Performance metrics for inertial confinement fusion implosions: Aspects of the technical framework for measuring progress in the National Ignition Campaign

Abstract

The National Ignition Campaign (NIC) uses non-igniting "THD" capsules to study and optimize the hydrodynamic assembly of the fuel without burn. These capsules are designed to simultaneously reduce DT neutron yield and to maintain hydrodynamic similarity with the DT ignition capsule.We will discuss nominal THD performance and the associated experimental observables. We will show the results of large ensembles of numerical simulations of THD and DT implosions and their simulated diagnostic outputs. These simulations cover a broad range of both nominal and offnominal implosions. We will focus on the development of an experimental implosion performance metric called the experimental ignition threshold factor (ITFX). We will discuss the relationship between ITFX and other integrated performance metrics, including the ignition threshold factor (ITF), the generalized Lawson criterion (GLC), and the hot spot pressure (HSP). We will then consider the experimental results of the recent NIC THD campaign. We will show that we can observe the key quantities for producing a measured ITFX and for inferring the other performance metrics. We will discuss trends in the experimental data, improvement in ITFX, and briefly the upcoming tuning campaign aimed at taking the next steps in performance improvement on the path to ignition on NIF. * Electronic address: spears9@llnl.gov

Core conditions for alpha heating attained in direct-drive inertial confinement fusion

“…The pressures are identical, whereas hot-spot radius, burn widths, and ρR scale by ∼ 4×. The hot-spot temperature is ∼30% higher for the larger implosion and the compression neutron yield increased by 425× to 2.25×10 16 , similar to the analytic prediction. The DEC2D run was then repeated with the alpha-particle energy deposition turned on.…”

“…Applying the scaling in Eq. (2) from 26.2 kJ to 1.9 MJ of laser energy, we estimate that OMEGA implosion 77068 extrapolated to 1.9 MJ would produce 2.4×10 16 neutrons from compression alone. To estimate the enhancement caused by alpha heating, we calculate the Lawson parameter for shot 77068.…”

“…The yield enhancement caused by alpha heating from Eq. (5) leads to an amplification of 2.1× and to a total neutron yield including alpha heating of about 5×10 16 or 140 kJ of fusion energy. Note that the accurate power indices and parameters in the scaling laws are critically important for large extrapolations in energy.…”

“…Here we use the expression of Refs. [8,14,15] that is also related to the Livermore experimental ignition threshold factor (ITFx) [7,16,17]: where ρR is in g/cm 2 , the no-α neutron yield is in 10 16 , and the stagnating DT mass at bang time is in milligrams. The latter is often approximated with 1/2 of the unablated DT mass [8,15] …”

“…In the absence of alpha heating, the fusion yield from hydrodynamic compression alone would have been ∼ 4-5×10 15 . By extrapolating recent OMEGA results in size to match the 1.9 MJ of incident NIF laser energy, we find that the best-performing direct-drive OMEGA implosion to date would lead to a neutron yield of 4.5×10 16 neutrons and a level of alpha heating corresponding to about a 2× yield enhancement, similar to the indirect-drive HF targets at the same laser energy. The larger fusion yield in direct drive, results from the larger size and fuel mass of the 1.9 MJ direct-drive targets.…”