Effect of plasma configuration on carbon migration measured in the inner divertor of JET using quartz microbalance

The deposition of carbon in the louvre area of the inner divertor of JET was measured by means of a Quartz Microbalance diagnostic (QMB) during 806 exposures (total of about 6479 s) in various divertor conditions. Exposure time was controlled by means of a shutter. The overall integrated frequency shift of the deposition crystal from the start of the measurements to the end was 23 640 Hz, corresponding to an average carbon deposition flux of 5.5 × 10−8 g/cm2 s or 2.8 × 1015 C/cm2 s. Extrapolating this to the total time of 26.4 hours of divertor plasmas with the MKII GB SRP divertor (gas box divertor with septum replacement plate) yields a total amount of 35.4 g of carbon layers deposited on the louvre region. It was found that the deposition increases significantly with decreasing distance of the strike point position to the louvre entrance. Elmy-H-mode discharges with the strike point on the horizontal target dominate the carbon layer formation on the QMB.