Korea Superconducting Tokamak Advanced Research Plasma Research Articles

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

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.

Status of KSTAR Electron Cyclotron Heating System

An 84-GHz, 500-kW electron cyclotron (EC) heating (ECH) system is under installation for ECH-assisted start-up in the Korea Superconducting Tokamak Advanced Research (KSTAR) facility. An 84-GHz, 500-kW gyrotron, and 1.5-MVA power supply system have been installed at KSTAR, and the initial test of the gyrotron has been carried out with a short-pulse condition of 20 μs and maximum beam parameters of 80 kV and 25 A that generate an output radio-frequency (rf) power of 500 kW. The planned 2-s-long operation with 500-kW rf output power is beginning with a long-pulse test of the gyrotron power supply. The launcher system was fabricated in collaboration with Princeton Plasma Physics Laboratory. It will inject 500-kW rf power into the KSTAR plasma with a highly flexible steering mirror system, allowing toroidal and poloidal beam deposition scans. KSTAR will employ 170-GHz EC current drive (CD) in ITER-relevant experiments such as the suppression of the neoclassical tearing modes and the creation of an electron internal transport barrier. The Japan Atomic Energy Agency will provide a 170-GHz, 1-MW gyrotron on loan in 2008 in accordance with a Korea-Japan fusion collaboration agreement, and it will be used for the 170-GHz, 1-MW ECCD system in 2010. This paper describes the current status of the installation and initial conditioning tests of the 84-GHz gyrotron system as well as the development plan of the 170-GHz ECH and CD system. Also, this paper discusses the CD efficiency and the steering range of the second-harmonic X-mode injection at 170 GHz and 5 MW from an equatorial launcher.

Design concepts for KSTAR plasma control system

We present design concepts and features of the plasma control system (PCS) in Korea Superconducting Tokamak Advanced Research (KSTAR). A design structure of the PCS is proposed as an effort to achieve research objectives of the KSTAR project. The PCS architecture is characterized by the real-time data communication using reflective-memory network, the hierarchical organization of dedicated controllers, and the integrated generation of actuator signals. Discussions are made of the functions and present design choices of the KSTAR PCS.

Numerical study for design of the passive stabilizer and its impact on MHD stability in the proposed KSTAR plasma

The effect of the passive plate stabilizer on the ideal magnetohydrodynamic (MHD) stability is numerically studied in order to guide its design in the proposed Korea Superconducting Tokamak Advanced Research (KSTAR). The parametric study is systematically performed by taking into account the two major variables: the plate’s distance from the plasma surface, and the poloidal (toroidally continuous) vacuum gap required for the access of neutral beam ports and other diagnostics near the outboard mid-plane. Extensive calculations are carried out on the plasma beta limits for low- n ( n being toroidal mode number) MHD modes in several major operating regimes. The results lead to a practical and optimistic design point: plasma-wall separation ≈10 cm, and the outboard gap having a vertical separation of about 80 cm between the upper and lower plates. This is one of the choices that will make it feasible to achieve the advanced operation with bootstrap current fraction of over 80% at β n =5 in the reversed shear mode in KSTAR.