Development of an outer-off-midplane lower hybrid wave launcher for improved core absorption in non-inductive plasma start-up on TST-2

A new outer-off-midplane lower hybrid (LH) wave launcher was developed for improved core absorption in the TST-2 spherical tokamak. In the previously developed outer-midplane and top launch scenarios, plasma current was driven only very near the plasma edge (r/a > 0.7), resulting in limited driven current density. Strong thick-target x-ray radiation has also been observed for the top launch scenario, which indicated LH wave driven fast electron losses. The outer-off-midplane launch scenario was designed to drive current at the core (r/a similar to 0.5) at low phase velocity to suppress production of unnecessarily high energy electrons. In the non-inductive plasma start-up experiment with the new outer-off-midplane launch scenario, improved core electron heating was confirmed by the Thomson scattering diagnostic that showed the electron temperature was about twice as high as the previous two scenarios. Reduction of the LH wave driven fast electron losses was confirmed by the measured x-ray radiation intensity that was substantially lower than the top launch scenario. It was also easier to maintain the optimum level of the electron density compared to the outer-midplane launch scenario that indicated the undesired LH wave interactions with the scrape-off-layer plasma may have been reduced due to strong core absorption.