Erosion and deposition behaviour of a-C:H layers in the private flux region of the JET MKII-HD divertor

Material deposition was measured in the private flux region of JET by means of a quartz microbalance (QMB) for ∼11 900 s of successive plasma discharges with moderate additional heating (2–3 MW). The QMB was located under the load bearing septum replacement plate with a view towards the inner divertor. In total a ∼270 nm thick hydrogenated amorphous carbon layer was measured assuming a density of 1 g/cm3 correlated with a fluence of 2.36 × 1023 D+ ions into a toroidal section of 1 cm length of the inner divertor. The area of the QMB in the private flux region was deposition dominated when the inner strike point position was on the vertical tiles, line-of-sight with the quartz crystal, and turned into erosion solely by moving the strike point from vertical tile 3 to horizontal tile 4. The most likely reason is a change of the C/D flux ratio of the particles impinging on QMB turning this area from a deposition to an erosion dominated region.