Fusion

Fusion is the process that powers the sun. If we can harness fusion power on Earth, it would provide effectively limitless, carbon-free, safe energy. There are two approaches. In inertial confinement fusion (ICF), high power lasers (or other ‘drivers’) compress a pellet of frozen deuterium-tritium fuel to very high density and temperature, confined for short times associated with the fuel’s inertia (nanoseconds). The other approach, presently the more advanced, is magnetic confinement fusion (MCF). Here the hot, low density fuel is held in a toroidal chamber using magnetic fields for confinement times of seconds.

When the fuel is heated to fusion temperatures (100,000,000K), the electrons are stripped from the nuclei, creating an ionised gas called a plasma. Plasmas are susceptible to a range of waves and instabilities that drive turbulence and degrade confinement. In MCF this determines the device size. For example, the 16Bn Euro ITER facility is large enough to give the required confinement despite the turbulence, providing a fusion yield of 10 times the applied heating power. Scheduled for completion in the 2020s, ITER will provide the first plasma with heating dominated by the energetic alpha particles produced by the fusion reactions, allowing the final physics questions to be answered to build a demonstration power plant, DEMO. For example, how do the alpha particles affect the plasma stability and turbulence, and how do we exhaust them from the plasma once they have cooled to avoid dilution extinguishing the fusion burn? The other fusion product is a 14MeV neutron to be captured in a blanket to extract its energy and react it with lithium to produce tritium. Understanding how materials behave under this energetic neutron irradiation, combined with exposure to hot plasma, is something we still know little about because ITER will be the first device to create these conditions. ITER will also address a range of fusion technologies, such as heating systems, tritium breeding blankets and exhaust handling: issues that integrate plasmas with materials.

The flagship ICF facility is NIF in the US. It tried to achieve fusion conditions during 2012, but did not succeed. The reasons require more research, but again plasma instabilities are a likely cause. Once the issues at NIF are resolved the priorities for future laser-based systems (e.g. HiPER) can be defined on the route to inertial fusion energy. Then the materials issues discussed above for MCF apply to ICF also. ICF creates extreme states of matter with high energy density that are of fundamental physics interest as well as having important applications beyond energy.

Fusion research interfaces with several fields. There are synergies with the nuclear industries where the next generation fission reactors will have high energy neutrons and so share some materials issues with fusion. Space plasmas share phenomena also found in MCF plasmas while energetic astrophysical phenomena can be simulated in the lab using high power lasers. In industry, low temperature plasmas with similar characteristics to those at the edge of a MCF plasma have applications in manufacturing, from advanced coating technologies to computer chips.