Effect of poloidal phasing on ion cyclotron resonance heating power absorption

摘要

Two ion cyclotron resonance heating (ICRH) systems are planned for ITER, each system containing 24 antennas distributed as a two by four array of poloidal triplets. The ITER antennas are designed to operate at a poloidal phase difference between the upper and lower triplet of Delta theta(pol) = +/- 90 degrees in the antenna currents. Since current tokamak experiments normally operate at Delta theta(pol) = 0 degrees, experience from ICRH schemes with Delta theta(pol) not equal 0 is lacking. In this paper, the effects of poloidal phasing on ICRH power absorption and coupling are studied using the novel code FEMIC, which is described here. Simulations of the ITER antenna and the JET ITER-like antenna show that increasing the poloidal phase difference increases the destructive interference of the fast magnetosonic wave near the equatorial plane. This causes a degradation of the on-axis heating performance and reduces the total coupled power to the plasma. Best on-axis heating was obtained for Delta theta(pol) = 0 degrees, resulting in peaked profiles. By increasing the poloidal phase difference the absorption profiles tend to become less peaked or hollow on-axis. The effect is localized and occurs for rho(pol) less than or similar to 0.1, i.e. near the magnetic axis. The total coupled power was found to be asymmetric around Delta theta(pol) = 0 degrees due to the plasma gyrotropy, where the maximum coupled power occurs within -33 degrees less than or similar to Delta theta(pol) less than or similar to - 22 degrees on ITER and JET. The exact location of the maximum depends on the width of the pedestal. The strength of the asymmetry increases with the pedestal width.