Impurity transport studies using fast imaging of injected carbon on the MAST tokamak

The operation of next-generation fusion reactors will be significantly affected by impurity transport in the scrape-off layer (SOL). Current modelling efforts are restricted by a lack of detailed data on impurity transport in the SOL. A short burst carbon injector has been designed and installed on the MAST tokamak. The injector creates short lived carbon plumes originating at the MAST divertor lasting less than 50 micro-seconds. The injector uses high voltage capacitor banks that are discharged across concentric carbon electrodes located in the lower divertor. This results in a very short plume duration allowing resolution of the plume evolution and precise localisation of the plume relative to the X-point on MAST. Carbon was injected into Ohmic L-mode plasmas on MAST and emission from the plumes was imaged using 2 fast filtered cameras positioned at different viewing locations. The majority of images were filtered at the Carbon II(515nm) and Carbon III(465nm) emission lines while a D-alpha and a Carbon I filter were also used to give a measure of the plasma perturbation and initial source. A Langmuir probe was also incorporated into the injector to give a measure of the strike point location. Injection was performed at various distances from the separatrix by varying the timing of the ablation on a microsecond timescale. The plumes can be seen to expand towards the X-point parallel to the magnetic field over 3 frames with a frame rate of 75kHz or 13μs. Significant perpendicular expansion is also visible on some pulses, which could be due to cross-field transport or neutrals being ionised far from the point of injection. This data will be used to constrain simulations using the onion skin model (OSM) code. Presented here are filtered images from the experiments run on MAST.