New method for reshaping of the WEST Lower Hybrid launcher to reach long pulse operation

Abstract

The aim of WEST (tungsten-W Environment in Steady-state Tokamak) experiments is to master long pulses (1000s) and submit the ITER-grade tungsten divertor to power fluxes of 10 MW/m2. The lower hybrid current drive (LHCD) system, composed of a fully active multijunction (FAM-LH1) launcher and a passive active multijunction (PAM-LH2) launcher, with a total capability of 7 MW/1000s, is essential to meet these goals. The LH1 launcher was reshaped before the start of WEST to match the new plasma shape and thus improve its coupling capability. During the 2019–2021 experimental campaign, long pulse operation (∼ 55 s) was successfully and routinely performed with PLH = 3 MW and good LH coupling properties. Thanks to infrared monitoring during long pulses, the new toroidal shape of the LH1 launcher was found to avoid overheating on the front face edges. It also highlighted the existence of hot spots on the LH2 launcher's front face edges, limiting its power capability in high power and long pulse scenarios. A reshaping inside the vacuum vessel of the LH2 launcher was therefore performed during the shutdown in 2021. This paper describes the reshaping methods used for the two launchers and the tools developed for the reshaping of LH2 inside the tokamak.