"Burning plasmas" are very hot fully ionized gases whose temperature is maintained dominantly by self-heating from fusion reactions within the plasma. The governments of China, Europe, India, Japan, Russia, South Korea and the United States (representing more than half the world’s population) have assessed that we are scientifically and technologically ready to explore the “burning plasma” state [see the report Burning Plasma, Bringing a Star to Earth, National Academies Press, Washington DC, 2004], and have embarked on an international partnership to construct the ITER Project, near Aix-en-Provence, France. The mission of ITER (“the Way” in Latin) is to “demonstrate the scientific and technological feasibility of fusion energy for peaceful purposes.”
The basic nuclear reactions that are contemplated for use in ITER, and in future fusion power plants, are: D + T → α + n + 17.6 MeV n + 6Li → T + α + 4.8 MeV
ITER will enable research on key scientific questions that bear on the feasibility of fusion power as well as intellectually fascinating questions such as the dynamics of a complex self-organized system, non-linear interactions between supra-thermal charged alpha particles and thermal plasma, and evolution of the interacting physical phenomena of turbulent transport and global stability at the physical scale of a fusion power plant.
Fusion is attractive as an energy source because the basic raw materials, deuterium and lithium-6, are abundant, and because it is physically impossible for a fusion power plant to explode like Chernobyl or melt down like Three Mile Island. The radioactive waste from fusion will not require geological storage like Yucca Mountain. No large energy storage, long-distance energy transmission nor large land use would be required. The catch, however, is that we are only part way to developing fusion. Extensive worldwide development of experimental and theoretical understanding of fusion plasmas has provided the physics basis for ITER construction. Research now going on in the domestic fusion programs of each of the ITER parties is key both to assuring the success of ITER operations and to moving beyond ITER to demonstrate that fusion is economically practical.
To put ITER in context, it is valuable to compare fusion power production, energy gain, and pulse duration between 1975 and today, ITER’s goals, and the goals of a practical power plant. (The figures above are for simultaneous accomplishments, and are rounded.)
As can be seen, ITER is a very dramatic step beyond present capabilities, and today’s results represent an equally dramatic step beyond the first results with strongly heated plasmas in 1975. Extensive experimental scaling and large-scale computer models point to success for ITER and possibilities for further improvements in both stability and confinement, opening the way to options to improve the operations of ITER and thus to go further towards a power plant, based on research in ITER. Fusion power levels depend quadratically on the plasma pressure. β, the ratio of the particle pressure niTi + neTe to the magnetic field pressure (B2/2µo), is a key parameter determining the global stability limits of plasmas, which determine the power producing capabilities. ITER can explore β-limits and opportunities for improving pressure limits. Fusion gain (fusion power production divided by the plasma heating power supplied externally) depends on the parameter niTiτE, where 1/τE is the decay rate of the energy stored in the hot plasma when all heating is turned off. Again in this area ITER is well positioned to explore opportunities for improvements in understanding and performance.
ITER is also well positioned to explore opportunities for very long-duration plasma pulses, which are desirable from the power-plant perspective. The pulse duration for ITER shown in the table above is defined by the ability to sustain the current in the ITER plasma through induction. A solenoid generates changing magnetic flux to drive an electric field around the plasma torus. This process is well understood, and the projected pulse length of ITER is all but guaranteed. The 300–500 second pulse will be long enough that all of the plasma-facing components will come into thermal steady state, so key steady-state physics and engineering issues can be explored. ITER is also designed to study non-inductive means to drive current efficiently and so can advance this science towards fusion power plants. ITER will utilize non-inductive techniques explored in the superconducting devices now on line in China and South Korea, and in construction in India and Japan.
Achieving the goals for a fusion power plant will require advances beyond plasma physics, and will require joint physics and engineering innovation. The very high heat and particle fluxes that will emerge from fusion plasmas will need to be handled in a way that allows the plasma-facing components to operate for two to five years without replacement. The high-energy neutron flux that will emerge from fusion plasma needs to be captured in a lithium bearing “blanket” to produce the needed tritium, as well as high quality heat. This blanket also needs to maintain its required properties for two to five years without replacement. ITER will make dramatic advances in both of these areas. Furthermore, Europe and Japan are working on the engineering design and engineering validation of a beam-driven high-energy neutron source to test the materials being developed for fusion.
We certainly do not claim that fusion energy research is easy, nor that economically competitive energy production is assured. The international fusion research community has made tremendous strides, but the distance yet to cover is considerable. The world will continue to be in need of safe new energy sources as our population grows, as the standard of living hopefully rises everywhere, and as limits must be respected for emission of CO2 from energy systems worldwide. Thus, the international collaboration to develop fusion as a new energy source, with ITER its centerpiece, is not only an exciting scientific and technological challenge, but also a key part of the nation’s and the world’s long-term approach to energy.
Robert J. Goldston was Director of the DOE Princeton Plasma Physics Laboratory from 1997 until recently. Ned R. Sauthoff is Head of the U.S. ITER Project Office at the Oak Ridge National Laboratory.