Study on Pre-Ionization Using Second-Harmonic Electron Cyclotron Waves for the KSTAR First Plasma

Abstract

Korea Superconducting Tokamak Advanced Research (KSTAR) is adopting electron cyclotron heating (ECH)-assisted startup for the flexibility and reliability of KSTAR operation through preionization by using EC waves. An 84-GHz ECH system is being installed on the KSTAR tokamak, and the maximum available ECH power is presently 500 kW with a maximum duration of 2 s. A plasma breakdown voltage (or loop voltage) in the range of 4 V to 5 V is needed when fundamental harmonic EC power is used. Currently, second-harmonic ECH-assisted start-up is under consideration because a low toroidal field of BT ∼ 1.5 T is desirable for safety and because the high-beta experiments will be done in the initial operation phase. There have been limited studies of the efficacy of plasma breakdown by second-harmonic pre-ionization. The studies reported in this paper address the effectiveness of second-harmonic pre-ionization by 84-GHz EC-waves and alternatives to it for KSTAR. The optimum and threshold conditions for its successful start-up are calculated using a 0-dimentional time-dependent plasma-evolution code. The feasibility of second-harmonic breakdown of the KSTAR plasma is studied based on the recent experimental results in DIII-D tokamak on second-harmonic breakdown where 60 GHz and 110 GHz EC power was used. In this feasibility study, the ionization growth rate in G. L. Jackson’s theoretical model, which was used for the comparison with International Thermonuclear Experimental Reactor (ITER) second-harmonic pre-ionization, is also compared with KSTAR second-harmonic pre-ionization based on the DIII-D experimental results.