Analysis of a 8-Strap Plasma-Facing Launcher with Load-Tolerant External Matching Units for the ICRH System of DTT

The Italian Divertor Tokamak Test (DTT) facility, a new machine for fusion research, has been proposed by the Italian Fusion Community to study the problem of power exhaust with different divertor configurations in conditions close to the future DEMO reactor. In DTT three additional heating systems are foreseen: Negative Neutral Beam Injector (NNBI), Electron Cyclotron resonance Heating (ECRH) and Ion Cyclotron Resonance Heating (ICRH). This paper reports a preliminary design of the most promising antenna concept for the DTT ICRH system, and the comparison between two load-resilient external matching schemes. The ICRH system has a modular structure and, in the first phase of DTT, the goal of coupling a RF power of 3 MW to the plasma in the frequency range 60-90 MHz can be reached with a single module, which mainly consists of two antennas, four external matching units (EMUs) and four generators. As far as the antenna is concerned, an array of 8 straps was found to give better performances, compared to other types of ICRH antennas, in terms of power coupling capability, VSWR and minimum conductance (G-{min}). The antenna design has been firstly optimized with CST Microwave Studio at the central frequency of 75 MHz, using a movable water tank to emulate the magnetized plasma loading in front of the array. Then the antenna performances with realistic plasma load conditions have been carefully evaluated with suitable kinetic profiles. The most attractive matching scheme has been assessed by means of HFSS and CST, where a circuit simulation of the overall (end-to-end) RF system has been implemented. ICRH matching networks, realized with high power RF components in rigid coaxial cable (line stretchers, stubs, hybrid couplers, T-junction), are aimed at limiting the VSWR at the generators under the safely threshold of 1.5 : 1 by re-circulating the high reflected power toward the plasma without exceeding the maximum voltage of about 35 kV in the transmission line. Two different EMUs based on the conjugate-T and on the hybrid coupler have been studied by comparing the resilience to the strong loading variation due to the plasma instabilities in terms of S parameters, input VSWR, and electric fields. Finally adverse effects of asymmetries, as well as poloidal and toroidal mutual coupling between straps have been investigated too.