Worldwide there are great hopes that fusion technology will be a clean alternative source of energy. Since the 1950s, scientists all over the world have been attempting to use nuclear fusion for peaceful purposes. Despite the fact that the process works perfectly on our sun, it is difficult to reproduce these extreme conditions on earth. For these experiments two types of large fusion reactors are used: tokamak and stellarator. Both reactor types work on the same principle. The differences lie in the shape and arrangement of the coils that generate the toroidal magnetic field. The tokamak reactor has a symmetrical design. Stellarators on the other hand, have a complex asymmetrical reactor form.
How does it work?
Basically, it is a matter of fusing together two hydrogen isotopes at a time to form a new helium nucleus. In the course of this process, not just helium and a single neutron are formed but also an extraordinarily large amount of energy. Fusion can only take place in high vacuum at a high plasma temperature of some 100 to 150 million Kelvin. The aim of the heating process is to separate the nuclei from the electrons to form a plasma. The charged nuclei are guided within the reactor with the aid of an extremely strong magnetic field (of several Tesla) maintaining separation from the reactor walls. When two atomic nuclei approach each other or collide, they fuse and release a large quantity of energy. A part of this energy is needed to keep plasma at the same temperature in order to retain the system without any additional energy input. The generated heat is then used to produce steam which drives a generator.
Vacuum system requirements
One of the important requirements for operating a fusion reactor is a strong, reliable, and powerful vacuum system.
- The plasma vessel of a fusion reactor must be evacuated to a base pressure of < 10-6 Pa (< 10-8 mbar) and the pressure in the cryostat system should be below 10-3 Pa (< 10-5 mbar). The process gases in the experiment vessel are usually hydrogen, deuterium or tritium.
- Due to the high magnetic field of several Tesla (short T) the vacuum equipment need to be installed at a distance of 4 to 9 meters to the plasma vessel. Even at this distance, the magnetic field can still reach strength of 100 mT. All components must therefore be provided with magnetic shielding. It is especially important for turbopumps, to prevent the rotor from heating up as a result of eddy current
- All vacuum components used require their electronics to be installed at a distance from the actual pump or measuring device. This is because modern digital electronics are damaged by radioactivity.
For many years, Pfeiffer Vacuum has been a globally well-established and highly competent partner for fusion experiments. It is especially important for us to design solutions in close cooperation with the user. The most essential part of the process is determining the best possible product combination for the respective application. Pfeiffer Vacuum HiPace turbopumps in use in such Projects as Wendelstein 7-X shown they are suitable for the fusion experiments. The special bearing principle and internal construction of the HiPace turbopumps ensure that a large quantity of heat can be transferred. Therefore the HiPace pumps can achieve high thermal operating reliability in external magnetic field and can be installed in close proximity to the plasma vessel. Due to the shorter distance between pumps and the vacuum chamber, a higher effective pumping speed of pumps can be reached in the plasma vessel. In order to avoid the damage of electronics in harsh conditions Pfeiffer Vacuum offers a wide range of external control devices for pumps and gauges. The distance which can be covered between drive electronics and pumps or gauges exceeds 100 m. The cables are developed to the customers’ needs and are free of halogens.