Studies of various lower hybrid wave launch scenarios for non-inductive start-up on TST-2

Efficient non-inductive start-up of a spherical tokamak may be achieved by using lower hybrid waves that have high current drive efficiency. On the TST-2 spherical tokamak, three lower hybrid launchers are installed at the outer-midplane, top and outer-off-midplane to explore efficient start-up scenarios. The previously installed outer-midplane and top launchers were both found to drive current far off-axis where confinement of fast electrons were expected to be low. The outer-off-midplane launcher was newly developed to increase current drive by improving absorption closer to the magnetic axis. The outer-off-midplane launcher driven plasma was confirmed to have higher core electron temperature than those of the previous two launchers, and reduced fast electron losses compared to the top launcher.