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Nuclear fusion: overcoming the Greenwald limit and paving the way for clean, sustainable nuclear energy. Find out more!
At CPG, we have spoken many times about the challenges that people researching in the field of nuclear fusion must overcome for the first commercial reactors to be successful. We discussed the need to develop new types of steel capable of minimally activating when faced with the impact of high-energy neutrons; about the importance of stabilizing the plasma and controlling turbulence, etc.
However, so far we have only briefly touched on the reason why each new experimental nuclear fusion reactor is larger than the last. In fact, when the assembly of ITER (International Thermonuclear Experimental Reactor) is completed, the fusion machine being built by an international consortium led by Europe in the French town of Cadarache, will be the largest experimental reactor on the face of the Earth. And it will not be, of course, by chance.
Nuclear fusion and the Greenwald limit
In experimental nuclear fusion reactors, such as ITER, scientists confine charged deuterium and tritium nuclei using a magnetic field. What happens is that, no matter how powerful this field is, it always has an intensity limit, and the particles, when produced, acquire very varied energies. Some have a lot of energy, and others, however, acquire little energy. Reactor engineers are able to contain the average energy, but those particles that exceed this energy value have the ability to escape the magnetic field.
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The problem is that if too many particles escape, too much energy is lost and it is not possible to sustain the fusion reaction over time. Fortunately, this challenge can be solved by modulating the magnetic fields and increasing the size of the plasma. This is the reason why each experimental reactor is larger than the last. Scientists believe ITER is the appropriate size because the more particles there are around one that wants to escape, the more likely it is that it will impact another in its escape path and change direction or give up its energy.
In search of stability in the fusion reaction
Ultimately, what scientists working on fusion look for is that the energy that escapes is small enough so that there is no decreasing level of energy within the reaction. This has already been achieved in JET, but it was achieved for a short time, as it is not possible to maintain the effort for a prolonged period due to the lack of size, seeing it in a very simplified way. Anyway, good news has just happened. A research group from the American company General Atomics published an article in Nature that makes a significant contribution in this area.
The Greenwald limit establishes the maximum density value that the fuel can reach inside the vacuum chamber of a nuclear fusion reactor. In theory, when this value is exceeded inside a tokamak reactor, a disruption can occur, which is an event in which the plasma destabilizes, magnetic confinement is interrupted and the fusion reaction stops. A disruption can cause serious damage to the inner walls of the vacuum chamber, depending on the energy of the particles that escape confinement and impact with them.
Exceeding the Greenwald limit does not guarantee that a disruption will occur, but physicists and engineers working on tokamak reactors have until now considered this parameter a barrier they could not ignore. The contribution that General Atomics scientists made is very relevant because they were able to empirically prove some working conditions that allowed them to sustain plasma stability with a density 20% above the Greenwald limit for 2,2 seconds.
In their experiment, they used a tokamak reactor with a radius of 1,6 meters (ITER will have a radius of no less than 6,2 meters) and a gas that contains deuterium nuclei (ITER's fuel will incorporate both deuterium nuclei and tritium). As we have seen, it is very important that the plasma density is high enough to minimize the probability of significant energy losses caused by particles that manage to escape magnetic confinement. And now, researchers working on tokamak reactors know that it is possible to surpass the Greenwald limit to work with the density that requires sustaining the fusion reaction. There's no doubt this is great news.
Picture: General atomics
Source: Nature
