C. R. Seon

Development of the prototype pneumatic transfer system for ITER neutron activation system

The neutron activation system (NAS) measures neutron fluence at the first wall and the total neutron flux from the ITER plasma, providing evaluation of the fusion power for all operational phases. The pneumatic transfer system (PTS) is one of the key components of the NAS for the proper operation of the system, playing a role of transferring encapsulated samples between the capsule loading machine, irradiation stations, counting stations, and disposal bin. For the validation and the optimization of the design, a prototype of the PTS was developed and capsule transfer tests were performed with the developed system.

Diagnostic neutron activation system for KSTAR

The Korea Superconducting Tokamak Advanced Research (KSTAR) device has begun deuterium plasma operation and increased neutron generation from D-D fusion reaction in the plasma is expected as the heating power of the plasma increases. A neutron activation system utilizing the pneumatic transfer of encapsulated metal samples has been implemented to monitor the fusion neutron source strength and the total fusion power from the KSTAR plasma. The prototype pneumatic transfer system for the ITER neutron activation system was slightly modified to be used for KSTAR, and a High Purity Germanium (HPGe) detection system was used to count gamma photons from the activated samples. The Monte Carlo code MCNP and the inventory code FISPACT were also used for the calculations to evaluate the total number of neutrons emitted from the D-D fusion reactions in the KSTAR plasma. The analysis of data from an activation measurement obtained from the 2011 KSTAR campaign shows the neutron flux at the irradiation station was 2.2 × 108 cm−2s−1, and the total neutron yield was 4.7 × 1013 n/s for a typical NBI-heated, H-mode KSTAR plasma shot.

Development of two-channel prototype ITER vacuum ultraviolet spectrometer with back-illuminated charge-coupled device and microchannel plate detectors

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.

Control of core argon impurity profile by ECH in KSTAR L-mode plasmas

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.

Test of prototype ITER vacuum ultraviolet spectrometer and its application to impurity study in KSTAR plasmas.

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.

Dust particle growth in rf silane plasmas using two-dimensional multi-pass laser light scattering

We measured a spatio-temporal distribution of particle size and a spatial growth rate in a capacitively coupled silane plasma using in situ multi-pass laser light scattering. The two-dimensional measurement was accomplished using a low power He–Ne laser and a set of spherical mirrors across the plasma that enables us to span multiple beam paths over the plasma region in the vertical direction from the electrode sheath to the bulk plasma. In temporal, the measurement result shows two particle growth periods in which the fast particle growth (nucleation) is followed by the slow particle growth (coagulation). In spatial, the fastest particle growth occurred at the highest vertical position that corresponds to the furthest position from the sheath. The particle coagulation modeling indicates that it is consistent with the largest proto particle creation rate in the plasma bulk.

Density measurement of particles in rf silane plasmas by the multipass laser extinction method

Measurement of the time evolution of the particle number density was investigated in rf silane plasmas by using the multipass laser extinction method. A He–Ne laser beam underwent multiple reflections on one horizontal plane of the plasma. The extinction signal increased in proportion to the beam pass numbers. A 1011cm−3 density of 8nm radius particles was measured at 10s in a 32mTorr and 50W discharge using nine passes. The primary particle density was obtained by comparing the measured particle sizes with the calculated sizes from the light extinction signals and the Brownian free molecule coagulation model.

Role of particle size and gas pressure on the nonlinear oscillatory behavior of a dust particle in a direct current discharge

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 μm) 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.

Correlation between nanoparticle and plasma parameters with particle growth in dusty plasmas

Since plasma parameters are altered by dust particles, studying how plasma parameters are related to dust particle growth is an important research issue in dusty plasma. In this paper, the correlation between plasma parameters (electron temperature and ion flux) and particle parameters (particle radius and density) is investigated in silane plasma both experimentally using a floating probe and theoretically by solving balance equations including an additional electron and ion loss to the dust. The results reveal that while the ion flux shows two peak values in the early discharge phase and at the end of coagulation phase, the electron temperature shows a sudden increase in the coagulation step and a gradual decrease in the molecular accretion step. Moreover, the calculated results with the secondary electron emission taken into account produce the best fit with the experimental results. Thus the study confirms that the secondary electron emission plays a crucial role in the coagulation of the dust particles.

Role of hydrogen in evolution of plasma parameters and dust growth in capacitively coupled dusty plasmas

The temporal behavior of naturally produced dust parameters (radius and density) and plasma parameters (electron temperature and ion flux) was investigated in radio frequency SiH4/H2/Ar plasmas. As a result, the electron temperature and ion flux were shown to be strongly correlated with the three-step dust growth pattern. In addition, the generation of dust particles was suppressed by mixing more hydrogen gas due to the plasma chemistry, and consequently, the dust growth rate in the molecular accretion growth, which is known to be proportional to the growth rate of thin film deposition, increased.