Analysis of the FAST ICRH antenna with salted water load using HFSS code

In most tokamak machine for researches on Fusion Energy usually the dominant auxiliary heating system is the Ion Cyclotron Resonance Heating (ICRH). In FAST (Fusion Advanced Studies Torus), a compact tokamak with high toroidal magnetic field (8.5 T) and plasma current (up to 8 MA), the design foresees an ICRH system able to couple a RF power up to 30 MW through six antennas, in the 60 - 90 MHz frequency range. FAST is a new tokamak machine proposed as a Satellite Tokamak for accompanying the International Thermonuclear Experimental Torus (ITER) programme. Each antenna consists of eight straps arranged in a four rows by two columns array. Many of the RF characteristics of the antenna can be simulated with a good accuracy, using a commercial code as HFSS, by replacing the magnetized plasma with a high permittivity dielectric. Salty water solution can advantageously be used as dummy load. All the relevant parameters of the antenna have been carefully evaluated in a large range of straps´ loading by varying the distance between straps and dummy load. For a given maximum voltage sustainable in the line without arcing or breakdown, the power handling capability of the RF system and the improvements in the minimum conductivity (gmin) of the antenna array, produced by inserting in each transmission line a service stub, used for a cooling issue, have been studied. The effects of asymmetries between central and side straps and the impact of the presence of the Faraday Shield (FS) is also investigated. Finally the resilience to the loading variation of a conventional matching unit, made by shifter and stub, is compared to a load tolerant matching based on the Conjugate-T concept.