Study of Physical and Chemical Sputtering of Beryllium in the JET ITER-LikeWall
JET is equipped with a first wall material combination comparable to the ITER selection acomprising beryllium (Be) in the main chamber and tungsten (W) in the divertor and some recessed wall areas. Be is selected owing to its low atomic number, its low tritium retention and excellent getter properties, but material erosion limits the lifetime of plasma-facing components PFCs made of Be. Initial ERO modelling of the shaped Be first wall modules close to the ITER separatrix predict high erosion rates and limited armour lifetime. However, uncertainties in the modelling, in particular in the atomic data and sputtering yields, still exist and further bench- mark under ITER-comparable tokamak conditions is required. The ITER-Like Wall in JET (JET-ILW) demonstrated successful plasma operation, strong reduction of the C content (x20), and high plasma purity (Zeff ~ 1.2). Equipped with its bulk Be limiters, the JET-ILW allows the study of Be erosion by optical emission spectroscopy and observation of various transitions of BeI (e.g. 457nm), BeII (e.g. 527nm) and the BeD A-X band under different plasma conditions and surface temperatures. The total Be sputtering consists of the bare physical sputtering and the chemical assisted physical sputtering ­ sometimes referred as swift chemical sputtering. However, the composition of the total sputtering, its dependence on the impact energy and temperature, the strength of the chemical assisted physical sputtering are not known for a high temperature plasma edge conditions as present in the JET-ILW or in future ITER. Here, deuterium plasmas in limiter configuration have been used to vary the local electron temperature (Te) in the scrape-off layer (SOL), or better, scanning the impact energy of the impinging deuterons (Ei), as well as, to vary the PFC surface temperature (Tsurf) by plasma impact as they are only inertially cooled. The increase of Tsurf is expected to inhibit the sputtering channel via BeD which thermally decomposes at about 540K according to studies in PISCES.