Thermal Response of Tritiated Codeposits from JET and TFTR to Transient Heat Pulses

Abstract

High heat flux interactions with plasma facing components have been studied at microscopic scales. The beam from a continuous wave neodymium laser was scanned at high speed over the surface of graphite and carbon fiber composite tiles that had been retrieved from TFTR and JET after DT plasma operations. The tiles have a surface layer of amorphous hydrogenated carbon that was codeposited during plasma operations and laser scanning has released more than 80% of the codeposited tritium. The temperature rise of the codeposit was much higher than that of the manufactured material and showed an extended time history. The peak temperature varied dramatically (e.g. 1436°C compared to >2300°C) indicating strong variations in the thermal conductivity to the substrate. A digital microscope imaged the codeposit before, during and after the interaction with the laser and revealed 100-micron scale hot spots during the interaction. The heat flux produced a pattern of beads on a mixed Be/C deposit. Heat pulse durations of order 100 ms resulted in brittle destruction and material loss from the surface, whilst a duration of ≈ 10 ms showed minimal changes to the codeposit. These results show that reliable predictions for the response of deposition areas to off-normal events such as ELMs and disruptions in next step devices need to be based on experiments with tokamak generated codeposits.