Study of the ohmic losses of a traveling wave antenna section in view of application on DEMO

A multi-section Traveling Wave Antenna (TWA) is one of the presently proposed options for Ion Cyclotron Resonance Heating (ICRH) of DEMO. This option would reduce the antenna power density while enabling sufficient heating in low coupling situations by increasing the number of radiating straps. Such an antenna consists of several sections each fed in a resonant ring configuration. In order to guide future engineering requirements, a detailed computation of the ohmic losses due to finite conductivity of the antenna components of one TWA section is performed for different loading conditions. The geometry of the TWA section is compatible with the presently proposed layout of DEMO and is modeled by the finite element code Microwave Studio (MWS) taking into account the finite resistivity of its constituent materials. As MWS cannot handle plasma loading, it has been implemented in the code by a layer in front of the antenna with the real part of the permittivity of water and open boundary conditions to reproduce single-pass absorption conditions and capture the main antenna loading characteristics of a plasma. The loading conditions are then varied changing the distance between the antenna and this layer. An equivalent antenna resistance characterizing those ohmic losses as a function of frequency, material resistivity and loading has been computed. This resistance can be linked to the skin depth resistance and compared to the radiation resistance in order to assess the performance of the antenna for a given choice of material. These results are then extrapolated to a real plasma loading based on ANTITER results for different plasma profiles and water dielectric load distance.