Design and analysis of a PAM launcher at 4.6 GHz for a new LHCD system on EAST

Abstract

To improve the current drive (CD) capability in long-pulse (up to ~ 1000 s) H-mode operation, it has been decided to develop a new lower hybrid current drive (LHCD) system at 4.6 GHz with an active cooling passive active multijunction (PAM) launcher on EAST. In this paper, both the RF and the physical properties of this PAM are studied numerically. The same nominal parallel refractive index (N|| = k||c/ω, where k|| is the parallel wavenumber, c the velocity of light, and ω the wave frequency) of 2.04 as the existing 4.6 GHz full active multijunction (FAM) is chosen. Ray-tracing calculations indicate that good accessibility could be achieved when the LH waves radiate with this nominal N|| in typical long-pulse H-mode plasmas. The coupling performance in terms of power reflection coefficient (RC), power spectrum, maximum electric field, power directivity (DP) and global CD capability is evaluated with the ALOHA code based on the linear coupling theory. Good coupling performance with averaged RC ≦ 1% and DP ~ 70% could be expected with the density (ne) in front of the PAM close to the cut-off value (ne_co). The simulated RC remains below 6.5% over a wide density range 0.5 ≦ ne/ne_co ≦ 10, which is similar to the plasma edge conditions produced by edge localized mode (ELM) activity. A detailed comparison with the existing 4.6 GHz FAM launcher is also performed.