Newsweek BY HANNAH OSBORNE ON 10/23/17
Scientists believe that fusion energy—which generates electricity in the same way that the sun creates energy—has the potential to provide the world with an almost limitless, clean source of power.
But while it is known that fusing two lighter atomic nuclei to form a heavier nucleus releases energy, it is far more difficult to harness that power. In order to do so, they would have to create plasma hotter than the sun that could be stably confined.
So far, researchers working across the globe have managed to achieve these temperatures to produce the plasma using two types of device—a tokamak and a stellarator (explained below). But as of yet they have been unable to generate more power from the fusion than it takes to create the reactions in the first place.
In a study published in Nature Physics in June, lead author Yevgen Kazakov, a researcher at the Laboratory for Plasma Physics in Brussels, Belgium, and colleagues showed how they had developed a new way to heat fusion plasma in tokamaks. By using radio frequency heating, they were able to raise ions to energies far greater than had previously been achieved.
NASA
The technique involves three ion species—hydrogen, deuterium and helium-3. Normally, only two species are used. By adding in a third, of which there were only trace amounts, researchers could focus in the energy on this species and heat it up to far higher energies.
Scientists now plan to build on this technique in the effort to achieve fusion energy.
In an email interview with Newsweek, John Wright, from the Massachusetts Institute of Technology (MIT), one of the study authors, spoke about the challenges he and other scientists are facing, and how they are working to overcome them.
People say nuclear fusion is always 30 years away—realistically will it be achieved within this time frame?
The answer to this question is always dependent on political and social will and funding to an extent. However, I am confident saying that the path to nuclear fusion has never been clearer. What is needed now is a next step experiment that enables us to test the robustness of the tokamak design to steady-state fusion plasmas.
The ITER device [which will be the world’s largest fusion experiment] being constructed in the south of France by an international consortium is expect to begin operations late next decade. If it operates as expected, it will demonstrate net fusion power output in bursts of thousands of seconds.
During the period since ITER’s design, construction, and operation, technology and plasma physics have and will continue to progress. For example, recent developments in the field of high field high temperature superconductors may permit the construction of tokamaks with higher magnetic fields and hence smaller and cheaper construction than ITER. Therefore, in concert with ITER construction and operations, other tokamaks should be built in parallel that focus on integrating new developments and capabilities to address other technical challenges outside of ITER’s mission.
These experiments can easily happen within 30 years. With luck, and societal will, we will see the first electricity generating fusion power plants before another 30 years pass. As the plasma physicist Artsimovich said: “Fusion will be ready when society needs it.”
What is the biggest difficulty in nuclear fusion?
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Using the tokamak, the type of magnetic fusion confinement device discussed in the Nature Physics paper, we can already achieve the conditions needed for nuclear fusion. This configuration is well tested and its performance well understood and is the basis for most fusion programs around the world. While there are several technical challenges that must be addressed for economic fusion power, the biggest difficulty for the nuclear fusion program is the time it requires to address these one at a time.
ITER is the first experiment to be built in over 30 years to address one of these issues—in this case the physics of a burning plasma in which its temperature is maintained by its own fusion reactions. The scale and cost of ITER is such that it requires a multinational consortium to build over a period of a couple decades.
A technical challenge that our paper tries to address using present day tokamaks such as Alcator C-Mod and JET is the understanding and control of the very energetic fusion product ions that must heat the core plasma as they make their way to the wall. Our work shows a method to efficiently raise the energy of a third species of ions to levels comparable to that of those produced by fusion in order to study their behavior in present day devices.
Why is temperature so important?
Fusion reactions take place at temperatures of 100s of millions of degrees Celsius. The products of a fusion reaction are at tens of billions of degrees Celsius. Temperature, therefore, plays two important roles in fusion.
Firstly, we must efficiently create and maintain a high temperature to enable the fusion reaction to take place. Our Nature Physics article focused on one method of doing this with microwaves launched from an antenna that heat the ions resonantly known as Ion Cyclotron Resonance Heating (ICRH).
Through experiment and simulation, we established that a new method of using a third ion species at concentrations of less than one percent of the total plasma could be used to efficiently heat that species to very high energies and in turn heat the whole plasma. This method may have applications to more efficient heating of the plasma to the temperatures needed to begin the fusion burn.
The second important role of temperature is in the very high energy of the fusion products. Our heating technique is also capable of heating a small component of the plasma to temperatures comparable to that of the fusion products and so provides a way to study how the high energy fusion products interact with the plasma in experiments before burning fusion fuels are used. Like any fire, fusion burns hotter if the fire is bigger or better insulated, and our method addresses both of these aspects of fusion.
How does the latest study go towards helping reach these temperatures?
Our study uses two main ion species to control the level of efficiency at which a third species is heated. The result is a very efficient method to heat this third ion species to tens of billions of degrees to mimic fusion products or to heat the bulk plasma to 100s of millions of degrees to create the conditions to initiate a fusion burn.
What are the main differences between a tokamak and a stellarator?
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A tokamak and a stellarator both have an overall toroidal (donut-like) shape. The tokamak is uniform in the toroidal direction (the long way around the donut). This symmetry improves its confinement—its efficiency and holding the plasma in its magnetic field—at the price of the need to produce a toroidal current needed to complete the confining magnetic field. Creating this current continuously is known as the steady state problem for tokamaks.
A stellarator has non-uniform shape and magnetic field in the toroidal direction that eliminates the need for toroidal current—hence is more robustly steady state than the tokamak. But this asymmetry reduces its confinement properties making fusion gain more difficult. It also is inherently more complex to construct. So the difference can be summarized as: stellarators give up the confinement benefits of axi-symmetry to solve the steady state challenge of tokamaks.
Which approach do you think is best for fusion?
While fusion research is focused on the tokamak with some efforts with the stellarator, there are many other approaches being pursued—some with private funds. This level of interest reflects the urgency felt to create new carbon free energy sources.
Of all these concepts, only the tokamak has demonstrated the properties necessary for fusion energy. With the completion and operation of ITER, the tokamak will be the device that first demonstrates a burning fusion plasma with net power gain. So in the near term, the tokamak provides the quickest path to fusion energy. But it is important to continue developing the stellarator and other concepts as secondary paths in the hopes they may eventually prove to be more efficient.
What is the next thing scientists will need to overcome?
Concepts other than the tokamak need to demonstrate the basic conditions for fusion: maintaining a hot enough core plasma to generate fusion reactions with a cool enough edge plasma to avoid damaging the wall materials. For tokamaks, the next thing scientists need to overcome are technical issues that affect the economics of a fusion reactor as a power plant.
The main obstacles are: 1) survivability of device components to minimize the need for replacement and refurbishment during, 2) efficient generation of the stabilizing toroidal current needed to complete the confining magnetic field, and 3) net production of Tritium fuel from Lithium in the reactor structures.
How could fusion help the planet?
Fusion can enable the transition to a carbon neutral power infrastructure. It produces no long lived radioactive waste and has a fuel that is plentiful and ubiquitous. Fusion can complement other carbon free energy technologies such as wind and solar by providing reliable base load power that can fit into the existing electrical grid infrastructure. After all, wind and solar derive their power from fusion in the Sun.