Overview on spallation target design concepts and related materials issues

From a modest beginning of a few kW of beam power spallation sources have now evolved into systems that must be able to handle several MW, mostly delivered in short pulses of less than microsecond duration. The high radiation field and high instantaneous heat deposition which spallation targets, in particular for the new high power sources, are subject to have led to several different design concepts which aim at circumventing or reducing the deleterious effects on the materials in the targets. Efficient cooling and high neutron source density are competing requirements which can be best reconciled by moving the target material out of the reaction zone and removing the heat elsewhere before returning the material back into the proton beam. One option is the use of a flowing liquid metal, which has been the method of choice in most of the recent spallation source designs, but requires solutions to a variety of new problems, such as liquid metal corrosion, cavitation erosion and e.g. in the case of PbBi, or Pb, high temperature gradients. Using a rotating solid target is an option in certain cases but still has to cope with the instantaneous load levels. While it may help to keep the average heat load and radiation damage in the target material low and thus extend the target life time by more than an order of magnitude, it still has its own design and materials issues. Opportunities to carry out research in this field are rather limited because the effects can hardly be simulated off line and, apart from spallation targets in operation, almost no facilities are available.