Uranium fusion implosion capsules see improved performance over gold

Inertial confinement fusion (ICF) lies at the heart of much of today’s fusion research, relying on the rapid compression of deuterium-tritium fuel mixtures to reach densities high enough to achieve fusion. While they enjoy regular use in such experiments, gold cylinders housing the fuel-filled capsules, called hohlraums, are prone to preheating the nuclear fuel inside, reducing compressional pressure during such experiments. New work at the world’s largest center for ICF research looks to improve on the materials used in ICF experiments.

Dewald et al., from the National Ignition Facility (NIF), demonstrated the first use of bare depleted uranium for hohlraums in experiments of ignition scale capsule implosions. Their results validate the expectations from ICF models.

During indirect-drive ICF, 192 laser beams strike the walls of the hohlraum. Up to 2 megajoules of energy is then converted to X-rays, causing the fuel-filled capsule to implode. When the laser beams strike hohlraums, however, a flux of suprathermal X-rays (from the M-band 4-3 line transitions of gold with X-ray photon energy greater than 2 keV) preheats the fuel, counteracting the compressional force and leading to lower fusion yields.

Implosions using bare depleted uranium hohlraums achieved hot spot X-rays that were triple those of their gold counterparts. The overall fusion neutron yields were double for a 40 percent decrease in M-band X-ray flux, consistent with two-dimensional hydrodynamic simulations. The authors show analytically that the M-band flux reduction is consistent with improved in-flight capsule compression observed via X-ray radiography, and ultimately with the neutron yield increase.

The uranium hohlraums have yielded such a significant performance improvement over gold that the NIF has adopted them in large portion of their current highest neutron yield ICF experiments.

Source: “First demonstration of improved capsule implosions by reducing radiation preheat in uranium vs gold hohlraums,” by E. L. Dewald, R. Tommasini, N. B. Meezan, O. L. Landen, S. Khan, R. Rygg, J. Field, A. S. Moore, D. Sayre, A. J. MacKinnon, L. F. Berzak Hopkins, L. Divol, S. LePape, C. A. Thomas, M. Farrell, A.Nikroo, and O. Hurricane, Physics of Plasmas (2018). The article can be accessed at https://doi.org/10.1063/1.5039385 .