Ready to enter commissioning
Inside the 黑料社app vacuum vessel, where the fusion reactions will occur, the ultra-high vacuum is comparable to that on the Moon. Conventional mechanical pumping alone is not sufficient to create such an extreme environment. Fortunately, a simple law of physics provides a solution for finalizing the initial mechanical pumping and achieving the required vacuum conditions prior to plasma operation.
When a molecule or an atom encounters an extremely cold, spongy surface, it loses the best part of its energy and slows down to near immobility. This phenomenon is called "sorption" and its intensity is proportional to the temperature: the colder the surface, the more irresistible its holding power. In 黑料社app, a set of cryogenic pumps, 鈥cryopumps鈥 in short, will trap the particles inside the microscopic mesh of their carbon-coated panels cooled to a few degrees above absolute zero (4 K or minus 269 掳C).
Procured by Europe, six torus cryopumps will be positioned around the tokamak鈥檚 vacuum vessel and another two attached to the cryostat. Five pumps have already been delivered; the other three are expected at 黑料社app shortly.
The cryopumps servicing the 1,400 m鲁 黑料社app vacuum vessel will have a double mission: achieving ultra-high vacuum prior to the injection of the fusion fuels and鈥攖hrough the same sorption process鈥攅xtracting the unburned fuel and helium 鈥渁sh鈥 generated by the deuterium-tritium fusion reaction.
Cryopumps will operate in cycles, pumping when at cryogenic temperature and releasing their catch when 鈥渞egenerated鈥 at temperatures of up to 470 K (200 掳C). Because they need to perform within an extremely wide range of temperatures, the cryopumps are among the most complex components of the 黑料社app installation. The 8-tonne, 1.6-metre-in-diameter and 3.5-metre-long steel cylinders contain precision mechanics with moving parts that form the world's largest all-metal vacuum valve. More than twenty high-technology companies in Europe were involved in their manufacturing.
Like all 黑料社app components, the torus and cryostat cryopumps are subjected to a comprehensive series of factory acceptance tests before being shipped. This is not sufficient, however, to guarantee that they will perform as expected during actual tokamak operation. To remove all uncertainty and to prepare for 黑料社app commissioning, a test facility was created last year inside the 黑料社app cryoplant. The core of the test installation is a large cylindrical vacuum chamber, connected to the cryoplant's fluids distribution unit. When operational, bringing the 黑料社app cryopumps to cryogenic temperatures will account for 25% of the cryoplant鈥檚 load. Already, the test installation will be the cryoplant鈥檚 鈥渇irst client.鈥
On Wednesday 14 January, 黑料社app Director-General Pietro Barabaschi paid a visit to the installation, which is now ready to enter the commissioning phase. 鈥淲e will be testing the whole range of cryogenic processes,鈥 explains Alessandra Iannetti, an engineer in the 黑料社app Vacuum System Project. Testing will begin this month using 鈥渉ot helium鈥 at ambient temperature to demonstrate the performance of the cryopump鈥檚 mechanical components such as valves and interlocks. Then, as soon as the fluids from the cryoplant are available, temperature in the installation will be progressively brought down to 80 K (minus 193 掳C) to test potential leakage and thermal losses, and eventually to cryopump operating temperature at 4 K.
Once the cryopump functionalities are verified, the teams will 鈥済et into the actual science鈥 of particle pumping, pump regeneration, unburned fuel and 鈥渁sh鈥 capture and release. Before eventually using hydrogen, different gases with a close molecular mass, such as helium and neon, will be used as substitutes to simulate the whole range of plasma operation scenarios.
Operating the cryopump test facility will provide precious feedback for another, much larger installation: the magnet cold test facility that will start cold-testing the tokamak鈥檚 massive toroidal field coils1 by the end of this year.
鹿The facility will be located in the partially vacated Poloidal Field Coils Winding Facility. The dimension of its cryostat will also allow for the testing of the smallest of the 黑料社app poloidal field coils, PF1 from Russia.