H.L. Yang

The design of the KSTAR tokamak

Abstract The Korea Superconducting Tokamak Advanced Research (KSTAR) Project is the major effort of the Korean National Fusion Program (KNFP) to develop a steady-state-capable advanced superconducting tokamak to establish a scientific and technological basis for an attractive fusion reactor. Major parameters of the tokamak are: major radius 1.8 m, minor radius 0.5 m, toroidal field 3.5 Tesla, and plasma current 2 mA with a strongly shaped plasma cross-section and double-null divertor. The initial pulse length provided by the poloidal magnet system is 20 s, but the pulse length can be increased to 300 s through non-inductive current drive. The plasma heating and current drive system consists of neutral beam, ion cyclotron waves, lower hybrid waves, and electron–cyclotron waves for flexible profile control. A comprehensive set of diagnostics is planned for plasma control and performance evaluation and physics understanding. The project has completed its conceptual design phase and moved to the engineering design phase. The target date of the first plasma is set for year 2002.

ECH pre-ionization and assisted startup in the fully superconducting KSTAR tokamak using second harmonic

This letter reports on the successful demonstration of the second harmonic electron cyclotron heating (ECH)-assisted startup in the first plasma experiments recently completed in the fully superconducting Korea Superconducting Tokamak Advanced Research (KSTAR) device whose major and minor radii are 1.8 m and 0.5 m, respectively. For the second harmonic ECH-assisted startup, an 84 GHz EC wave at 0.35 MW was launched before the onset of the toroidal electric field of the Ohmic system. And it was observed that this was sufficient to achieve breakdown in the ECH pre-ionization phase, allow burn-through and sustain the plasma during the current ramp with a low loop voltage of 2.0 V and a corresponding toroidal electric field of 0.24 V m−1at the innermost vacuum vessel wall (R = 1.3 m). This is a lower value than 0.3 Vm−1 which is the maximum electric field in ITER. Due to the limited volt-seconds and the loop voltage of the Ohmic power system, the extended pulse duration of the ECH power up to 180 ms allowed the plasma current to rise up to more than 100 kA with a ramp-up rate of 0.8 MA s−1.

Progress of KSTAR 5-GHz Lower Hybrid Current Drive System

Abstract A 5-GHz steady-state lower hybrid (LH) current drive (LHCD) system is planned to support steady-state and advanced tokamak operation on the Korea Superconducting Tokamak Advanced Research (KSTAR) experiment. As an initial phase, a pulsed 5-GHz, 500-kW LHCD system has been installed in KSTAR for basic experimental studies of the LH coupling and flux saving in the plasma current ramp-up, prior to long-pulse noninductive operation in KSTAR. A Toshiba-made klystron developed in collaboration with Pohang University of Science and Technology in 2006 is utilized for the initial KSTAR LHCD system. The LH launcher is designed as a fully active waveguide grill type with a parallel refractive index n[parallel] value ranging from 1.8 to 4.3 and with high directivity. In the initial stage, the LH launcher consists of eight columns of four-way power splitters and two columns of dummy waveguides, one on each side. The operational n[parallel] value is fixed at 2.1 but can be adjusted by replacing waveguide components external to the vacuum vessel. Since the target operation pulse duration of the initial LHCD system is 2 s with an output power of 500 kW at the klystron window, the prototype klystron was recently successfully conditioned to a radio frequency power of 514 kW for a maximum pulse duration of 3 s using a matched dummy load (voltage standing wave ratio of 1.16:1). This paper presents the progress of the initial KSTAR LHCD system and the performance test results of the prototype klystron. The research plan aiming at steady-state LHCD operation in KSTAR is also described in this paper.

Electron Cyclotron Heating and Current Drive Program for KSTAR Based on the 170-GHz Gyrotron

Abstract Electron cyclotron heating and current drive (ECH/ECCD) has become an essential tool for fusion plasma research in toroidal devices. In the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak, development of a high power and multifrequency ECH/ECCD system is in progress. The multiple frequency sources employed in KSTAR (84 GHz and 110 GHz have been used, and 170 GHz and possibly 140 GHz are planned) support the wide range of operating magnetic fields from [approximately]1.5 to 3.5 T. In particular, 170-GHz power, which will be used on ITER, corresponds to the second harmonic of the cyclotron frequency for the KSTAR operating range from 2.5 to 3.5 T. This frequency will be mainly used for control of the local plasma current profile, in order to manipulate the internal magnetohydrodynamic instabilities such as the sawtooth and neoclassical tearing mode, which can be harmful to steady-state high-beta operation. This paper presents the status of the KSTAR ECH/ECCD program and the ray-tracing calculations of the 170-GHz electron cyclotron wave propagation for various plasma conditions in KSTAR. In the ray-tracing simulation, the TORAY-GA ray-tracing code is used to study the dependence of the ECH/ECCD on the plasma profiles as a function of the beam aiming angles.

Mode Content Study of Propagating Waves Using Burn Patterns in the KSTAR 84-GHz ECH System

Abstract In order to transmit a wave efficiently in an electron cyclotron heating (ECH) system, it is important to suppress mode conversion loss caused by coupling in the matching optics unit and misalignment in the transmission line. To understand the cause of mode conversion loss, it is necessary to analyze the mode content in an oversized circular corrugated waveguide. For mode analysis of the propagating wave in the corrugated waveguide, several methods based on the phase-retrieval process and the iterative process are suggested. But, in the Korea Superconducting Tokamak Advanced Research 84-GHz ECH transmission line, a well-known method using burn patterns was used for better coupling of the output beam from the gyrotron onto the axis of the corrugated waveguide by adjusting a large ellipsoidal mirror in an L-shaped chamber, a so-called L-box. During the adjustment of the mirror in the L-box, evidence of the existence of higher modes other than HE11 was found. For the mode content study, the radiation intensity distribution was measured using thermal paper as a function of the distance along the waveguide at a high power level. The mode content of the wave was estimated by comparing the measured burn patterns and calculated patterns at different locations. This paper describes the results of mode content estimation using burn pattern images as a function of the mode mixture ratio.

Status and characteristics of diagnostics on Korea Superconducting Tokamak Research (KSTAR)

Diagnostic plan for KSTAR device to support the project goal and physics mission is presented in this paper. Merits and weaknesses of some specific diagnostics which could impact the physics mission of KSTAR are discussed in detail.