Development of an off-midplane launch lower hybrid antenna for improved core current drive in non-inductive start-up plasmas on TST-2.

Non-inductive plasma start-up using lower-hybrid waves (LHW) has been studied on the TST-2 spherical tokamak. Two different LHW launching scenario has been investigated using the outer-midplane antenna and the top antenna. According to the numerical calculation, plasma current is driven near the plasma edge and the orbit losses of fast electrons is thought to limit the current drive efficiency in these two scenarios. The off-midplane launch scenario was investigated for improved current drive closer to the magnetic axis using ray-tracing and Fokker-Planck code. The current drive simulation showed that plasma current twice as high as presently achieved may be obtained by the off-midplane launch scenario. The driven current was found to be robust against variation of density and temperature. Finally, substantial synergistic improvement may be obtained by simultaneous injection of both the outer-midplane and off-midplane launch. A new off-midplane antenna was fabricated and installed in TST-2. [ABSTRACT FROM AUTHOR]