Erosion Issues for Long-Pulse Operation of a Volumetric Neutron Source

Abstract

The purpose of a volumetric neutron source is the development and testing of the nuclear components of a fusion reactor. The main issue in this case is very long pulse operation, such as 2 weeks at a time, to elicit the nuclear effects to be studied. Operation at this pulse length will cause extreme erosion if the edge plasma cannot be tailored appropriately. Typical erosion rates that can be expected at some of the plasma-facing components such as the divertor target or the divertor baffles, without specifying a particular type of device, are analyzed. Accurate predictions of erosion and redeposition require not only knowledge of the erosion mechanism but also detailed knowledge of the plasma parameters, plasma flows, and their spatial distributions, as well as temperature distributions of plasma-facing components and other parameters. It is, therefore, a very difficult task to predict erosion/redeposition rates and patterns for future machines. Nevertheless, some estimate is needed of expected erosion rates, crude as they may be, so future machines for long-pulse operation can be designed. For that purpose, physical sputtering is examined only as a basis for erosion estimates and does not take into account the important processes of chemical sputtering and radiation-enhanced sublimation or the complicated redeposition processes. Even with this simplified approach, one can grasp the order of magnitude of erosion rates that will be encountered when a plasma device is operated for long pulses and at high-duty cycles.