In principle, vacuum compatibility of the interface between the beam line and the target is achieved when the proton beam can reach the target material without too much interference. The value of about 10-3-10-4 mbar for the maximal pressure that is tolerable above the target surface is determined by the rate of plasma formation. The latter occurs from the interaction of the proton beam with the gases and vapours that are emanated from the spallation target. An intense plasma will cause sputtering on the structural materials, give rise to an additional thermal load on the beam line walls and may eventually lead to beam clogging. The need to avoid plasma formation thus implies that the vacuum pumping speed, including geometrical limitations must be sufficient to reach required pressure under the given gas load. The most important contributors to the atmosphere above the target material are the outgassing of dissolved elements, liquid metal evaporation and emanation of volatile spallation products. Experiments combined with theoretical calculations on this topic performed at SCK•CEN in Mol, Belgium (VICE) and at the polytechnic university of Milan, Italy have shown that at 400 °C, outgassing of LBE can be reduced to a level where it is comparable to the liquid metal evaporation rate. On the other hand the production rate of spallation products is not sufficient to cause any difficulties for as far as the vacuum interface problem is concerned. Nevertheless, the absence of the target window removes the containment barrier for the radioactive spallation products that is otherwise present. For this reason a closed vacuum system must be foreseen. The solution taken for MYRRHA is a scheme in which the vacuum above the target is maintained by a set of cryopumps in order to minimise the amount of moving parts in the spallation target. Periodically, each of the cryopumps is regenerated by a turbopump that is backed by a small molecular getter pump. The latter may then be treated as radioactive waste. The device may possibly require some initial cooling because of the amount of radioactive materials that are concentrated in the getter pump. |