Tokamak JET launches new deuterium-tritium campaign

After 18 months of preparation and repair, the world's largest tokamak, JET, is restoring work with the aim of launching next year with deuterium-tritium plasma, i.e. real thermonuclear launches. Similar experiments have not been conducted on tokamaks since the mid-90s, and it is time to test the accumulated new ideas experimentally.


Composite image of the JET tokamak vacuum chamber (about 8 meters in diameter) and plasma during experiments.

It was here, on JET in 1997, that the thermonuclear reaction power was set for magnetic traps - 16 megawatts for about 100 milliseconds. The duration then, however, was limited by the duration of the operation of the neutral injection system , which is responsible for the external heating of the plasma. Today, these restrictions are much softer, so there are plans to keep the 16-megawatt burning for ~ 5 seconds. Again, longer is impossible, because There is a certain limit on the total exposure of the vacuum chamber design to thermonuclear neutrons.



Profiles of record-breaking thermonuclear experiments and a planned future

An important change in comparison with 1997 was the conversion of the reactor to a full metal lining - carbon-fiber and graphite elements disappeared. The latter, in their time, helped to reduce plasma pollution by materials with high atomic numbers and pass the so-called “radiation barrier” on the way to thermonuclear temperatures. However, over time, it became clear that in terms of operation, the metal wall is better - less dust, less tritium that is “stuck” in the structure. This is especially true of the divertor, an element to which plasma is “drained” to remove heat and contaminants from the fusion region.


The ITER divertor element, recently manufactured by Europe, is lined with tungsten blocks and active cooling. The direct part has (at an acute angle) a plasma flow of 5-10 megawatts / m ^ 2

In addition to the interaction of tritium with a promising (planned and at ITER) full metal wall, solutions for suppressing ELM instabilities using special guns firing frozen pellets from a DT mixture, well, and many ideas of tokmashniki on plasma behavior will also be tested.

In the course of the “experimental DT campaign No. 2 - DTE-2”, also for the first time in history, plasma experiments are planned on pure tritium. Since the mass / charge ratio of tritium is one and a half times more than that of deuterium, modeling and experiment can be compared on a variety of phenomena sensitive to this ratio.

According to the plans for the next few months, the machine will be commissioned, and then an approximately 5-month calibration series of physical experiments on deuterium. After about a 1-month check by the UK's atomic surveillance, the readiness of all systems to work with tritium will begin a 3-month physical TT program. This will be followed by additional safety drills, another acceptance, and finally, the four-month DTE-2 itself.


The very first launch of JET after a break on hydrogen plasma. Slow 40 times.

The long and complex entry into this program of experiments is associated both with the nuisance of tritium itself and with induced radioactivity as a result of a thermonuclear reaction.

Tritium is volatile, like any hydrogen, flammable and highly radioactive gas. To work with it, it is necessary to install all equipment in sealed glove boxes, pipelines to be surrounded with sealed second shells, a building should be equipped with a pressure reduction system (to reduce the likelihood of leakage to the outside) and oxygen content (to prevent fires, which will be a nightmare in case of tritium). In total, the site may contain no more than 20 grams of tritium stored as uranium hydride (treytida?) And delivered to the heating system. But burned in all experiments will be only about 1 milligram. Such a big difference between the “warehouse” and the needs is explained by the fact that a very small fraction of tritium burns during passage through the plasma, and the rest, unfortunately, is contaminated with deuterium and antimony, after which the mixture must be sent to the isotope separation — there is no such system on the JET site.


Calculated values ​​of dose rate (radioactivity) inside the JET vacuum chamber as a result of thermonuclear activation. However, such activation quite quickly falls by 2-3 orders of magnitude.

The second most important engineering task here (and in the future - at ITER) will be working with an activated structure. At the end of DTE-2, the radiation background in the center of the vacuum chamber will reach 80 mSv / h (8 roentgens per hour), therefore, a remote-controlled robotics will be used for work inside. In the course of preparation, we already trained on replacing tiles, installing new ones, installing various sensors, etc.


Remote controlled robot inside JET. It was used during the dismantling of the activated elements after DTE-1.

Finally, we should mention another “fashionable” idea - liquid lithium walls, which solve many engineering problems in the durability of facing a chamber before the damaging effects of neutrons and plasma: for the first time, JET will test the interaction of such a wall and deuterium-tritium plasma.

In my opinion, on the one hand, similar programs are important for preparing the launch of a full-fledged deuterium-tritium campaign on ITER, and on the other hand, they emphasize the incredible difficulties in working with DT-reaction. In conditions when thermonuclear power engineering is not a “saving straw” for civilization, it is difficult to expect bets on DT-reactors.