Thermal Response of Tritiated Co-deposits from JET and TFTR to Transient Heat Pulses
Authors: C.H. Skinner, N. Bekris, J.P. Coad, C.A. Gentile, A. Hassanein, R. Reiswig, and S. Willms
Date of PPPL Report: May 2002
Presented at: the International Workshop on Hydrogen Isotopes in Fusion Reactor Materials, May 22-24, 2002, in Tokyo, Japan. Conference proceedings to be published in a special issue of Physica Scripta.
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 (Tokamak Fusion Test Reactor) and JET (Joint European Torus) after D-T plasma operations. The tiles have a surface layer of amorphous hydrogenated carbon that was co-deposited during plasma operations, and laser scanning has released more than 80% of the co-deposited tritium. The temperature rise of the co-deposit was much higher than that of the manufactured material and showed an extended time history. The peak temperature varied dramatically (e.g., 1,436 degrees Centigrade compared to >2,300 degrees Centigrade), indicating strong variations in the thermal conductivity to the substrate. A digital microscope imaged the co-deposit before, during, and after the interaction with the laser and revealed 100-micron scale hot spots during the interaction. Heat pulse durations of order 100 ms resulted in brittle destruction and material loss from the surface, whilst a duration of approximately 10 ms showed minimal changes to the co-deposit. These results show that reliable predictions for the response of deposition areas to off-normal events such as ELMs (edge localized modes) and disruptions in next-step devices need to be based on experiments with tokamak generated co-deposits.