Please cite this article in press as: J.H.C.M. Belo, et al., Optimization of a lower hybrid current drive launcher for ITER, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.01.056 ARTICLE IN PRESS G Model FUSION-7756; No. of Pages 7 Fusion Engineering and Design xxx (2015) xxx–xxx Contents lists available at ScienceDirect Fusion Engineering and Design jo ur nal home p age: www.elsevier.com/locate/fusengdes Optimization of a lower hybrid current drive launcher for ITER Jorge H.C.M. Belo a,∗ , Marc Goniche b , Julien Hillairet b , João P.S. Bizarro a a Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal b CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France h i g h l i g h t s • Reflection, directivity and E-fields of LHCD PAM launchers for ITER investigated. • Wide range of antenna parameters (junction lengths; phase-shifter heights) regarded. • Broad range of edge plasma considered: from the cut-off density to ELM activity. • Trade-offs between plasma density, reflection coefficient and E-field are necessary. • Additional margins for integration of the launcher in ITER may be achieved. a r t i c l e i n f o Article history: Received 5 October 2014 Received in revised form 16 December 2014 Accepted 12 January 2015 Available online xxx Keywords: Passive active multijunction (PAM) Lower hybrid current drive (LHCD) ITER a b s t r a c t An international R&D program for lower-hybrid current drive (LHCD) in ITER is being conducted to deliver 20 MW (CW) using 500 kW klystrons at 5 GHz, with N ||peak = 2.0 ± 0.2 for different plasma scenarios. The launcher is based on the passive-active mulitjunction (PAM), a concept more resilient to conditions expected at the plasma edge, notably densities close to cut-off (n ec ) and ELM activity, which lead to significant and abrupt reflection of RF power from the plasma, but even under which it may still attain extremely low power reflection coefficients at the input (R ∼ 1%). It has also a robust and shielded struc- ture; is suitable for long-pulse operation; and has been validated experimentally on FTU and Tore Supra. Here the focus is on the PAM section of the launcher, and the objective is to explore, under broad plasma loading – from n ec to 10 n ec – the impact that design parameters such as the junction lengths, phase-shifter heights, and output waveguide widths have on its performance, particularly on R and on the E-fields inside its waveguides; and to explore also a configuration with a different phase-shifter arrangement, the so-called alternative design. © 2015 Published by Elsevier B.V. 1. Introduction The main objective of ITER is to demonstrate that an energy amplification factor Q of 10 can be sustained for 400 s. In addition, it is planned, in a second phase, to extend the plasma duration to 3000 s with Q = 5. This requires a modification of the plasma cur- rent profile and, moreover, external sources driving current in the outer part of the plasma. Of the current drive (CD) solutions devel- oped so far, lower hybrid (LH) is recognized to be the most efficient for driving current far off-axis. Furthermore, modeling of LHCD for the ITER steady-state scenario has shown that up to 1 MA could be driven with 20 MW of RF power, when the wave parallel index of refraction N || is properly adjusted (N ||peak = 1.9–2.0) to maximize ∗ Corresponding author. Tel.: +351 218417696. E-mail address: jbelo@ipfn.ist.utl.pt (J.H.C.M. Belo). the LHCD efficiency while keeping the penetration and absorption of the wave at the required position (normalized radii r/a ∼ 0.7) [1]. Besides, LHCD is fundamental for the inductive flux saving needs. It is under this backdrop that an international R&D program for LHCD in ITER is being conducted, under which a conceptual design of the full ITER LHCD system – from the klystron RF power sources to the LH multijunction launcher – was produced [2], aiming to be considered as an option for the ITER operational phase. Still, this program revealed that there may be tight margins for integration of the launcher in ITER associated with the length and weight of the antenna, and with neutron loading on the RF windows. With the other components of the launcher but the multijunction already highly optimized, this raised a question: is it possible to increase these margins with the multijunction while optimizing its perfor- mance? This paper focus on this key component: the PAM multijunc- tion – the final link of the LHCD system – and on the work being http://dx.doi.org/10.1016/j.fusengdes.2015.01.056 0920-3796/© 2015 Published by Elsevier B.V.