ScienceWise - Jan/Feb 2007

How to Avoid Making Waves

Article Illustration
Article Illustration
A view along the magnetic confinement coils

The fusion of deuterium and tritium, two isotopes of hydrogen, to create helium in large-scale fusion reactors offers the promise of vast quantities of electricity with almost no greenhouse gas emissions. The most practical way to achieve this is to magnetically confine a ring or loop of plasma within a reactor and heat it to tens of millions of degrees.

In order to make the process as efficient and elegant as possible, one would like the fusion reactions themselves to heat the plasma rather than having to use an external heating method such as high power microwaves. Theoretically, this is possible since one of the fusion products is helium in the form of highly energetic alpha particles. These alpha particles could, in principle, transfer their energy to the remaining deuterium and tritium fuel. There is however a significant obstacle to implementing this idea in practice.

Electrically conductive fluids, such as plasmas, have special properties resulting from the electrical and magnetic forces generated by their many moving charges. One such property is the presence of so called Alfvén waves – a travelling oscillation of ions along magnetic field lines. As coincidence would have it, the speed of the alpha particles produced by fusion is close to that of the Alfvén wave modes.

Confined superheated plasma is a turbulent, unstable and delicate thing and the presence of any large amplitude Alfvén waves could easily disrupt the flow to the point where the plasma dissipates and the reaction ceases. Because of this, an understanding of the physics of Alfvén waves in confined plasmas is of critical importance to the international efforts to develop viable fusion power. Prototype fusion power reactors are multi billion dollar undertakings designed for efficiency and robustness, which generally makes them far from ideal to conduct experiments on the impact of different coil configurations on Alfvén waves. However, this is where Australia is able to make an important contribution to the international fusion effort.

The ANU hosts the only large-scale stellarator plasma confinement facility in the southern hemisphere, the H-1NF. Although the plasma contained within H-1NF does not undergo fusion reactions, the confinement system does have a highly flexible design coupled with sophisticated and innovative diagnostic systems. This allows different coil configurations and containment parameters to be tried with comparative ease. This flexibility coupled with highly advanced diagnostics allows more accurate measurements to be made on parameters such as plasma rotation and density profiles than any other plasma device in the world. And it is precisely these factors that are so critical to the physics of Alfvén waves.

But generating the data is only half the battle. Moving superheated plasma is such a complex system that the signals produced by the multitude of sensors are extremely hard to interpret. A substantial part of the research effort is the development of advanced data mining algorithms to filter key features from the mass of individual measurements. Recent results are beginning to clearly show characteristic Alfvén wave signatures in the data, which is an exciting beginning. Director of the H-1NF, Dr Boyd Blackwell sees such strong collaboration between mathematical scientists, theorists and experimentalists as the key to developing an improved understanding of the physics of plasma confinement.

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