Experiments on trace argon impurity transport in L-mode discharges were performed on Korea superconducting tokamak advanced research (KSTAR) with electron cyclotron resonance heating (ECH). Ar emission was measured by soft x-ray (SXR) arrays and vacuum UV (VUV) diagnostics. A significant reduction in the core Ar emissivity was observed with core ECH. The reduction was the largest with on-axis heating and became smaller with outward heating positions. The diffusivity and convection velocity of Ar were obtained by analysis of the SXR data with the SANCO impurity transport code for the on-axis ECH and the non-ECH shots. In the on-axis ECH case, both diffusivity and convection velocity increased. Furthermore, the convection changed its direction from inward to outward in the plasma core (r/a < 0.3), resulting in a hollow profile of the total Ar density. Together with the reduction in the SXR signals, the hollow impurity profile in the core and the reversal of the convection velocity consistently confirm that ECH can reduce impurity accumulation in the core region. Neoclassical impurity transport and linear stability of micro-turbulence were calculated and discussed in relation to the possible transport mechanism.

Experimental and theoretical studies were conducted to simultaneously study the role of particle size and gas pressure on nonlinear behavior of dust oscillations in a plasma. Oscillation spectra were experimentally obtained by using four different sized (1.5, 2, 3, and 4 m) particles at 250 mTorr, which is high pressure compared to previously reported works. The measured results were in good agreement with theoretical calculations based on a self-consistent collisional plasma model and a parametric dust oscillation model. In addition, particle size and gas pressure dependence of force profile and oscillation spectrum was investigated by numerical calculation in order to understand the role of particle size and pressure separately. It is concluded that occurrence of the subharmonic resonance and the net trapping force profile are mainly determined by particle size while gas pressure is mostly responsible for the superharmonic resonance and hysteresis.

A vacuum ultraviolet (VUV) spectrometer of a five-channel spectral system is designed for ITER main plasma impurity measurement. To develop and verify the system design, a two-channel prototype system is fabricated with No. 3 (14.4-31.8 nm) and No. 4 (29.0-60.0 nm) among the five channels. The optical system consists of a collimating mirror to collect the light from source to slit, two holographic diffraction gratings with toroidal geometry, and two different electronic detectors. For the test of the prototype system, a hollow cathode lamp is used as a light source. To find the appropriate detector for ITER VUV system, two kinds of detectors of the back-illuminated charge-coupled device and the microchannel plate electron multiplier are tested, and their performance has been investigated.

Experiments with a small amount of Ar gas injection as a trace impurity were conducted in the Korea Superconducting Tokamak Advanced Research (KSTAR) H-mode plasma (= 2.8 T, = 0.6 MA, and = 4.0 MW). 170 GHz electron cyclotron resonance heating (ECH) was focused along midplane with fixed major radial position of = 1.66 m and scanned its power from 0 to 800 kW. The emissivity of the Ar 16+ (3.949 Å) and Ar 15+ (353.860 Å) spectral lines were measured by the X-ray imaging crystal spectroscopy (XICS) and vacuum UV (VUV) spectrometer, respectively. The peak emissivity of Ar 15+ was reduced by ECH, an effect largest with 800 kW compared to 600 kW of ECH. The Ar 16+ emission increased with higher heating power. The ADAS-SANCO impurity transport code was used to evaluate the Ar transport coefficients. It was found that the inward convective velocity found in the plasma core without ECH was decreased with the ECH, while diffusion remained approximately constant resulting in a less-peaked Ar density profile. Theoretical results from the NEO code suggest that neoclassical transport is not responsible for the change in transport, while the microstability analysis using GKW suggests a dominant ITG mode during both ECH and non-ECH plasmas.

Articles you may be interested inEdge impurity rotation profile measurement by using high-resolution ultraviolet/visible spectrometer on J-TEXTa) Rev. Sci. Instrum. 85, 11E423 (2014) To optimize the design of ITER vacuum ultraviolet (VUV) spectrometer, a prototype VUV spectrometer was developed. The sensitivity calibration curve of the spectrometer was calculated from the mirror reflectivity, the grating efficiency, and the detector efficiency. The calibration curve was consistent with the calibration points derived in the experiment using the calibrated hollow cathode lamp. For the application of the prototype ITER VUV spectrometer, the prototype spectrometer was installed at KSTAR, and various impurity emission lines could be measured. By analyzing about 100 shots, strong positive correlation between the O VI and the C IV emission intensities could be found.