Design of a high-power load for millimetre-wave Gaussian beams

The design of a compact matched load for high-power measurements and testing of gyrotrons and transmission lines in ECRH plants for fusion research applications is currently in an advanced phase. The aim is to provide more than 95% absorption and precise calorimetric measurement of the input power in CW. This work is based on the results of tests at high power and short pulse length (140 GHz, 0.5MW, 0.5 s) on loads installed on the ECRH plant of the FTU Tokamak in Frascati. The loads consist basically of hollow spheres of copper with the inner wall covered by plasma-sprayed lossy ceramics. Tests at higher power and longer pulses on the ASDEX-Upgrade ECRH plant showed, after a number of successful pulses, progressive damage on the absorbing layer, marked by the appearance of electrical arcs. The absorber degradation, showing specific damage patterns, due to exposure to high-power millimetre waves, has been analysed in detail and strategies are proposed, in order to improve the power-handling capabilities and the energy extraction rate. New measurements of millimetric absorption and thermal conductivity have been performed on samples of different ceramics, for choosing the best absorbing layer. A modified expander mirror surface with a better deposition profile, numerically computed with a multi-reflection model of the sphere, is designed to avoid radiation accumulation close to the entrance port. Improved cooling channels, which in principle can exploit the increased heat transfer rate due to surface boiling, as used in high performance cooling circuits such as plasma-facing components, will provide 1-MW CW power capability. In this paper, some technical solutions for the construction and the constraints on the allowable deformation during pulses are given. (author)