This study investigates erosion, deposition and material migration in the divertor of the fusion tokamak JET. Nuclear fusion provides a potential method for sustainable energy generation without large carbon emissions or long-term radioactive waste. Toroidal chambers with magnetic field coils known as tokamaks are used to contain the plasma of hydrogen fuel. The fuel ions can erode the plasma-facing materials, leading to degradation of plasma performance, limitation of vessel lifetimes and fuel retention. Plasma-material interaction is particularly significant in the divertor region of the tokamak. The carbon walls of JET-C have been replaced with the beryllium/tungsten walls of JET-ILW in anticipation of their use in the larger ITER tokamak. Determination and analysis of the different erosion/deposition characteristics provides vital information for the efficient, economic and safe operation of ITER.
A combination of diagnostics and modelling techniques have been applied to produce a detailed study of the important processes and results. Rotating collectors provide time-resolved deposition measurements through varying the surfaces deposited on; quartz microbalances (QMBs) use piezoelectric crystals to measure changes in deposited mass. A simple, geometrical model is used to describe the rotating collector depositions over long timescales, incorporating experimental data from sources such as spectroscopy. More detailed, higher time resolution modelling of erosion, deposition and transport in the JET-ILW divertor is performed with a Monte Carlo code written for this study.
The rotating collectors demonstrate a replacement of carbon by beryllium as the dominant impurity deposit in JET-ILW relative to JET-C and an associated reduction in deuterium retention. The total deposition rate on the JET-ILW collectors is reduced by an order of magnitude. In general, time-dependent modelled and experimental collector deposition profiles show good qualitative agreement. A lack of carbon deposition in the remote JET-C outer divertor for corner strike points is determined from the collector modelling and QMB measurements; similar behaviour is not observed for beryllium in JET-ILW. Additionally, there is a reversal of deposition asymmetry between the inner and outer divertor corners in JET-ILW. These different distributions of deposits are attributed to the different chemical properties of carbon and beryllium and their associated responses to elevated temperatures.
Local beryllium surface coverages have a considerable impact on erosion and deposition behaviour in JET-ILW due to reduced impurity concentrations. Monte Carlo modelling is used to assess the impact of varying strike points, beryllium fluxes, beryllium coverages and plasma temperatures/densities. Further insight is gained through comparison of modelling and experimental results. Peaking of the beryllium influx is investigated using divertor spectroscopy and modelling, revealing the importance of the limiter phase and initial divertor phase for beryllium erosion, deposition and transport.
