The largest experimental reactor of this type tokamak for nuclear fusion that exists is called JT-60SA and it is in Naka, a small city not far from Tokyo (Japan). The construction of this mill began in January 2013, but it was not done from scratch; he did it taking the JT-60 reactor as a starting pointits precursor, a machine that came into operation in 1985 and that for more than three decades has achieved very important milestones in the field of fusion energy.
The assembly of the JT-60SA was completed in early 2020, and from the end of 2023 it is ready to start the first tests with plasma. This machine is a device tokamak that just like JET and the future ITER resorts to the magnetic confinement of the ionized plasma. Although the ultimate goal of fusion is to use deuterium and tritium, JT-60SA initially uses only deuterium for its experiments, as it is not designed to handle the high neutron loads of tritium (that will be an ITER task). Either way, this machine is titanic. Colossal.
In fact, it has a height of 15.4 meters and a diameter of 13.7 meters. However, the most impressive are the “specifications” that allow us to form an idea about its performance. And it is capable of confining a plasma with a volume of 130 m³, as well as generating a toroidal magnetic field of 2.25 Tesla and sustaining a current inside the plasma of 5.5 MA (5.5 million amperes). These figures are impressive, and presumably when ITER is ready to begin the first plasma tests its figures will be even more astonishing.
An engineering prodigy
During the last two years, the Japanese and European engineers working on the JT-60SA reactor have installed several extraordinarily sophisticated systems in this machine that will play a leading role during the next experiment campaign. One of these systems is made up of two ring-shaped coils 8 meters in diameter that have been expressly designed to control the confinement of the plasma that is moving at very high speed inside the vacuum chamber. An amazing note: these two devices were wound directly inside the reactor.
However, another of the technological solutions that these engineers have installed in the reactor in recent months is even more amazing. Every time the researchers who operate this very complex machine carry out an experiment with it They need to know with maximum precision possible temperature and electron density of the plasma. The main problem they face is that it is not possible to obtain this data by taking direct measurements.
The interaction between the laser and the plasma is what allows engineers to indirectly calculate temperature and density
For the fusion of deuterium and tritium nuclei to take place, the plasma containing them must reach a temperature of at least 150 million degrees Celsius, and any sensor that comes into contact with it at this temperature will not survive. This is why the JT-60SA reactor engineers have been forced to develop an extraordinarily sophisticated diagnostic system. Thomson dispersion measurement equipment components have been designed and manufactured in Italy, Romania and Japan.
Broadly speaking, this device manages to measure the temperature and density of the plasma electrons by analyzing the light it emits with a high-power laser beam dispersed, precisely, by the plasma electrons themselves. In some way the interaction between the laser and the plasma is what allows engineers indirectly calculate temperature and density. The JT-60SA reactor will have two Thomson dispersion diagnostic systems. The core one has been developed in Japan, and the plasma edge one has been devised in Europe.
This enormous effort has been worth it. The reactor is almost ready to start the next experiment campaign. All that remains is to carry out a gradual start-up that will allow testing the main systems of this machine, and at the end of 2026 the experiments will begin. They will last for six months. Most impressively, this campaign will take the JT-60SA to an unprecedented level of current, enabling longer, steady-state plasma pulses to be sustained. The researchers operating the reactor are confident that everything they will learn during these experiments will be very valuable in bringing the future ITER to a successful conclusion. Let’s hope that the performance of the JT-60SA will finally live up to expectations.
Image | QST
More information | Fusion For Energy
GIPHY App Key not set. Please check settings