Speaker
Description
Fusion reactors are generating energy by nuclear fusion between deuterium and tritium. In order to evacuate the high gas throughputs from the plasma exhaust, large pumping speed systems are required. Within the European Fusion Programme, the Karlsruhe Institute of Technology (KIT) has taken the lead to design a three-stage cryogenic pump, featuring an 80 K thermal radiation shield, and two charcoal coated pumping stages; at 15K – 22K for hydrogenic species adsorption and at 5K for helium (a product of the fusion reaction) adsorption. This configuration can provide a separation function of hydrogen isotopes from the remaining gases, thus tritium can be internally recycled and reinjected, limiting its inventory in the machine.
The pumping speed relates directly to the interaction between the gas and the sorbent, characterized by the sticking coefficient which depends on a complex way on the nature of the couple gas-sorbent, the sorbent and gas temperatures, the gas pressure, the specific flow rate to the sorbent surface, and the surface coverage. Since literature related to this topic is scarce and inconsistent, a dedicated experiment was conducted in the large cryogenic vacuum TIMO-2 facility at KIT. A test pump consisting in a charcoal coated panel equipped with electrical heaters for temperature regulation between 5K and 25K, and housed by an 80K thermal shield and inlet baffle, has been tested under various gases, gas mixtures and gas flows with two different geometrical configurations.
The influences of the panel temperature, the specific flow rate to the charcoal surface and the incoming gas temperature on the pumping speed have been characterized. After a description of the experimental set-up, experimental results are discussed and open questions are addressed. In a future work, supporting Monte Carlo simulations should allow for derivation of the sticking coefficient values by comparisons with the experimental results.