Manufacturing and Testing of Actively Cooled Test Limiters for TEXTOR Made of an LPPS Tungsten-Coated TZM Heat Sink

To investigate the erosion and redeposition phenomena of fusion-related materials under stationary conditions, actively cooled test limiters were developed for the Tokamak Experiment for Technology Oriented Research (TEXTOR). The test limiters allow experiments under stationary conditions within a plasma pulse length of 10 s. Heat loads of typically 10 MW/m² are removed by pressurized water; the volume flow is 10 m³/h, the pressure is 15 bar, and the minimum coefficient of heat transfer is nearly 70 000 W/m²K. The limiters were manufactured as low-pressure plasma-spraying thermally sprayed tungsten-coated heat sinks made of the molybdenum alloy TZM. The required properties of the tungsten coating were developed by the use of a statistically based optimization routine. Optimized, actively cooled limiters were successfully tested in Forschungszentrum Jülich's Material Research Ion Beam Test Facility (MARION) with hydrogen beams. Maximum heat loads of up to ~17 MW/m² were applied without any failure of either the heat sink or the cooling system. The steady state of the surface temperature was measured within 2 s. Analytical and numerical models describing the effects of heat load distribution and spatial temperatures were found to be in excellent agreement with numerical predictions. In an additional experiment, loss of coolant was simulated. Transition boiling was generated, and after repeated heat loads higher than 10 MW/m², cavitational damage of the heat sink occurred. Concerning the material selection for heat sinks of hypervapotrons and other cooling systems based on enhanced boiling of the cooling liquid, this result might be of special interest.