Experimental observations and modelling of carbon transport in the inner divertor of JET

In JET, hydrocarbon transport in the inner divertor has been studied on a shot to shot basis by means of a quartz microbalance (QMB) mounted at the entrance of the inner divertor pump duct. Movement of the strike point down the vertical divertor target in the direction of the QMB increases the deposition. Largest carbon deposition is found with the strike point located at the corner of the horizontal divertor plate which offers a direct line-of-sight to the QMB. Monte-Carlo modelling with the ERO transport code reproduces the dependence of material deposition at the QMB on the plasma configuration. A further enhanced carbon flux to the QMB is detected if the strike point is moved for the first time to a location where a carbon layer was built up in former shots. This can be understood in terms of a much larger erosion yield for re-deposited carbon compared with bulk material.