The need for new, environmentally friendly energy sources accelerates as we enter the next half-century, driven by the likelihood of permanent changes in the availability and price of fossil fuels. Consensus is growing that global warming from increased carbon dioxide in the atmosphere is real and significant. Oil supplies in the near future will not readily meet demand, and natural gas and natural-gas liquids will not easily fill this supply/demand gap. The likely results: increased reliance on coal, and an associated increase in the level of carbon dioxide in the atmosphere. Can we find replacements for carbon-dioxide-emitting energy sources that do not place added burdens on the atmosphere?
Increased reliance on efficiency, nuclear fission, and renewables could fill the energy supply/demand gap during the first half of this century,1 but the limitations on these sources and the widening supply/demand gap especially will be challenging during the last half of the century. If properly funded, a commercial fusion reactor prototype could be available by the 2040s, and fusion power substantially deployed during the second half of this century.
Projections of future world energy demand from the International Panel on Climate Control (IPCC) are based on a range of assumptions used for energy analysis.2 The top curve in Figure 1 is a basic IPCC (business as usual) energy-demand projection through the next century. The next curve shows how much of this energy can come from fossil fuels (assuming the present mix) where atmospheric carbon dioxide is limited to 750 parts per million (ppm). The third curve is the difference between these two curves-the requirement for non-carbon-dioxide-emitting energy sources required to achieve the 750 ppm limitation. This carbon dioxide level is roughly twice the present level.
The bottom curve in Figure 1 shows a fusion scenario with a deployment rate based on experience with other energy technologies.
The advantages of a fusion-based energy system include its relatively unlimited fuel source, with limited hydrocarbon or greenhouse-gas emissions and limited nuclear waste. Fusion is, by its nature, a base-load electrical power source, and is deployable within the existing distribution system-limiting the need for additional power transmission facilities. In addition, electricity derived from fusion can be used to produce hydrogen as a transportation fuel, if we choose to implement the infrastructure to support hydrogen-powered transportation. The challenge over the next several decades will be completion of an economically competitive fusion power plant.
During the past several decades, funding of small to moderate-sized experiments for domestic and foreign fusion programs has helped the development of a full-scale reactor, but larger experiments will be required to complete the development and advance the technology for a commercial power reactor.
The magnetic confinement configuration that is most widely studied and developed, and has achieved the best performance, features a donut-shaped plasma and is called a Tokamak. An electromagnetic field coil configuration provides the magnetic fields to contain a plasma with temperatures in the 100 million degrees centigrade range required for fusion reactivity. Various methods have been developed to heat the plasma, producing temperatures up to 30 times hotter than the center of the sun.
The Tokamak Fusion Test Reactor (TFTR) at the Princeton Plasma Physics Laboratory (PPPL) and the Joint European Torus (JET) at Culham, England, were constructed in the early 1980s with the capability to operate with a deuterium/tritium fuel mixture. In the mid-1990s, both of these devices demonstrated a significant production of fusion power. The TFTR, shown in Figure 2, achieved a temperature of 370 million degrees centigrade. A discharge lasting about one-half second reached a peak fusion power production of 10 MW.3 The JET produced a peak of 16 MW of fusion power with a discharge of about three times that time scale.4 In addition, JET made 4 MW for 5 seconds. The TFTR was decommissioned in 1997 due to U.S. fusion budget constraints, while the JET is continuing to produce excellent experimental results.
The fusion energy produced in an individual magnetic confinement fusion experiment has risen by a factor of more than 1 trillion over the last 30 years (see Figure 3). As a comparison, computer speed has risen by a factor of one hundred thousand during this same time period.
These achievements stimulated the technical and political development of the International Thermonuclear Experimental Reactor (ITER). The ITER is planned as a joint international project among six partners (Japan, the European Union, Russia, South Korea, China and the United States). A decision has been made to site ITER at Cadarache in southern France, with experimental operations scheduled to begin in 2014.
The ITER device, shown in Figure 5, would be the first full-scale fusion reactor, although not a prototype power plant. The experiment will aim to produce up to 500 MW of fusion power for up to 400 seconds, with the reacting fuel almost fully self-heated by the alpha particles produced in the fusion reaction. The experimental program will include test blanket modules to develop and demonstrate heat transfer and tritium breeding. The ITER configuration features superconducting magnets for long-pulse operation. Full remote handling will be required because of activation of the structure.
A demonstration U.S. power plant would enable the commercialization of fusion energy in about 35 years. Early in its lifetime, it would show net electric power production, and ultimately would demonstrate the commercial practicality of fusion power. The basic configuration of the demo could be similar to ITER. However, the physics and technology development preceding the demo would determine its final configuration and facilitate steady state operation, producing about 1 GW of electrical power.
Advances in steady state magnets, tritium breeding blankets, remote maintenance, and fusion materials are required for construction and operation of a demonstration power plant. In addition, these technologies must be integrated with a reacting plasma.
ITER will advance steady state magnets, remote maintenance, and technology systems integration. While some blanket testing will occur during the project, full blanket development will require additional facilities. For example, sufficient irradiation testing volumes and neutron fluence (neutron flux times irradiation time) will be needed for blanket materials development.
The most economical approach to materials testing would be to employ a deuteron accelerator to irradiate a lithium target, producing a high flux of neutrons within a modest testing volume. This test volume would be sufficient for materials tests, but not adequate for blanket systems tests.
For the demo to meet its ambitious goals, additional technology development will be required beyond that of ITER. A much smaller device, the Component Test Facility (CTF), will provide the technology development bridge between ITER and the demo. CTF would use a driven deuterium-tritium plasma with low-fusion output, enabling blanket materials development in a relevant fusion environment on the smallest possible scale. While the fusion power output will be modest, the flux and fluence will be relatively high because of the modest size of the device. This is a faster, much less expensive, less risky approach than testing in a large device that would be limited by tritium consumption, and which would have a very large blanket needing replacement for multiple tests. If necessary, more than one CTF could be constructed.
ITER will have about 20 percent of the power rating required by a commercial power reactor of similar size. In addition, the ITER plasma will not be sustainable for steady state operation. The present research program in the United States is directed toward providing the physics and technology base to bridge the performance gap among ITER, CTF, and the demo, along with broadening the configuration options available for these devices. The United States is carrying out programs to strengthen the physics base. The magnetic confinement approach is being pursued by the General Atomics Corp., Oak Ridge National Laboratory, MIT, PPPL, and a number of other laboratories nationally and worldwide. The U.S. Department of Energy's Office of Science funds fusion research in the United States.
PPPL is constructing and testing potential configuration improvements. One of these, the National Spherical Torus Experiment (NSTX) at Princeton,5 is a compact variant of the well developed Tokamak, with the potential for use as a CTF. The configurational feature that facilitates reduction in size is a plasma envelope patterned after a fat donut with a very small hole in the center. The need for a very small space in the center of the reacting chamber facilitates a compact overall configuration. A second configurational improvement being developed at Princeton is the compact stellarator.6 A comparison between a conventional configuration, a spherical torus, and a stellarator is shown in Figure 4. The stellarator uses three dimensional shaping to facilitate steady state operation.
Fusion energy research has brought a level of development that supports the construction and operation of the ITER. A plan is in place for the remaining development required to construct a demonstration fusion power plant. The plan leads to significant fusion power production after mid-century, filling the widening energy supply/demand gap during this period and for the long term.