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What Is Magnetic Fusion?
While it has long been known that it is nuclear fusion that makes the Sun shine, the first concerted attempts to harness fusion power on Earth began in the 1950s, first in secret but collaboratively among many nations by 1958. These first efforts, and the fusion research described in this interim report, employed strong magnetic fields to confine the hot gases that produce fusion power. By the 1960s, the invention of the laser led to a different approach in which lasers quickly heat a tiny quantity of fuel that explodes as it burns.1 This report deals only with magnetic fusion, which has had the best performance to date, leading to governmental discussions in the 1990s on how to advance magnetic fusion energy research as a world-wide endeavor—what is now the International Thermonuclear Experimental Reactor (ITER) project. Several books describe this history.2,3,4
A magnetic fusion reactor can be thought of as a miniature Sun confined inside a vessel that can be highly evacuated. A strong magnetic field confines the high-pressure plasma and limits contact between the surrounding vessel and the high-temperature plasma undergoing fusion reactions. The first fuel will probably be a mixture of deuterium (D), a form of hydrogen in all water, and another form of hydrogen called tritium (T) that would be manufactured inside the reactor. The energy potential in the tiny amount of deuterium in a gallon of water is equivalent to 300 gallons of gasoline. It is this abundance of fusion fuel, together with environmental advantages, that has inspired governments to support fusion research over many decades. D-T fuel produces harmless helium, together with neutrons that can make the reactor vessel radioactive, but with much less chance for danger to the public than fission reactors, according to studies reported in Chapter 15 of The Fusion Quest. This interim report discusses only fusion using D-T fuel, which is the focus of the world-wide research program. Magnetic fusion energy using either deuterium (D-D) or deuterium and helium-3 (D-3He) fuel is more challenging due to requirements for higher temperature, reduced impurity concentration, and improved confinement.5
The challenge has been that producing fusion on Earth requires temperatures even hotter than stars and in the range between 100 and 200 million degrees. Gases this hot become ionized, consisting of a “plasma” of free ions and electrons, like the gases conducting electricity in a neon sign but requiring 10,000 times higher temperature. Magnetic fields are needed to confine hot, high-pressure plasmas by way of electric currents inside the ionized plasma. Many arrangements of magnets to confine a hot plasma have been tested. The most successful, and the one employed in ITER, is the tokamak configuration, originally developed in Russia and further explored and improved upon by research in the United States and elsewhere.
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1 For more information on inertial fusion energy, see National Research Council, An Assessment of the Prospects for Inertial Fusion Energy, The National Academies Press, Washington, D.C., 2013.
2 J.L. Bromberg, Fusion, MIT Press, Cambridge, Mass., 1982.
3 T.A. Heppenheimer, The Man-Made Sun, Little-Brown, Boston, Mass., 1984.
4 T.K. Fowler, The Fusion Quest, Johns Hopkins Press, Baltimore, Md., 1997.
5 P.E. Stott, The feasibility of using D–3He and D–D fusion fuels, Plasma Phys Contr F 47:1305, 2005.
The tokamak is a descendant of the linear “pinch” known since the 1930s. In a pinch, a current flowing through a plasma column confines itself by its own magnetic fields produced by the current. Bending the current column into a circle prevents leakage out the ends, and doing this inside a “toroidal” or doughnut-shaped vessel keeps the air out. Coils above and below the toroidal vessel provide the magnetic force that bends the column into a circle. Other “toroidal field coils” wound on the vessel itself add a stabilizing twist to magnetic field lines inside the column. Thus, the tokamak has three sources of magnetic field: the pinch current that mainly confines the pressure; the “poloidal” coils that bend plasma current into a circle; and the strong “toroidal field coils” that twist the current into a highly stable confined plasma.
Fusion performance is measured by the pressure of the plasma, P, and the timescale for plasma energy escape, τE. The fusion power density produced from the fusion of deuterium and tritium (D-T) is equal to 0.08 P2 megawatts per cubic meter (MW m-3) when P is expressed in atmospheres. The record volume-averaged plasma pressure for magnetic fusion is 2.0 atmospheres and was set in October 2016 in the Alcator C-Mod device at the Massachusetts Institute of Technology. The plasma pressure expected in ITER is 2.6 atmospheres resulting in a peak fusion power density exceeding 0.5 MW m-3. Commercial fusion energy systems would need to have plasma pressures between 3 and 8 atmospheres. The energy escape time, τE, determines whether or not the plasma is self-sustaining or whether external power must be injected to keep the plasma hot and at high pressure. ITER is designed to produce τE ~ 3.7 seconds, and the product of the average pressure and τE is PτE ~ 10 atm·sec. If the electron and ion pressures of the plasma are equal, the plasma becomes a “burning plasma” when the product PτE is greater than about 8 atm·sec. This is when the energetic alpha particles generated from fusion reactions in the plasma are able to balance the energy escaping from the plasma. The highest previous levels of plasma confinement product were achieved in tokamak experiments conducted in the 1990s: The Tokamak Fusion Test Reactor6 achieved 0.3 atm·sec, the Joint European Tours7 reached 0.7 atm·sec, JT-608 reached 0.65, and DIII-D reached a confinement parameter of PτE ~ 0.5 atm·sec.9
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6 Hawryluk et al., Results from deuterium-tritium tokamak confinement experiments, Rev. Mod. Phys. 70:537, 1998.
Practical problems include how to get heat out of this circular device, how to prevent neutron damage to the magnet coils, and how to respond if, despite all, the strong current ring tries to short-circuit to the wall (called a “disruption”). What makes it worth dealing with these difficult issues is the remarkable fact that plasmas inside tokamaks can adjust themselves to reduce leakage of heat across the magnetic field. This “H-mode,” or “high-confinement mode,” of tokamak operation, was discovered experimentally in the 1980s10,11 and has been widely reproduced even as essential aspects of it remain enigmatic. ITER’s baseline operating scenario is an H-mode plasma. Critical research efforts in the U.S. and abroad are focused both on ensuring that the ITER plasmas will attain and maintain H-mode performance and on developing alternative operating scenarios for ITER, which do not rely upon uncertain H-mode physics to attain the energy confinement that is required to create a burning plasma.
Whether a tokamak in the image of ITER will be the best path to a commercial reactor is much less certain, hence the need for continuing innovation to explore other paths. One such path is the stellerator being pursued in Germany and Japan. The stellarator is also a toroidal magnetic system but one not requiring the pinch current—nor the associated cost of maintaining it—as in tokamak reactors.
In addition to toroidal magnet configurations, a number of linear magnet configurations have been studied, all of which employ external power to create a closed magnetic field configuration of the plasma inside the linear magnets. Additionally, the tandem mirror configuration uses neutral beam injection and electron cyclotron resonance heating to modify electrostatic potentials and reduce plasma leakage out the ends. The only linear device large enough to compete with tokamak performance was the superconducting Mirror Fusion Test Facility that completed construction at the Lawrence Livermore National Laboratory in February 1986, only to be shut down before operating because of declining magnetic fusion budgets.
Whatever the final magnet shape, the fact that magnets might confine a plasma producing fusion energy on Earth completes a long journey, beginning with Michael Faraday’s invention of the magnetic dynamo in 1831 and ending with Einstein’s discovery that mass becomes energy, very soon leading to speculations about nuclear fusion long before fission was discovered. It was Faraday’s discovery that prompted Maxwell to create the theory of light that eventually posed the puzzle that led to Einstein’s E = mc2.
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7 Keilhacker et al., High fusion performance from deuterium-tritium plasmas in JET, Nuc Fusion 39:209, 1999.
8 H. Kishimoto et al., Advanced tokamak research on JT-60, Nuc Fusion 45:986, 2005.
9 Lazarus et al., Higher fusion power gain with profile control in DIII-D tokamak plasmas, Nuc Fusion 37:7-12, 1997.
10 Wagner et al., Regime of improved confinement and high beta in neutral-beam-heated divertor discharges of the ASDEX Tokamak, Phys Rev Lett 49:1408, 1982.
11 Wagner et al., Development of an edge transport barrier at the H-mode transition of ASDEX, Phys Rev Lett 53:1453, 1984.
