Hohlraum.

The most commonly used fuel for inertial fusion is a mixture of deuterium and tritium. Both are isotopes of hydrogen. Deuterium is a stable isotopes with one proton and one neutron in its nucleus, while tritium is radioactive and contains one proton and two neutrons. At room temperature, both are gases.

Capsule Structure

A precisely manufactured capsule is used to contain the fusion fuel and serve as the target for inertial-fusion lasers. Its outer shell, called the ablator, can be made from materials such as hydrocarbon plastic, beryllium or high-density carbon (synthetic diamond). During the laser pulse, the outer surface of the ablator is rapidly removed by ablation, producing an inward reaction force that compresses the remaining shell and fuel.

The ablator surface must be extremely smooth, because small imperfections can seed asymmetries and hydrodynamic instabilities during implosion. Ignition capsules are manufactured with surface irregularities controlled at the nanometre scale. Typical inertial-fusion capsules are only a few millimetres in diameter and contain a very small amount of fuel.

Filling

Hohlraum with a cone for fast ignition.

The capsule is filled with deuterium–tritium gas through a very small fill tube and then cryogenically cooled. A thin, uniform layer of solid D–T fuel forms on the inner surface of the capsule, while the central region contains D–T vapour. This cryogenic fuel layer must have an exceptionally uniform thickness and smooth inner surface to achieve a symmetric implosion. In D–T targets, the small amount of heat produced by tritium beta decay helps redistribute the fuel and smooth the layer, while precise thermal control is used to maintain the required geometry.

The capsule and frozen fuel layer are inspected using high-resolution imaging techniques, including X-ray radiography, to verify their shape and uniformity. Before the shot, the cryogenic target is maintained at about 18–20 K so that the solid D–T layer remains stable.

Hohlraum

Cryogenic target.

In indirect drive, the fuel capsule is placed inside a cylindrical cavity called a hohlraum, typically made from a high-Z material such as gold or uranium. Laser beams enter through openings at both ends, known as laser entrance holes, and strike the inner walls. The heated walls reradiate much of the absorbed laser energy as X-rays that irradiate the capsule.

The X-rays fill the hohlraum and create a relatively uniform radiation field around the capsule, driving its implosion from all sides. The capsule must be held very close to the centre of the hohlraum, while the geometry and surface quality of both the hohlraum and capsule are manufactured to extremely tight tolerances to preserve implosion symmetry.

D–T fuel capsule.

Saturn target developed at the Laboratory for Laser Energetics for polar-direct-drive experiments on OMEGA, with the capsule suspended by fine spider-silk fibres.

Precise target positioning at the centre of the chamber is essential. At NIF, the cryogenic target positioner maintains the target location to within about 2 micrometres while keeping it at approximately 18—20 K with extremely high temperature stability.

The fuel capsule and, in indirect-drive experiments, the hohlraum are single-use components and are destroyed during each shot. Present research targets are highly specialised and expensive to manufacture because of their extreme precision and complex assembly. A future inertial-fusion power plant would therefore require automated mass production of targets at a tiny fraction of today’s research-target cost.