Structure

Interior of the NIF target chamber.

At the National Ignition Facility, inertial-confinement fusion experiments take place at the centre of a spherical target chamber about 10 metres in diameter. The chamber is assembled from 10-centimetre-thick aluminium panels and covered with approximately 0.3 metres of boron-containing concrete, which helps absorb neutrons and provides biological shielding. Numerous openings allow the laser beams to enter the chamber and provide access for diagnostics and target-positioning systems. Final optics and protective debris shields must be kept extremely clean to preserve beam quality and protect sensitive optical components. Diagnostic instruments view the target through dedicated ports or can be positioned close to the chamber centre to measure the implosion and fusion reaction. The chamber is maintained under vacuum during experiments. After each shot, residual gases are removed by the vacuum system, while target debris and shrapnel are contained by protective surfaces and debris shields and must be managed during subsequent operation and maintenance.

Size

Exterior of the NIF target chamber.

The large size of an inertial-fusion target chamber, compared with the millimetre-sized fuel capsule, has several reasons. A very small amount of D–T fuel can release a substantial amount of energy: complete fusion of 1 mg of D–T would produce about 340 MJ, equivalent to roughly 80 kg of TNT. Modern experiments already achieve fusion yields of several megajoules, although only a fraction of the fuel is burned. The chamber must therefore be large and robust enough to withstand intense bursts of X-rays, neutrons and target debris while also providing sufficient space for laser-beam access, diagnostics and shielding. Its size is also determined by the large number of laser beamlines, diagnostic systems and target-handling components that require dedicated access ports around the chamber.

Inertial Fusion Power Plant

The Shiva laser target chamber during maintenance.

In an inertial-fusion power plant, the reaction chamber would need systems to absorb the energy released by fusion, protect the chamber structure and breed tritium for the fuel cycle. Lithium-containing blankets or flowing liquid-wall concepts have been proposed for this purpose. Such materials could absorb neutron energy, transfer heat to a power-conversion system and breed tritium through neutron interactions with lithium. A flowing liquid layer could also help protect the chamber wall from repeated fusion pulses and remove some target debris.