Nuclear fusion is perhaps one of the most sought after sources of energy, but it also remains one of the most technically challenging and elusive. Mechanical Engineering Professor Riccardo Betti is one of the world\'s leading experts on nuclear fusion and a frequent advisor to the federal government on the future of fusion. We asked him about the tremendous advances in the field to date and the hurdles ahead.
Betti is the director of the Fusion Science Center for Extreme States of Matter at the University of Rochester\'s Laboratory for Laser Energetics. He is the vice chair of the U.S. Department of Energy\'s Fusion Energy Science Advisory Committee, chair of the DOE\'s High Energy-Density Laboratory Plasmas panel, and a member of the Board of Physics and Astronomy of the National Academies of Science. He recently won the Edward Teller Medal for his research into laser-driven fusion.
Why is nuclear fusion such a potentially valuable source of energy?
Fusion is kind of the holy grail of energy for the future: It is clean, and there is an unlim¬ited supply of fuel.
The process burns hydrogen isotopes, called deuterium and tritium. Deuterium is of virtually unlimited availability because you can extract it from seawater. There is enough deuterium in one kilometer cubed of seawater to create an amount of energy equivalent to the amount of energy of the entire world\'s oil supply. There is enough energy in deute¬rium to power the planet forever.
And it is clean because unlike fission, which is the breaking of atoms in a normal nuclear reactor, it does not make actinides—very dangerous nuclei that are produced in the fission process and emit a lot of dangerous radiation. This kind of waste is not produced in fusion reactions. Instead, they produce helium, which is a harmless noble gas.
Could you explain, briefly, the process of inertial confinement fusion?
There\'s a [plastic] shell a couple millimeters in diameter filled with hydrogen isotopes, deuterium and tritium. You need to freeze it to a low temperature so that the deuterium and tritium are solid. Then you shine the laser on that pellet. The laser energy is absorbed by the surface of the pellet. All of this energy it had in the beginning is turned into pressure—about 500 gigabars or 500 billion atmospheres of pressure. The pressure that you need to achieve in these implosions is twice the pressure in the center of the sun.
Just like in a diesel engine, you have to compress the fuel enough for it to self-ignite. The center of this implosion is very hot. Ignition will start there, then it will propagate to the nearby nuclei until the burn has spread throughout the whole capsule. When the whole thing is burning you get a lot of energy out.
What is the state of research into inertial confinement fusion?
The ignition process has been demonstrated in weapons, but not in the laboratory yet. This is the task of the new laser at [the National Ignition Facility or NIF] at Lawrence Livermore National Laboratory in Livermore, Calif. The Omega laser at LLE doesn\'t have enough power and energy to trigger ignition, but a lot of aspects of the science can be studied on a laser the size of Omega, and a lot of prepara¬tion work can be done on the Omega laser. [Omega and the National Ignition Facility] work together in a program called the National Ignition Campaign. Omega is the main contributor outside Livermore to the National Ignition Campaign.
Rochester is doing three things. One is helping the National Ignition Campaign to achieve ignition. That is, I would say, job number one.
Second, if you want to make electricity, you need to demonstrate that you can get 100 times as much energy out as you put in. There are lots of inefficiencies in the process. Fusion creates heat, and only about one-third of that can be transformed into electricity. In addition, lasers are very inefficient in converting electricity into light. So in order to have an economically viable power plant, the energy gain from each imploding target has to be at least 100.
The Livermore laser can only get gains up to 10 or 20 because they use a system called indirect drive, where they don\'t shine the laser on the target itself; they shine the laser on a heavy metal shell, and that transforms the la¬ser light into X-rays. That process is inefficient, so they lose two-thirds of the laser energy. Then they use the X-rays to drive the implosion. And the reason they do that is because that\'s the way weapons work, and that\'s what they are interested in. However, if you want to make energy, you don\'t want to go that route. You would be better off shining the laser directly on the target: that is called direct drive. So Rochester studies direct drive, which is the most efficient inertial fusion process to make energy. We can study that effectively because that\'s what our laser is designed for.
Rochester can also study a different way of igniting a capsule that can make the gain even higher. It\'s called fast ignition. You compress the fuel with one laser and then ignite it with another laser. That gives you more control and higher gains, but you need a very fast, high-power laser. We just built a new high-power laser called Omega EP that has a power of 1 PetaWatt. It can send a short pulse to the compressed target and ignite it separately from the compression.
Alternative energy as a whole is a hot topic today. Most readers will have heard of other alternative energy sources such as solar, wind, and hydroelec¬tric. How does fusion compare to those as far as upsides, downsides, challenges, and timeline for implementation?
There are some aspects of fusion that are very attractive and others that are not. One of the very attractive aspects is what is called power density—that means that you can make a lot of power in a small volume. Utility industries love this because they like to build a small building and make all of the energy they need there. They don\'t like low power density systems like solar and wind because they take a lot of land to make energy.
Fusion is a renewable energy because its fuel availability is basically unlimited, just like solar and wind. The real problem is the technology.
The energy of fusion comes from the neu¬trons produced in the fusion reaction. Every fusion reaction produces very energetic neutrons. You need to stop these neutrons and collect their energy. To do this, you need to build a system that, when bombarded with neutrons, survives for a long time, and they haven\'t found the type of material that could function in a fusion power plant yet.
Also, interfacing a high technology device like a laser with the harsh environment of a reac¬tor chamber is very difficult to do.
Another technical difficulty is that you need to inject this pellet of solid deuterium and tritium into the target chamber at a very high speed and shoot it with a laser with high ac¬curacy. You can accurately shoot a pellet with a laser, but you have to do that 10 times a second in a fusion reactor.
So there are a lot of high technology and ma¬terials science issues that go along with making a system that makes electricity reliably and competitively with other energy sources.
In your opinion, is it only a matter of time before controlled nuclear fusion is a source of energy, or are there still questions as to whether it will ever be harnessed for that purpose?
I don\'t have much doubt that eventually fusion will become a major energy source. It would be very hard at this point, though, to give an accurate time scale for this. A running joke in the field is that every time someone asks how long it will take for fusion to be a reality they are told that it is 30 years away—for 60 years, it has been 30 years away—so I refuse to give any time scales.
About the University of Rochester
The University of Rochester (www.rochester.edu) is one of the nation's leading private universities. Located in Rochester, N.Y., the University gives students exceptional opportunities for interdisciplinary study and close collaboration with faculty through its unique cluster-based curriculum. Its College of Arts, Sciences, and Engineering is complemented by the Eastman School of Music, Simon School of Business, Warner School of Education, Laboratory for Laser Energetics, Schools of Medicine and Nursing, and the Memorial Art Gallery.