Modelling radio-frequency plasma cleaning of fusion optics

Metallic mirrors are to be used extensively within ITER for diagnostics and real time control. Erosion of the first wall within ITER will cause particles to be redeposited around the machine, including on these first mirrors, which will cause a reduction in reflectivity and a degradation in quality of signal received by the detectors. Powering these mirrors to form capacitively-coupled plasmas (CCPs) with an induced self bias, and using the ions within the plasmas to bombard and remove the deposits, has shown some experimental success in recovering mirror reflectivity. In this work the ion energy distribution functions (IEDFs) from an Ar CCP formed on a 5 cm radius metallic mirror are modelled and investigated using the hybrid plasma equipment model. Initially a geometry variation is done showing that a simple increase in reactor volume can significantly impact the spatial distribution of the ion flux to the mirror surface leading to non-uniform etch rates across the surface, even after the maximum bias has been achieved. The ion energies need to be sufficient to remove depositions (focussing on the first wall material of Be which forms a surface oxide BeO) but not subsequently damage the underlying mirror. In order to achieve this both the voltage (50–1000 V) and the frequency (13.56–60 MHz) have been varied within the model showing trends that may lead towards IEDF optimisation. The increase in voltage increases the self bias linearly and the plasma density super-linearly, whereas increasing the frequency barely effects the self bias while increasing the plasma density sub-linearly. Both increases cause an increase in ion flux for these reasons but both also decrease the homogeneity of the ion flux across the mirror surface which will be required should the energies be above the threshold for the mirror. These results are also unique to the geometry being investigated and thus the conclusion is that it would be prudent to model individual mirror geometries to find optimal parameters. This becomes especially clear with the introduction of a perpendicular magnetic field into the simulation that significantly reduces electron transport within the plasma.