Analysis of Tungsten Melt Layer Motion and Splashing under Tokamak Conditions at TEXTOR

Full text: Tungsten(W) is foreseen as the plasma-facing component (PFC) material for the ITER Divertor in the activated phase and proposed for DEMO. The main challenges of high-Z PFCs are the plasma radiation losses and the possibility of melting under uncontrolled conditions, posing additional large constraints for the power handling. Melting can lead to large W influxes into the plasma, reduce the lifetime of the PFCs and degrade the power handling capability due to subsequent surface irregularities. Melt layer dynamics and splashing are investigated mainly in plasma gun and electron beam experiments, with limited data from tokamaks. A dedicated R and D programme has been started under the IEA Implementing Agreement on Plasma Wall Interaction in TEXTOR. Sets of castellated W plates with different gap width and shaping were exposed in the TEXTOR PWI test facility to power fluxes of typically 30 MW/m2. The main objectives were to analyze the formation of the melt layer, its motion and stability under the plasma impact and the magnetic field (2.25 T), in particular with respect to the possible bridging of the gaps and melt splashing. The melting of W led in all cases to a large material redistribution with the liquid W moving perpendicular to the magnetic field. The motion under TEXTOR conditions is driven mainly by a jxB force, with the current determined by the thermionic emission of the molten W. The force exceeds the plasma pressure force significantly. The motion of liquid W caused during one single melt event a restructuring of the surface with hills at the end of the castellation of up to 0.5 mm in height. However, no bridging of the gaps by molten W was observed. In a longer melt event (∼ 2 s) with higher impact power W droplet formation and ejection occurred. Melt droplets were ejected into the plasma and charged, returning them to the W target. Post mortem analysis near the molten area found W droplets with sizes between 4 μm and 100 μm and an estimated mass of 2 mg in total. Splashing led to a periodical accumulation of W in the plasma core leading to first minor then to a major disruption. The results of the melt layer behavior and material redistribution were used to benchmark the MEMOS-3D code. (author)