Scientists successfully demonstrate a new method to make nuclear fusion pellets

The conventional approach

To create fuel for fusion reactors, isotopes of hydrogen, namely deuterium, and tritium, are frozen into a solid spherical shell. Since the isotopes are gaseous in their native state, scientists use extremely low temperatures to bring them into a solid state where they can be layered.

The shell is then bombarded with lasers to subject it to extremely high temperatures and pressures, following which it collapses and then ignites to undergo fusion.

While this approach can release enormous amounts of energy, a fusion-based power plant would require millions of such shells every day to supply power reliably. The frozen shell approach is too expensive and not economically feasible.

Dynamic Shell Approach

Researchers Igor Igumenshchev and Valeri Goncharov at the Laboratory for Laser Energetics (LLE) at the University of Rochester have demonstrated a radically different approach that could allow fusion energy plants to make their fuel pellets.

Called dynamic shell formation, it uses liquid droplets to create targets for lasers to target and bring about ignition. The concept was first described by Goncharov, a distinguished scientist and assistant professor at Rochester's Department of Mechanical Engineering, in 2020. However, it had not been demonstrated even in the lab.

In the dynamic shell formation approach, a liquid droplet of deuterium and tritium is injected into a foam capsule. Lasers are then trained on the tablet, which develops into a spherical shell. The shell then implodes and collapses, resulting in ignition.

Since dynamic shell formation does not need cryogenic layering of hydrogen isotopes, it is relatively cheaper to execute and can also be scaled.

The researchers used the OMEGA laser at LLE to demonstrate that this could be achieved experimentally. However, they used a sphere of plastic foam with the same density as a liquid deuterium-tritium fuel instead of hydrogen isotopes for their demonstration.

The lasers required would need longer and higher energy pulses to ignite the dynamic shell into a fusion reaction. Nevertheless, the experiments demonstrate that active shell formation is practically feasible.

“This experiment has demonstrated the feasibility of an innovative target concept suitable for affordable, mass production for inertial fusion energy,” said Igumenshchev, a senior scientist at LLE, in the press release.

The research findings were published in the journal Physical Review Letters.

Abstract

In the dynamic-shell (DS) concept [V. N. Goncharov et al., Novel Hot-Spot Ignition Designs for Inertial Confinement Fusion with Liquid-Deuterium-Tritium Spheres, Phys. Rev. Lett. 125, 065001 (2020).] for laser-driven inertial confinement fusion the deuterium-tritium fuel is initially in the form of a homogeneous liquid inside a wetted-foam spherical shell. This fuel is ignited using a conventional implosion, which is preceded by a initial compression of the fuel followed by its expansion and dynamic formation of a high-density fuel shell with a low-density interior. This Letter reports on a scaled-down, proof-of-principle experiment on the OMEGA laser demonstrating, for the first time, the feasibility of DS formation. A shell is formed by convergent shocks launched by laser pulses at the edge of a plasma sphere, with the plasma itself formed as a result of laser-driven compression and relaxation of a surrogate plastic-foam ball target. Three x-ray diagnostics, namely, 1D spatially resolved self-emission streaked imaging, 2D self-emission framed imaging, and backlighting radiography, have shown good agreement with the predicted evolution of the DS and its stability to low Legendre mode perturbations introduced by laser irradiation and target asymmetries.