Implosion Physics
The members of the Omega Experiments Group conduct laser–fusion implosion experiments1 in support of the National Inertial Confinement Fusion Program. Thermonuclear fusion is the process by which nuclei of low atomic weight such as hydrogen combine to form nuclei of higher atomic weight such as helium. Two isotopes of hydrogen, deuterium (composed of a hydrogen nucleus that contains one neutron and one proton), and tritium (a hydrogen nucleus containing two neutrons and one proton) provide the most energetically favorable fusion reactants.
LLE studies the direct-drive inertial confinement fusion concept2 on the Omega Laser Facility3 and the National Ignition Facility4, which involves the heating and compression of fusion fuel by the direct illumination of intense laser beams. A small spherical cryogenically cooled target containing a layer of deuterium–tritium ice is irradiated by intense laser irradiation that ablates the outer layer of the target and, due to the rocket effect, implodes and compresses the fuel while heating the gas inside the shell. At stagnation, the gas inside the compressed shell is heated to thermonuclear temperatures and the implosion stops when the pressure in the hot spot becomes sufficiently high. After stagnation, the remaining fuel then expands outward and cools, and the fusion reactions cease. Thermonuclear ignition is defined as the point when the ratio of the fusion energy out and the laser energy in exceeds unity.
The Four Stages of the Target Implosion
Understanding implosion physics depends on advanced diagnostics.
Advanced diagnostics are used for each stage of the implosion (laser irradiation, acceleration, deceleration, and stagnation)
- Improvements in laser beam pointing, target placement, and laser power balance are underway to achieve 1% rms on-target, overlapped intensity balance
- Energy coupling is inferred from UV, x-ray, and nuclear diagnostics
- Multidimensional effects on hot-spot formation and stagnation are quantified using:
- 3-D gated x-ray imaging
- 3-D neutron time-of-flight measurements
- spatially resolved and temporally resolved x-ray continuum measurements
- x-ray spectroscopy of tracer elements
- particle (neutrons, deuterons) imaging
EOS: equation of state
The Concept of Hydrodynamic Scaling: How to Study Ignition-Relevant Implosions on OMEGA
The physics goals and the laser and target requirements are derived from ignition target designs at the MJ NIF energy scale.5,6
Hydrodynamic equivalence combined with ignition theory allows one to compare OMEGA-scale implosions to ignition-scale targets on a symmetric NIF illumination configuration with the same laser beam smoothing as on OMEGA. Hydrodynamically scaled implosions are energetically scalable and have identical implosion velocities, laser intensities, and adiabats. Hydro-equivalent implosions exhibit the same 1-D dynamics and the same hydrodynamic instability growth.
Optimization and Statistical Modeling
Statistical modeling is used to triple the fusion neutron yield in OMEGA implosions.7 Larger targets, thinner ice, and changes to the pulse shapes led to higher yields as predicted by the statistical mapping relations.
A Physics-Based Statistical Mapping Framework
Three-Dimensional Nuclear and X-ray Diagnostics: Detect and Mitigate Mode 1
Nonuniformities in the laser illumination and target can lead to an asymmetric compression of the target, resulting in a poorer implosion. The effects of asymmetric compression are measured with a suite of nuclear and x-ray diagnostics.10 The neutron-averaged hot-spot velocity and apparent ion temperature (Ti) asymmetry are determined from neutron time-of-flight measurements, while the areal density of the compressed fuel surrounding the hot spot is inferred from the scattered neutron energy spectrum. The low-mode perturbations of the hot-spot shape are characterized from x-ray self-emission images along three quasi-orthogonal lines of sight. Implosions with significant mode-1 laser-drive asymmetries show large hot-spot velocities (>100 km/s) in a direction consistent with the hot-spot elongation observed in x-ray images, measured Ti asymmetry, and areal-density asymmetry. Laser-drive corrections have been applied through shifting the initial target location to mitigate the observed asymmetry and to provide better performing implosions.
White arrows indicate projection of hot-spot flow velocity of x-ray detector plane.
SLOS-TRXI: single line-of-sight time-resolved x-ray imager
1 J. Nuckolls et al., Nature 239, 139 (1972).
2 R. S. Craxton et al., Phys. Plasmas 22 110501 (2015).
3 T. R. Boehly et al., Opt. Commun. 133, 495 (1997).
4 G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 43, 2841 (2004).
5 R. Nora et al., Phys. Plasmas 21, 056316 (2014).
6 R. Nora, Ph.D. thesis, University of Rochester, 2015.
7 V. Gopalaswamy et al., Nature 565, 581 (2019).
8 A. Lees, Bull. Am. Phys. Soc. 65, TI01.00005 (2020).