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In the framework of thermonuclear fusion research, increasing interest is focussed on high power, high frequency millimeter-wave generators like gyrotrons. Measurements of gyrotron power and energy allow an accurate energy balance in ECRH (Electron Cyclotron Resonance Heating) to be carried out during plasma heating. The expected increase of the power, up to 2 MW, and time, up to 10 s (or continuous wave), calls for new technological solutions in manufacturing bolometric loads, particularly in making use of absorbing layer ceramics very resistant to thermal shocks. In this particular case, also a good absorption capability at 140 GHz is requested.In a previous work [N. Spinicchia, et al., Proc. of 16th Congress Italian Association of Vacuum, Catania (1), ed. Compositori, Bologna, Italy, 2002, p. 259], we studied alumina based coatings with various techniques-ESEM (Environmental Scanning Electron Microscopy), micro-analysis with X-ray EDS, Differential Scanning Calorimeter (DSC), X-ray diffraction analysis (XRD). It has been shown that repeated exposures to high energy fluxes causes the degradation of the coating.The present work is devoted to studying and selecting an absorbing ceramic coating to be deposited on the inner surface of the load. The aim is to characterize the material with respect to absorbing capability at a frequency of 140 GHz and thermal shock and fatigue resistance caused by long-term exposure to high power millimeter-wave radiation. Bolometric loads are planned to resist to 1-2 MW power continuously.To this purpose APS (Air Plasma Spray) and VPS (Vacuum Plasma Spray) ceramic coatings have been studied to search for a convenient substitute to the actual Alumina-Titania coating (Al2O3-13TiO(2)). A number of physical parameters such as thermal conductivity, thermal expansion coefficient, coating porosity and melting point were also considered to select among the candidate coatings. (c) 2005 Elsevier B.V. All rights reserved.

High power gyrotron testing at long pulses is a necessary step in the development of mm-wave systems for fusion research. For this purpose a compact calorimetric load is being developed, with low overall reflectivity (< 5%) and aiming at 1 MW-CW capabilities, after the good performances of the loads (0.5 MW, 0.5 s per pulse) installed on the 140 GHz ECRH plant of the FTU Tokamak in Frascati. Tests at the same frequency but at higher power (0.7 MW, 1 s) and longer pulses (up to 2 s at 0.5 MW) on the ASDEX-Upgrade ECRH plant showed, after repeated exposures, damages on the absorbing layer, recognized as thermal effects due to the local enhancement of incident power: the effects visible near the entrance port are explained studying the superposition (with interference) of waves reflected inside the sphere. Samples of degraded and non-degraded coating have been analyzed with various techniques, revealing changes of the substrate's physical characteristics. Measurements of absorber temperature during and after the pulse, performed with an infrared detector looking directly into the load, allowed estimates of the peak temperature and thermal properties of the coating in the real working conditions. Improvements on power deposition, geometry and cooling are proposed, to be integrated in a new load design. (C) 2003 Elsevier Science B.V. All rights reserved.