M. Joung

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.

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.

Variation of plasma parameters on boundary conditions in an inductively coupled plasma source for hyperthermal neutral beam generation

The variations of plasma parameters on the boundary conditions, especially potential, of plasmas were measured in an inductively coupled plasma source developed to generate a hyperthermal neutral beam. Hyperthermal neutrals can be produced by Auger neutralization when ions with low energy are neutralized by impinging on a metal surface called a reflector. However, the reflector is a significant source of ion drain when it is biased to a negative potential. The plasma potential can be negative with respect to the grounded chamber potential while the reflector is negatively biased, but other plasma parameters, namely density and temperature, are not sensitive to the reflector bias. If the electron loss current into the chamber wall is governed by the space charge limited current law, sustainment of the plasma with a negative potential can be explained in terms of the charge balance equations for quasineutrality.

Effects of the collimator on the plasma parameters in a hyper-thermal neutral beam source

Summary form only given, as follows. Hyper-thermal neutral beam sources are developing for the next generation semiconductor manufacturing device. The collimator in the device contacts the plasma to extract ion beam and exchange the incident ion beam to neutral beam resulted in Auger effect on the collimator surface layer. Hence, the neutral beam energy is proportional to the voltage difference between the plasma potential and bias voltage on the collimator. However, the biased collimator affects on the plasma parameters, especially the plasma potential In this presentation, the effects of the collimator biasing on the plasma parameters have been investigated by using various diagnostics tools, Langmuir probe, capacitive probe, emissive probe and grid ion energy analyzer. Ar and He plasma were used for the research. In addition, the effects of the collimators physical size will be discussed on the plasma parameters.