The achievement is good, but not unique. Building a fusion reactor is easy in a way – the problem with billions of euros is that energy should also be produced and not just consumed.
The nuclear reactor as an energy consumer. High school student Cesare Mencarini built a nuclear fusion reactor, but this kind of fuser-type device doesn’t produce energy, it consumes it. Screenshot from Mencarin’s Linkedin page. Cesare Mencarini (screenshot)
A 17-year-old high school student Cesare Cariani has built a functioning nuclear fusion reactor in a school class, is written in various media, such as the British newspaper Mirror.
According to the news, it took Mencarini, who lives in Cardiff, UK but is originally from Italy, a year and a half to build this device. The fusion inside the device has not been ignited yet, but this step is coming soon, Interesting Engineering clarifies.
Not much is mentioned in the news about the technical details of the Mencarin reactor, except that the device produces neutrons and that it uses high electrical voltage to perform fusion.
According to a forum post written by Mencarin – the link of which was found on the author’s Linkedin wall – the device used a voltage of 5 kilovolts. In addition, a vacuum had to be absorbed inside it (0.025–1.2 millibars, or a vacuum quality of 99.9–99.999 percent).
These scraps of information suggest that the reactor is some sort of adaptation of a device known as a fuser.
In a certain sense, a fuser is really a fusion reactor. Inside it, the nuclei of heavy hydrogen, or deuterium, are collided with a high electric voltage so that some of the nuclei fuse with each other to form either helium-3 or tritium, or hydrogen isotope 3.
A proton is also released from the latter reaction, a neutron from the first reaction. As an electrically neutral particle, the neutron can be released from the electric fields of the device.
Fusers typically use a voltage in the order of 10 kilovolts. This voltage gives deuterium nuclei as much kinetic energy as a temperature of a good 110 million degrees Celsius.
Inside the fusor. A picture of a fusor built by another hobbyist. Samuel Low (CC BY)
At a voltage of ten kilovolts, only a small part of the nuclei colliding towards each other fuse, but the reaction still ignites to a noticeable extent. To completely break the electric potential barrier of two +1 atomic nuclei would require a voltage of around a megavolt
.
Fusion is easy, energy production is not
According to the Mirror, Mencarin’s device might be the first fusion reactor set up specifically for the school classroom, but this guess is hardly true.
After all, a lot of fusers have been built in the world – even homemade, also for school classes, and even by 13-year-old enthusiasts. In fact, enthusiasts have even set up internet discussion forums around the topic, where Mencarini himself has enjoyed himself.
If working fusion reactors can be built so easily, how is it possible that the production of fusion energy is so difficult and takes decades from state-level organizations?
At the heart of the conflict is whether the device consumes energy or produces it. The fusor needs significantly more energy than it produces, so it is definitely not the savior of the world’s energy problems. Building a fusion reactor suitable for energy production simply cannot be done by individual physics enthusiasts, but billions of euros of research has been needed for that.
However, fusors can be used as active neutron sources.
Mencarini says on his Linkedin wall that the project cost him about 8,000 pounds, or about 10,000 euros. Some of the steel components of the pressure chamber he built himself, some he ordered ready-made.
This is how you calculate the voltage yourself The electric force between two spherical charges F
on F =the²z₁z₂/r
² where kis Coulomb’s electric constant (= 1 / 4πe ₀ = 8.99 MNm²/C²), e elementary electric charge (0.1602 aC),z ₁ andz ₂ charge numbers (here both +1) and r
distance. Electric voltage, or potential U
is defined as energy per charge. The energy of the electric field is obtained by integrating the force over the distance. The details of the integration are omitted here, but the result is a very simple equation. In that case, the energy – and thus the voltage – is inversely proportional to the first power of the distance, not the second: U₂ =times₁/
r … and of course symmetricallyU ₁ =times₂/ r, although in this case there is no difference between the equations becausez ₁ =z
₂ = +1.
The complete overcoming of the electric potential of the two deuterons would mean touching, i.e. a distance of about 1.5 femtometers. By entering the numbers into the equation, we get 960 kV, which rounds to one megavolt.
The fact that the fusion starts slowly even at a hundred times lower voltages is due to quantum mechanical tunneling. When two particles are close enough to each other, they can with a small probability cross the potential wall separating them through quantum phenomena, even if the energy according to classical physics is not enough.The story was originally published in Tekniikka&Talous magazine
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