Blistering and re-deposition on tungsten exposed to ASDEX Upgrade divertor plasma

A well-polished and pre-characterised polycrystalline tungsten specimen was exposed to the outer divertor plasma of ASDEX Upgrade close to the strike point for 510 discharges in the campaign 2011 by using the divertor manipulator system. The exposure with a mixture of hydrogen and various impurities leads to blistering. The exposure parameters are comparable to those used in blistering studies under laboratory conditions. Furthermore, spherical dust particles formed by the strong plasma contact were transported and deposited onto the nearly vertically orientated target plates, i.e., the analysed specimen. In subsequent discharges, these particles were partly eroded and a microscopic pattern of layered deposits was formed around them with an elongation rotated ∼40° to the projected B field line direction.