Modelling of carbon migration during JET ¹³C injection experiments

JET has performed two dedicated carbon migration experiments on the final run day of separate campaigns (2001 and 2004) using ¹³CH₄ methane injected into repeated discharges. The EDGE2D/NIMBUS code modelled the carbon migration in both experiments. This paper describes this modelling and identifies a number of important migration pathways: (1) deposition and erosion near the injection location, (2) migration through the main chamber SOL, (3) migration through the private flux region (PFR) aided by E × B drifts and (4) neutral migration originating near the strike points. In H-Mode, type I ELMs are calculated to influence the migration by enhancing erosion during the ELM peak and increasing the long-range migration immediately following the ELM. The erosion/re-deposition cycle along the outer target leads to a multistep migration of ¹³C towards the separatrix which is called 'walking'. This walking created carbon neutrals at the outer strike point and led to ¹³C deposition in the PFR. Although several migration pathways have been identified, quantitative analyses are hindered by experimental uncertainty in divertor leakage, and the lack of measurements at locations such as gaps and shadowed regions.