Multiphysics Analysis of High-power Vacuum Feedthrough for DTT ICRH System

In the framework of the Divertor Tokamak Test (DTT) facility, which aims at the validation of an integrated solution for the power exhaust moving to the DEMOnstration power plant (DEMO), an Ion Cyclotron Resonance Heating (ICRH) system is under development. Its final configuration, which comprises two independent modules, shall couple 6MW to the DTT plasma, in a pulsed regime (pulse length 50 s, duty cycle 1.4%), in the 60-90MHz frequency range. The Vacuum Feedthrough (VFT), usually made with one or two alumina windows, assures the reliable separation between the pressurized Main Transmission Line (MTL) and Vacuum Transmission Line (VTL) sections, which is located close to the plasma-facing antenna. It is one of the most critical components of the ICRH system because it must protect the tokamak vacuum and withstand a high RF voltage in a resonant transmission line (RTL) based on rigid coaxial cable. Results of RF, thermodynamic, and mechanical simulations of RTLs, constituted by an impedance transformer, an RF divider (T-junction), and two output branches, each with a VFT, are reported. In our simulations, the effective antenna impedance is substituted with an equivalent resistive load (Rs), representing the coupling resistance locating the RTL outputs in a voltage node. Thermodynamic and mechanical simulations of matched RTLs, with two windows for each branch, were conducted in the worst condition, i.e., imposing the maximum working frequency (90 MHz) and the minimum value of Rs (0.5 Ohm). In all analyses, an RF power of 1.2MW (600kW each VFT) for a pulse of 50 s has been injected into the RTL. VFTs with conical and disk geometry types with Alumina 99 windows were studied. Considering only the unforced convection of the outer conductor, significant temperature and thermal stresses were obtained for both window types. The conical window reaches a maximum thermal stress of 140MPa, more than half of the Ultimate Tensile Strength of Alumina 99, i.e., 260MPa. Instead, given the same boundary conditions, the disk window reaches a maximum thermal stress of 35MPa. The thermal difference between the edges and the center obtained for both windows exceeds 30 degrees. The leading cause is the ohmic loss of the metals. The mechanical design is preliminary, but Multiphysics simulations already allow us to choose materials, geometry, and cooling method of VFT.