Load-Tolerant External Matching Unit based on a Wideband Hybrid Coupler for the ICRH System of DTT

In the first phase of the Divertor Test Tokamak (DTT) [1], the goal of coupling a RF power of 3 MW to the plasma [2] will be reached with a single ICRH module constituted by two antennas, four external matching units (EMUs) and four generators. The operation with high levels of RF power into ELMy H-mode plasmas requires a robust and reliable Load-Tolerant External Matching Unit (EMU). Currently the most appropriate solution, ensuring a VSWR at the generators under the safety threshold of 1.5:1, will be based on a Wideband two-section coaxial Hybrid Coupler operating in the frequency range of 60-90 MHz. The RF design by means of HFSS and CST-MS codes consists in the optimization of the characteristic impedances and lengths of the transversal and longitudinal branches aimed at achieving excellent performances (amplitude and phase balance at the output ports, and low reflection at the input port) over the entire frequency range. The analysis has been carried out for Zo=50 ? and 30 ? coaxial transmission lines in terms of S-parameters and electric field. Several good solutions have been determined with an amplitude imbalance about ±0.25 dB and an input reflection lower than -20 dB in a large part of the frequency range. Then a circuit simulation of the overall (end-to-end) EMU (Wideband Hybrid Coupler with shifter, stub and service stub in each of the two output branches) exploiting a suitable tool of HFSS, has been implemented. The procedure repeated at three different frequencies (65 MHz, 75 MHz, 85 MHz) allows to investigate resilience and amplitude balance of the EMU for a typical fast loading variation occurring during ELMs [3].