High-power millimeter-wave calorimetric beam absorbers

Beam absorbers play an important role both in electron cyclotron heating systems at high power and in millimeter-wave diagnostics that need a low level of stray or reflected power. In the first case short- and long-pulse loads are used, whose back-reflection can be kept within a few percent with proper techniques. In the second case, absorbers or scramblers are envisaged, to be put in hostile environments. At Istituto di Fisica del Plasma in Milan, a number of calorimetric loads have been developed, adopting several techniques for overall reflectivity reduction, which are suitable for beam sinking with calorimetric capability. They achieve a low overall reflectivity and high-power capability by a properly chosen power distribution in the absorbing wall provided by a dispersing mirror, by a smooth geometrical shape, by heat-resistant absorbing coatings of optimized thickness, and by accurate trapping of most of the escaping radiation with preload structures. Fundamental, when it becomes impossible to diffuse the incoming beam by the mirror alone, mostly because of side lobes at large angles, is the use of a newly developed phase-scrambling surface presented in this paper. It provides the necessary spreading, complementing all the other techniques into a set that can be helpful in designing millimeter-wave systems and diagnostics, in order to reduce spurious or unwanted signals.