Electromagnetic design of an ICRH system for IGNITOR

Abstract

An antenna array is designed for ion cyclotron resonance heating (ICRH) for the IGNITOR experiment, planned to operate in the 70–140 MHz frequency range. The design is based on the conventional strap antenna element, and the coupling properties of the antenna are calculated with a slab model of the plasma. A comprehensive analysis of the antenna system performance is undertaken, including considerations on the tuning and matching system. For the antenna simulation, we consider first a two-dimensional problem to obtain the parameters of the transmission line model for the strap current; the current profile thus obtained is then used in a three-dimensional simulation of the antenna that yields the radiation resistance; the input impedance of the straps is then obtained from the radiation resistance and the line parameters. Two methods to obtain the strap input impedance are compared. The first is based on a distributed concept of the radiated power, i.e. introducing a resistance per unit length; the second is based on loss-less transmission lines and lumped resistances at the discontinuities to account for the radiated power. Applying the same method as for the straps (with lumped resistances), the effect of the feeders has also been analyzed. The global antenna in each port consists of four straps to form a 2×2 poloidal and toroidal phased array; in our proposed design each strap is fed by a radio frequency (RF) power generator via a coaxial cable and a tuning and matching system. The power spectrum of the radiated parallel index and the power handling capability are optimized in the middle of the frequency range, at 91 MHz, for out-of-phasing in order to obtain a high heating efficiency and a high radiation resistance. Within the overall mechanical design constraints, the predicted radiation resistance is sufficient to withstand ICRH experiments with 4 MW of power injected in the plasma per-port as long as the distance between the Faraday shield and the straps is smaller than 0.02 m, with a maximum with-standing RF voltage in the system of 35 kV. The concept of input impedance and effective resistance are employed throughout to monitor the performance of the antenna system.