Analysis of the local re-deposition behavior of carbon at the main walls in TEXTOR by CD4 gas injection and Quartz Microbalance techniques

The local re-deposition of carbon that is chemically eroded from the main wall of fusion devices has been experimentally investigated using a specially designed Quartz Microbalance system. The system has been placed in the far SOL of TEXTOR with the front plate parallel to the field lines and CD4 has been injected in the vicinity. The total re-deposition efficiency on the front plate was about 1%. Modelling with ERO and the 3D-GAPS code reproduced the measured deposition efficiencies showing values of 2%. The remaining difference can be attributed to chemical re-erosion of the re-deposited C layer by the hydrogen atoms produced from the CD4 injection. The data show that re-deposition behavior on remote places does not suffer from enhanced erosion, as needed for the plasma wetted area.