What is fusion energy?
Fusion is a process by which two light nuclei join together (fuse) to form a heavier nucleus, and in doing so release considerable energy. Achieving this requires high temperatures such as those that drive the fusion processes which power the sun and stars.
The aim of fusion research and development is to create conditions on earth which are sufficient to generate many fusion reactions which may be harnessed to produce large amounts of thermal and/or electrical power. As there are no long-lived hazardous by-products and a plentiful supply of almost universally accessible fuel, fusion power has the potential to produce virtually limitless amounts of power in an environmentally friendly and economically viable way.
The Sun: A 5-billion year old fusion power plant.
Fusion of deuterium (2H) and tritium (3H).
A particularly favourable fusion reaction for use on earth is that involving deuterium (2H or D) and tritium (3H or T). Deuterium is an isotope of hydrogen, so-called ‘heavy hydrogen’, and is found in seawater (about 33 grams per tonne). Tritium, another isotope of hydrogen, may be produced by neutron reactions with lithium, which is widely available, both from ore from the earth (e.g. Western Australia) and also from seawater. In each D-T fusion reaction 17.6 million electron volts (MeV) of stored energy is released, along with a helium nucleus (He) and a neutron (n). This is about one million times the amount of energy released from a chemical reaction, such as the burning of fossil fuels. This is how so little fuel can produce so much energy when fusion is employed.
Experiments on present day experimental machines have already exceeded the temperatures required in a reactor, and produced short pulses of significant fusion power – up to 16 megawatts (MW) peak. The physics understanding of a toroidally confined plasma is not yet complete and substantial efforts are being made to do this because it will possibly lead to ways of improving the magnetic confinement scheme and making it a practical reactor (easy to operate, maintainable and cheaper). Plasma confinement studies are carried out in Australia on the National Fusion Plasma Research Facility H-1 (a type of magnetic configuration known as a heliac) at the Australian National University (ANU).
A simple toroidal magnetic field for plasma confinement.
Plasma and magnetic confinement
The positively charged D and T nuclei repel each other strongly, so in order for them to fuse as a result of their thermal motion, the temperature of the D-T mixture must be high. The temperature of the solar core is about 15 million degrees C, but to release net energy on earth from D-T fusion reactions, the temperature of the D-T fuel must be higher, typically 100 million degrees C. At these temperatures matter exists in what is known as the plasma state, in which atoms are fully ionised into the electrically-charged electrons and ions. In this ionized plasma state magnetic fields may be used to confine the motion of the nuclei (magnetic confinement).
Confining such a plasma is not trivial, at present the optimal method employs strong magnetic fields in a torus (doughnut) shape.
H-1 Heliac. The copper coloured circular conductor creates twist in the confining field lines. Only 18 of the 36 toroidal field coils are shown (grey).
Prospects for a stellarator fusion reactor
One may ask, given the high level of international interest in the ITER tokamak, and the potential for Australian involvement, “why is the H-1 Facility a stellarator?” The H-1NF configuration was chosen with an eye to the future: the step beyond ITER. Present performance parameters of tokamaks significantly exceed those achieved so far in stellarators, so ITER will be a tokamak. The international stellarator community formally endorsed this plan at a meeting of the Stellarator Executive Committee under the International Energy Agency (IEA) Implementing Agreement in the National Museum of Australia in 2002. However the fundamental advantage of the stellarator configuration – that it does not require a large current flowing in the plasma – combined with highly encouraging recent results has led some to propose that the subsequent device, the ‘DEMO’ reactor, be an advanced stellarator.
The superconducting stellarator ‘LHD’ in Japan has already achieved plasma durations of more than 30 minutes. In the last decade, significant breakthroughs were achieved in the computer optimisation of magnetic field coils to achieve ‘quasi-symmetry’, one of several symmetries in magnetic coordinate space, even though in real space, the shape is far from symmetric. Recent stellarator reactor designs are competitive in size and performance with advanced and spherical tokamaks. The next generation of superconducting and highly optimised stellarators will hopefully confirm, on a larger scale, the freedom from current induced plasma instabilities and the promising results on confinement and stability recently observed in Germany and Japan; the high density high confinement mode (HDH) exceeded operational limits of tokamaks both in rotational transform and plasma density. If so, we may well see a DEMO reactor in the form of a highly optimised stellarator.