Matched calorimetric loads for high power millimeter-wave gyrotrons

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.