Backlighting fusion

Anne L. Fischer, Contributing Editor, anne.fischer@photonics.com

Harnessing fusion energy requires achieving the milestone of fusion ignition in the laboratory. At Lawrence Livermore National Laboratory’s National Ignition Facility in Livermore, Calif., researchers are attempting indirect drive inertial fusion, involving a tiny capsule of heavy hydrogen fuel centered inside a hohlraum cavity.

This schematic shows how imaging an implosion was done in test. To the left are 15 laser beams that entered each end of the hohlraum. The proton backlighter was driven by 30 laser beams. As the backlighting protons passed through the laser-driven hohlraum, the plasma conditions and capsule implosions were sampled at various times.

Beams from 192 lasers with nearly 2 million joules of ultraviolet energy in a billionth-of-a-second pulse are directed to the inside walls of the hohlraum, which is heated, generating x-rays that implode the capsule to ignition conditions. Fusing atoms at the center of the capsule cause surrounding atoms to fuse, leading to ignition, a nuclear energy release greater than the laser energy required for the implosion. To achieve ignition, diagnostic tools are needed to see what happens inside the imploding capsule.

The capsule, containing deuterium-tritium, must be imploded with nearly perfectly spherical symmetry. Theoretical designs constrain the level of symmetry that must be achieved, and researchers are developing a variety of techniques to measure it. A challenge in achieving ignition is that the reactions take place inside a fuel capsule with an initial 2-mm diameter that has been compressed to such a degree that its temperature and pressure become much greater than those at the center of the sun.

A team led by Richard Petrasso, a scientist at MIT’s Plasma Science and Fusion Center, developed a fusion backlighting method using charged particles generated in a secondary implosion to probe the hohlraum and capsule dynamics. This technique was tested on the Omega Laser System at the Laboratory for Laser Energetics at the University of Rochester in New York. A capsule filled with deuterium and helium-3 was imploded, producing large numbers of energetic protons with energies of 14.7 and 3.0 MeV.

Collaborators from Lawrence Livermore National Laboratory, the Plasma Science and Fusion Center at MIT, the Laboratory for Laser Energetics and General Atomics in San Diego observed an asterisklike pattern in the electric fields within the hohlraum. In the experiment, 30 laser beams with a wavelength of 0.351 μm produced the radiation field in the hohlraum, while another 30 imploded the deuterium and helium gas-filled capsule used for the radiograph. A nuclear track detector recorded the proton images, determining the spatial distribution of the number of protons and their energy. Distribution is affected by the electric and magnetic fields within the capsule, while energy is determined by the amount of material they pass through. By taking an image and changing the timing of the proton sampling (the time when the backlighting protons start to pass through the target), the time evolution of the hohlraum conditions are probed (making a movie). The pattern recorded in the image results from the positioning of the incoming laser beams, but it will require further analysis to fully understand the implications regarding fusion.