Laboratories, and the Omega Laser Facility at the University of Rochester provide data that are informing design options for weapon Life Extension programs, validating models important for weapon design codes, and providing hostile environments for vulnerability and hardness testing.

However, there are limits to our current capabilities. To more completely access HED regimes in the laboratory, HED facilities capable of driving experiments with hundreds of megajoules of energy are required. The demonstration of ignition is the required technological threshold to that future multi-megajoule capability, and it is for this reason, as well as nearer-term benefits, that demonstrating ignition is a major Stockpile Stewardship Program (SSP) goal of the ICF program.

Each of the HED facilities plays a unique role in developing the underpinning science and technology towards enabling the first step to an igniting plasma. The experiments conducted in support of the ICF mission fall into two broad categories: (1) focused experiments that attempt to isolate and test our understanding of a specific code algorithm or material property such as radiative opacity; and (2) integrated experiments of varying complexity that serve several distinct functions. While each facility provides unique capabilities, together they enable the study of physical phenomena using different drivers in regimes of overlap that is essential for building confidence in results obtained in new HED regimes. Below we describe three such efforts that typify how these experiments are used in today›s SSP.

These examples are drawn from the NIF, but comparable experiments are performed on the Z and Omega facilities.

Ignition and Uncertainty Quantification

Ignition research and development involves some of the most complex integrated experiments performed at the HED facilities. In these experiments, solid cryogenic layers of deuterium-tritium (DT) fuel are imploded nearly symmetrically inside millimeter radius capsules to pressures required in an attempt to ignite DT fuel (see Figure 1). These experiments are heavily diagnosed with x-ray and nuclear instruments and are designed to test hypotheses aimed at understanding and improving target performance.

At present, implosions on the NIF are approaching the burning plasma regime in which the alpha particle energy deposited in the fuel is almost equal to the mechanical energy provided by the imploding capsule. This extra heating from the fusion alpha particles results in a yield amplification required for the experiment.

Analysis suggests that shell distortions resulting from drive asymmetries and hydrodynamic instability cause the assembled fuel to disassemble before the fusion self-heating from alpha particles has time to run away. Resolving these issues is the current focus of research. Similar efforts are underway exploring the physics of alternate compression schemes at the Z and Omega facilities.

An exciting emerging application of these ignition experiments is to test and advance our uncertainty quantification (UQ) methodology. A multi-disciplinary team of weapons scientists, ICF scientists, engineers, and computational scientists is developing new tools for application across the stockpile stewardship enterprise, using deep learning methods to combine simulation results and experimental observation. Ignition experiments at NIF are providing a stressing test of these new methods, particularly where little to no data yet exist.

Material Strength of Solids at High Pressure

Understanding the strength of materials at high pressure is important to the SSP. Strength is a quantitative measure of a material’s resistance to deformation and affects compressibility and the material’s evolution under deformation. It is a function of temperature, pressure and strain rate.

Microscopically, strength is the resistance to dislocation generation and transport. Direct observation of dislocations requires transmission electron microscopy (TEM)-level spatial resolution and is not available in current dynamic experiments. Instead, integrated experiments on the NIF assess material strength by measuring its effect in suppressing the growth of hydrodynamic instabilities such as the Rayleigh-Taylor (RT) instability (see "Laser Experiments Shed New Light on Supernova Physics"). Detailed measurements of the seeded instability growth are compared with hydrodynamic simulations to assess the veracity of various strength models.

Two-Shock Campaign

The recent NIF two-shock campaign, so called because it uses a two-shock x-ray pulse to implode a capsule, was designed to test the effectiveness of various implosion phase mix modeling techniques important for the SSP. In this platform, a plastic capsule is filled with tritium gas while a deuterated plastic tracer layer is situated at the inner surface of the capsule (see Figure 2). As the capsule decelerates during stagnation, the tritium makes a "hot spot" and copious TT (tritium-tritium) neutrons are produced.

At the same time, RT instability causes the deuterated capsule material to "mix" into the outer layers of the tritium gas, cooling the mixed layer of gas and reducing the TT neutron yield. The further the capsule material penetrates, the lower the TT yield.

Some of the capsule material that penetrates the gas mixes with the tritium at an atomic level. If this material contains deuterium from the tracer layer, the magnitude of the resulting DT neutron signal diagnoses how much of the tracer material has penetrated the gas and mixed atomically.

The DD neutron signal over and above that consistent with the DT signal is a separate measure how much tracer material penetrated the tritium hot spot but did not mix atomically. Together, these signals provide a tight constraint on models.