ELECTROMAGNETIC DESIGN OF AN ICRH SYSTEM FOR IGNITER

An antenna array is designed for ion cyclotron resonance healing (ICRH ) for the IGNITOR experiment, planned to operate in the 70-140 MHz fre quency range, The design is based on the conventional strap antenna el ement, and the coupling properties of the antenna are calculated with a slab model of the plasma. A comprehensive analysis of the antenna sy stem 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 l ine 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 o btained from the radiation resistance and the line parameters. Two met hods to obtain the strap input impedance are compared. The first is ba sed on a distributed concept of the radiated power, i.e. introducing a resistance per unit length; the second is based on loss-less transmis sion lines and lumped resistances at the discontinuities to account fo r 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 x 2 poloidal and toroidal phased array, in our proposed design each st rap is fed by a radio frequency (RF) power generator via a coaxial cab le and a tuning and matching system. The power spectrum of the radiate d parallel index and the power handling capability are optimized in th e middle of the frequency range, at 91 MHz, for out-of-phasing in orde r to obtain a high heating efficiency and a high radiation resistance. Within the overall mechanical design constraints, the predicted radia tion resistance is sufficient to withstand ICRH experiments with 4 MW of power injected in the plasma per-port as long as the distance betwe en the Faraday shield and the straps is smaller than 0.02 m, with a ma ximum with-standing RF voltage in the system of 35 kV. The concept of input impedance and effective resistance are employed throughout to mo nitor the performance of the antenna system. (C) 1998 Elsevier Science S.A. All rights reserved.