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| Fig. 1: A simplified depiction of the process behind inertial confinement fusion. (Source: Wikimedia Commons) |
Fusion research is often centered on magnetic confinement, a technique in which powerful magnetic fields confine fusion fuel as plasma inside toroid-shaped machines such as the tokamak. [1] However, the second major method used to achieve fusion is inertial confinement fusion (ICF), in which an extraordinary amount of energy is applied to a tiny pellet of deuterium and tritium to implode it to the point of fusion. The state of ICF research is not quite as developed as magnetic confinement fusion, but nevertheless ICF is an alternative that shows some promise.
Energy, typically from lasers, is applied symmetrically to the surface of a fuel pellet containing deuterium and tritium gas surrounded by a thin coating of heavy atoms.
Heat and pressure cause the outer layer to ablate (vaporize) and blast outward.
Reaction forces create inward-traveling shock waves that compress the core of the pellet, causing it to implode.
Under these increased temperatures and pressures the pellet core undergoes fusion, releasing thermonuclear heat. This heat travels outward toward the remaining layers, which undergo fusion in turn to create a condition known as ignition.
All of this takes place within a time frame on the order of nanoseconds. The energy applied by the lasers is necessary to overcome the electrostatic repulsion of the atoms in the deuterium-tritium fuel in order to force their nuclei to fuse. The fusion of the nuclei results in the conversion of mass to energy. [3]
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| Fig. 2: Closeup of a gold hohlraum used for indirect-driven ICF. Courtesy of Lawrence Livermore National Laboratory. |
There are two methods for applying the initial energy to the pellet: the direct-drive approach involves the application of energy directly to the surface of the pellet, usually by lasers, whereas in the indirect-drive approach the pellet is suspended inside a cylindrical container, known as a hohlraum and typically made from gold, which is heated instead. The indirect-drive approach is less energy-efficient because the hohlraum absorbs some of the incident energy and transmits only a fraction of it to the pellet, but on the other hand the hohlraum radiates this energy in a uniform manner about the pellet. The direct-drive approach suffers from asymmetry problems because many lasers must be controlled in concert to deliver energy as uniformly as possible. [3] The initial energy required to achieve fusion is tremendous. Although lasers can provide a great amount of energy, much of it is lost before reaching the pellet core in both direct-drive and indirect-drive methods. This inefficiency must be reduced in order to increase the energy yield of ICF to useful levels. [3]
Moreover, the aforementioned problem of symmetric energy application is important because of a phenomenon known as the Rayleigh-Taylor instability. Nonuniformity in the application of lasers will result in small bumps along the surface of the pellet that quickly balloon outward, rendering fusion impossible. [4] An analogy is squeezing a rubber ball with one's hands; because one's fingers cannot sufficiently cover the entire surface area of the ball, any efforts to compress it will result in the ball pushing out through such gaps. [3] The indirect-drive method attempts to address the Rayleigh-Taylor instability by uniformly heating the hohlraum, but it too is imperfect.
Two American ICF research facilities that are particularly notable are the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory and the Z-Machine at Sandia National Laboratories.
The NIF uses the indirect-drive method by applying nearly 200 laser beams to a hohlraum, which absorbs and emits the energy to the fuel pellet via blackbody radiation. The NIF currently achieves an efficiency of about 15% from lasers to the surface of the pellet (loss occurs because of the hohlraum). [3,5] Recent research at the Facility has included experiments on fast ignition, whereby the compression of the pellet is made to occur before its ignition in two discrete steps, which requires lower densities and temperatures. [6]
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| Fig. 3: Sandia National Laboratories' Z Pulsed Power Facility (Z-Machine) during an electromagnetic pulse. Courtesy of Sandia National Laboratories. |
Sandia's Z-Machine is an impressive machine that produces some of the strongest pulses of electrical currents in the world. ICF research conducted here also makes use of the indirect-drive approach, although electrical pulses rather than lasers are used to heat the hohlraum. Large capacitors are used to achieve pulses with extremely high energies, and this current also creates a strong magnetic field. This system is known as a Z-pinch because current flows in the z-direction (hence the name Z-machine). [7]
Although fusion is often hailed as the up-and-coming breakthrough source that will solve our energy and climate change issues, results are still lacking. An experiment yielding a net energy output (including power consumed by lasers) has yet to be achieved. The immense amount of funding poured into fusion projects and the equally immense amount of time and effort spent by scientists may compel some to purport grandiose statements, but frankly there are significant hurdles to be overcome before fusion becomes commercially viable. New York Times journalist Kenneth Chang said it well: "The running joke is that 'fusion is 30 years in the future - and always will be'." [1] Researchers can do nothing but push forward the state of the art and hope that this statement will not remain true for long.
© Russ Islam. 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] K. Chang, "Machinery of an Energy Dream," New York Times, 17 Mar 14.
[2] S. Pfalzner, An Introduction to Inertial Confinement Fusion (CRC Press, 2006), pp. 13-16.
[3] "Inertial Confinement Fusion: An Introduction," University of Rochester, March 2009.
[4] B. Olson, "Rayleigh-Taylor Instability and Fusion," PH241, Stanford University, Winter 2011.
[5] D. Handoko, "The National Ignition Facility," PH241, Stanford University, Winter 2014.
[6] "NIF Wakes up." Nat. Photonics 3, 177 (2009).
[7] W. W. Gibbs, "Triple-Threat Method Sparks Hope for Fusion," Nature 505, 9 (2014).
