The RF heating systems of italian DTT

The Divertor Tokamak Test (DTT) facility will be built to study a solution to the issue of power exhaust in conditions relevant for DEMO. The Italian DTT tokamak, by coupling to plasma up to 45 MW of additional power, will reach the needed condition of power flow to the divertor of 15 MW/m. The selected Heating Systems to achieve this goal are Electron Cyclotron Heating (ECH), Ion Cyclotron Heating (ICH) and Negative Neutron Beam Injector (NNBI). The power will be installed in two stages: a day-1 configuration with a coupled power of 25 MW and a second step where the completion of the 45 MW will be realized in 4 years from the day-1. At first stage 16 MW of ECH power and 4 MW of ICH will be installed, making the DTT plasma dominated by RF heating. The EC system is based on 170 GHz, 1 MW gyrotron, while for the transmission line a Quasi Optical approach has been chosen, with the feature to install the multibeam mirrors (8 beams on each one) under vacuum. The goal is to reduce the overall losses at ~10% avoiding atmospheric absorption and selecting the proper polarization for the longest section. The power will be injected into the tokamak using front steering individual antennas and capable to real time steer all the beams for the tasks assigned to EC waves. The first module of the ICH systems will be based on transmitters, capable of a wide frequency range (60-90 MHz), connected, though standard coaxial cables and RF components, to two movable antennas inserted in the equatorial ports of DTT. The selected range is done to exploit different heating schemes. The choice of the antenna type will be based on reliability (i.e. power density) rather than on its performance in term of peak coupled power. This led to choose a two-strap antenna with a power density of 3.5 MW/m2, shaped to fit the DTT scrape-off plasma and with an adjustable radial position. An external matching system is envisaged to cope with fast variation of antenna loading, e.g. due to edge localized modes.