Pre-ionization and plasma startup by second harmonic electron cyclotron wave for KSTAR first plasma

Second harmonic electron cyclotron heating (ECH)-assisted startup has been carried out for the fully superconducting KSTAR first plasma. The 84 GHz electron cyclotron (EC) wave, which is the second harmonic EC resonance frequency at R~1.7 m with the toroidal magnetic field of 1.5-T, is launched in the linearly polarized pure extraordinary (X2) mode from the low field side. An EC power $\sim$350 kW was sufficient to obtain a reliable KSTAR first plasma with a loop voltage of 2 V (0.23V/m) at the innermost vacuum vessel wall and the hydrogen pre-fill gas pressure of 3.5e-5 mbar. In this paper, the experimental results of the second harmonic ECH pre-ionization will be presented.