For centuries, it had been a mystery to man how the sun creates its energy. One of the first proposals was that the sun burns its fuel through ordinary chemical combustion just like wood burns on earth. It was soon concluded though that the sun would need an inconceivable amount of fuel in order to burn long enough for the geological structures to form that geologists had analyzed on earth. It was Einstein's theory of Relativity and his famous equation E=mc2 that represent the vast scientific advance that was required for full understanding of the processes producing the sun's energy. In 1939, Hans Bethe quantitatively described the reactions happening in the center of the sun in his paper "Energy Production in Stars". This paper won him the Nobel Prize for Physics in 1968. From this time on, especially since the early 1950's, it has become a goal for experimentalists and researchers to recreate the stellar fusion process here on earth. Over the course of the following decades, many fusion research devices were built all around the world, all of which were designed to investigate the science of terrestrial fusion. Most of these devices were magnetic confinement fusion (MCF) devices which use magnetic fields to confine the hot fusion plasma for the reaction. This technology stands next to inertial confinement fusion (ICF) which operates on a completely different principle. Here, the fusion fuel is compressed and heated in a very rapid and violent matter, resulting in a small supercritical explosion. In MCF, the reaction is carried out much closer to steady state.
In order to assess the performance of devices for fusion research, the so called fusion triple product has empirically been proven to be a useful figure of merit for near steady state fusion reactions. The triple product is the product of the electron density ne, the plasma temperature T and the energy confinement time τE. The energy confinement time is a measure of the energy loss of the system to the environment. It has been shown that for a self sustained burning fusion reaction, the triple product must exceed a value of at least 3 x 1021 m-3 keV s-1. [1] Although no reactor has yet achieved this, the fusion triple product has increased by a factor of about 10.000 since the inception of fusion research. [2]
The essential challenge of commercializing fusion power is achieving the conditions necessary for fusion to occur in a controlled setting, while maintaining proper confinement. The densities and pressures are limited by the strength of the confinement. The confinement is limited by the strength of the materials involved and by the strength of the magnetic fields. These limitations are given. This means that the greatest lever for successful reactions is high temperature. For terrestrial Deuterium-Tritium fusion, the ideal temperature lies around 100 million Kelvin. No material container could remain solid at these temperatures. This is why an immaterial container such as the magnetic field in MCF (or the inward coasting particle inertia in ICF) is necessary. The most successful magnetic fusion experiments have been conducted with toroidal confinement fields. Of the two main toroidal technologies, the Stellarator and the Tokamak, the Tokamak has empirically been proven to be the most promising design. Currently, the largest toroidal fusion research device is the Joint European Torus (JET) in Great Britain with a plasma volume of 90 m3 and a toroidal magnetic field strength of up to 4 Tesla. [3]
In order for a fusion power plant to be viable, it would have to produce more energy than is needed to keep the plasma heated. The ratio of output power to input heating power is labeled Q. In 1997, JET achieved a world record value of 0.7 for Q. [3] This figure is somewhat limited for a reactor of a given size due to the high heat loss rate. Since surface area (and therefore heat loss) of reactors scales like the square of the linear dimension but volume (and therefore the total contained energy) scales like the cube of the linear dimension, larger reactors will have a lower heat loss rate. For this reason a reactor larger in size than JET is required in order to achieve a Q of 1 or greater. The condition of achieving a Q of unity or greater is labeled breakeven.
Currently under construction in Southern France is the so-called "ITER" project. The ITER reactor will be a thermonuclear reactor twice as large as JET in linear dimension. It will have a plasma volume of 840 m3 and a magnetic field strength of 13 Tesla. The goal of the ITER project is to construct a Tokamak that for the first time will achieve a Q of greater than 10 and for a relatively long period of time of about 400 seconds and achieve a Q of greater than 5 in steady state operation. [4] This corresponds to a net gain in energy through the fusion reaction -- the essential requirement for commercial fusion energy.
Research at ITER will help verify several important technological aspects of fusion power. The most important issues are materials choices, tritium breeding concepts and superconducting magnets: The high heat and neutron flux coming from the plasma and the high stresses generated by the electromagnetic interaction between the plasma and the confinement magnets pose difficult challenges for the materials used in construction of the Tokamak device. Operation of ITER will show whether the current materials of choice can withstand steady state operation in a fusion tokamak. Furthermore, the superconducting magnets for the reactor are situated at temperatures of near absolute zero in close proximity to the 130 Million K plasma. It is an obvious challenge to keep the superconducting magnets at low enough stable operating temperatures considering this enormous heat gradient. Additionally, although superconducting magnets are a confirmed and readily implemented technology, they have never been used to create a field as large in strength and volume as the one to be deployed in ITER. Lastly, a commercial fusion power plant will have to produce its own tritium for the D-T reaction, since Tritium does not occur naturally. For this purpose, research will be conducted at ITER concerning Tritium breeding concepts where the neutron flux coming from the plasma will be used to transmute Lithium into Helium and Tritium. This Tritium will be collected and injected into the reactor, providing a reliable fuel source.
In addition to these serious technological challenges, there are also some more fundamental issues related to the physics of plasma confinement that ITER will have to deal with. A hot plasma is an inherently unstable entity. It is therefore extremely difficult to achieve a stable magnetic confinement and isolation of the plasma. There are countless instabilities that have been discovered experimentally that cause the plasma to cool down and lose energy. ITER will serve as an experimental institution for studying these plasma confinement issues and confirming the feasibility of long-term stable fusion plasma confinement.
The ITER project represents the transition between fusion research and commercial nuclear fusion energy production. The experiments conducted at ITER will verify the technology, science and engineering know-how required for a successful commercial power plant. Currently in the planning phase is the DEMO project -- the first demonstrational power producing commercial nuclear fusion power plant.
© Julian Kates-Harbeck. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] G. McCracken and P. Stott, Fusion - The Energy of the Universe (Elsevier, 2005).
[2] I. N. Golovin and B. B. Kadomtsev, "Status and Prospects for Controlled Thermonuclear Fusion," Atomic Energy 81, 793 (1996).
[3] J. Mlynár, "Focus on JET," European Centre of Fusion Research, EFD-R(07)91, March 2007.
[4] "Summary of the ITER Final Design Report," Intl. Atomic Energy Agency, IAEA/ITER EDA/DS/22, July 2001.