Fusion

There are two approaches to the commercial realisation of fusion energy. 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 deuterium-tritium fuel mix 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 late 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. These questions include, how do the alpha particles affect the plasma stability and turbulence; how do we exhaust the heat and particles from the plasma, and how do we efficiently drive current? Other fusion programmes, such as STEP at UKAEA and the Tokamak Energy private fusion approach, seek an accelerated pathway to magnetic confinement fusion.

The flagship ICF facility is NIF in the US, driving fusion power production with a set of high power lasers. Research aims to build further on its recent successes towards a commercially viable power plant, with plasma instabilities again a key issue. Experiments at NIF are key for addressing the priorities for future laser-based fusion systems (e.g. HiPER). Meanwhile, private fusion companies, such as First Light Fusion, are exploring new, innovative approaches to inertial fusion with accelerated timelines.

The deuterium-tritium fusion reaction produces a 14MeV neutron. Understanding how materials behave under this energetic neutron irradiation, combined with exposure to hot plasma, is something we still know little about. It is important because materials’ properties are altered by irradiation, and the amount of waste produced by a power plant will be closely linked to the materials that are used. In a fusion power plant, it will be important to capture as many neutrons as possible in a blanket designed to extract their energy and react them with lithium to produce tritium. This is a key technology that ITER will test, along with a range of others that integrate materials and plasma science, such as heating systems and exhaust handling. Many of the materials issues being addressed for MCF apply to ICF also, so there are multiple synergies here (and some differences).

The success of JET, producing a world record fusion energy of 59MJ in 2021, gives great optimism for the success of ITER and the prospects for commercialising magnetic confinement fusion.
NIF recently produced the world’s first burning plasma in an inertial fusion experiment, with 1.3MJ of fusion energy created from an injected 1.9MJ of laser 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.