Real-time RBS analysis of plasma erosion in DIONISOS

Abstract

One of the primary scientific challenges still facing the development of commercial nuclear fusion reactors lies at the plasma-material boundary. Plasma temperatures greater than 10 million degrees Celsius (10 keV) require clever magnetic field configurations to confine the plasma near the center of the toroid. However, the materials directly surrounding the plasma, known as the first wall, will be in contact with a cooler plasma, closer to 5 eV, and must be able to withstand intense neutron radiation as well as high heat fluxes. It is still unclear how some proposed first wall materials such as tungsten and molybdenum will behave in environments with these plasmas. Scientists must provide evidence supporting the lifetime, fuel retention capabilities, and neutron resilience of these materials in order to assure their high quality performance inside fusion reactors for many years. As a result, scientists must better understand how plasmas interact with surfaces of materials. This project contributes to this endeavor by studying plasma erosion in real-time using a helicon plasma source and an ion beam analysis technique known as Rutherford backscattering spectroscopy (RBS) to determine target thickness and composition. Copper coated aluminum targets were subjected to helium plasmas of varying fluxes and ion energies and were analyzed in real-time with RBS to determine the copper layer thickness as a function of time. This analysis will provide the frame work for studying fusion materials such as molybdenum and tungsten in the same way using hydrogenic plasmas. It is expected that the erosion rate will be proportional to the ion flux (a function of plasma density) and the sputtering yield (a function of ion energy), while being inversely proportional to the target density. The goal will be to develop a reliable method to characterize plasma regimes with reproduceable, well-behaved flux profiles and use them to controllably erode samples, while performing real-time RBS analysis of the surface layer.