Suk-Ho Hong

Characteristics of the First H-mode Discharges in KSTAR

Typical ELMy H-mode discharges have been obtained in the KSTAR tokamak with the combined auxiliary heating of neutral beam injection (NBI) and electron cyclotron resonant heating (ECRH). The minimum external heating power required for the L?H transition is about 0.9?MW for a line-averaged density of ~2.0 ? 1019?m?3. There is a clear indication of the increase in the L?H threshold power with decreasing density for densities lower than ~2 ? 1019?m?3. The L?H transitions typically occurred shortly after the beginning of plasma current flattop (Ip = 0.6?MA) period and after the fast shaping to a highly elongated double-null divertor configuration. The maximum heating power available was marginal for the L?H transition, which is also implied by the relatively slow transition time (>10?ms) and the synchronization of the transition with large sawtooth crashes. The initial analysis of thermal energy confinement time (?E) indicates that ?E is higher than the prediction of multi-machine scaling laws by 10?20%. A clear increase in electron and ion temperature in the pedestal is observed in the H-mode phase but the core temperature does not change significantly. On the other hand, the toroidal rotation velocity increased over the whole radial range in the H-mode phase. The measured ELM frequency was around 10?30?Hz for the large ELM bursts and 50?100?Hz for the smaller ones. In addition, very small and high frequency (200?300?Hz) ELMs appeared between large ELM spikes when the ECRH is added to the NBI-heated H-mode plasmas. The drop of total stored energy during a large ELM is up to 5% in most cases.

Initial phase wall conditioning in KSTAR

The initial phase wall conditioning in KSTAR is depicted. The KSTAR wall conditioning procedure consists of vessel baking, glow discharge cleaning (GDC), ICRH wall conditioning (ICWC) and boronization (Bz). Vessel baking is performed for the initial vacuum conditioning in order to remove various kinds of impurities including H2O, carbon and oxygen and for the plasma operation. The total outgassing rates after vessel baking in three successive KSTAR campaigns are compared. GDC is regularly performed as a standard wall cleaning procedure. Another cleaning technique is ICWC, which is useful for inter-shot wall conditioning under a strong magnetic field. In order to optimize the operation time and removal efficiency of ICWC, a parameter scan is performed. Bz is a standard technique to remove oxygen impurity from a vacuum vessel. KSTAR has used carborane powder which is a non-toxic boron-containing material. The KSTAR Bz has been successfully performed through two campaigns: water and oxygen levels in the vacuum vessel are reduced significantly. As a result, KSTAR has achieved its first L–H mode transition, although the input power was marginal for the L–H transition threshold. The characteristics of boron-containing thin films deposited for boronization are investigated.

First results on disruption mitigation by massive gas injection in Korea Superconducting Tokamak Advanced Research

Massive gas injection (MGI) system was developed on Korea Superconducting Tokamak Advanced Research (KSTAR) in 2011 campaign for disruption studies. The MGI valve has a volume of 80 ml and maximum injection pressure of 50 bar, the diameter of valve orifice to vacuum vessel is 18.4 mm, the distance between MGI valve and plasma edge is ~3.4 m. The MGI power supply employs a large capacitor of 1 mF with the maximum voltage of 3 kV, the valve can be opened in less than 0.1 ms, and the amount of MGI can be controlled by the imposed voltage. During KSTAR 2011 campaign, MGI disruptions are carried out by triggering MGI during the flat top of circular and limiter discharges with plasma current 400 kA and magnetic field 2-3.5 T, deuterium injection pressure 39.7 bar, and imposed voltage 1.1-1.4 kV. The results show that MGI could mitigate the heat load and prevent runaway electrons with proper MGI amount, and MGI penetration is deeper under higher amount of MGI or lower magnetic field. However, plasma start-up is difficult after some of D(2) MGI disruptions due to the high deuterium retention and consequently strong outgassing of deuterium in next shot, special effort should be made to get successful plasma start-up after deuterium MGI under the graphite first wall.

Development of a particle injection system for impurity transport study in KSTAR.

A solid particle injection system is developed for KSTAR. The system has a compact size, compatibility with a strong magnetic field and high vacuum environment, and the capability to inject a small amount of solid particles with a narrow injection angle. The target flight-distance of 10 cm has been achieved with a particle loss rate of less than 10%. Solid impurity particles such as tungsten and carbon will be injected by this system at the midplane in KSTAR. The impurity transport feature will be studied with a soft X-ray array, a vacuum ultra-violet diagnostic, and Stand Alone Non-Corona code.

First comprehensive particle balance study in KSTAR with a full graphite first wall and diverted plasmas

The first comprehensive particle balance study is carried out in the KSTAR 2010 campaign with a full graphite first wall and diverted plasmas. The dominant retention is observed during the gas puffing into the plasmas. Statistical analysis shows that deuterium retention is increased with the number of injected particles. Particle balance analysis in the whole campaign shows that the long-term retention ratio is ~21%, and the retention via implantation can be partially recovered by He-glow discharge cleaning (GDC), while long-term retention via co-deposition. The wall pumping capability is decreased with the D2 plasma due to fuel accumulation in the first wall, and He-GDC is effective in recovering the wall pumping. Boronization assisted by the D2 glow discharge using C2B10H12 strongly enhances the wall puffing and leads to negative retentions, but the wall pumping capability is recovered in 2–3 days by He-GDCs. Electron cyclotron resonance heating enhances wall outgassing during the discharge. During a diverted H-mode discharge, the retention rate decreases to a very low value, and a high divertor particle flux of ~1.5 × 1023 D s−1 is observed indicating the strong recycling divertor. The amount of recovered deuterium after discharges mainly depends on the plasma–wall interaction when the plasma is terminated, and disruptive discharges release more particles from the first wall.

Probe diagnostics in the far scrape-off layer plasma of Korea Superconducting Tokamak Advanced Research tokamak using a sideband harmonic method

Plasma characteristics in the far scrape-off layer region of tokamak play a crucial role in the stable plasma operation and its sustainability. Due to the huge facility, electrical diagnostic systems to measure plasma properties have extremely long cable length resulting in large stray current. To overcome this problem, a sideband harmonic method was applied to the Korea Superconducting Tokamak Advanced Research tokamak plasma. The sideband method allows the measurement of the electron temperature and the plasma density without the effect of the stray current. The measured plasma densities are compared with those from the interferometer, and the results show reliability of the method.

Characteristics of global energy confinement in KSTAR L- and H-mode plasmas

We evaluate the characteristics of global energy confinement in KSTAR (?E,KSTAR) quantitatively in three ways; firstly by comparing it with multi-machine scalings, secondly by deriving multiple regression equations for the L- and the H-mode plasmas, respectively, and lastly by comparing confinement enhancement of the H-mode phase with respect to the L-mode phase in each discharge defined as Hexp. The KSTAR database exhibits ?E,KSTAR of ?0.04 to ?0.16?s and of ?0.06 to ?0.19?s in L-mode and in H-mode plasmas, respectively. The multiple regression equations derived by statistical analysis present the similar dependency on PL and higher dependency on Ip compared with the multi-machine scalings, however the dependency on ? in both L- and H-mode plasmas draw the negative power dependency of ??0.68 and ??0.76 for H-mode and for L-mode database, respectively on the contrary to the positive dependency in all multi-machine empirical scalings. It is found that the energy confinement of both L-mode and H-mode of the discharges with Hexp?>?1.5 can be well-predicted by multi-machine scalings, ?E,89L and ?E,92H. Apart from this, the H-mode confinement with 1.5?

Ion Cyclotron Wall Conditioning (ICWC) on KSTAR

Abstract Ion Cyclotron Wall Conditioning (ICWC) has been performed in KSTAR. Fuel retention and removal, impurity removal have been investigated in a dedicated session. By varying pressure (mixture rate) and duty cycle, parameter study has been done. An average hydrogen retention rate of ~2 × 1020 H/sec is measured. The ratio of Himplanted/Dpumped is found to be ~5-15 depending on the operation conditions. Other impurity removal rate is of the order of ~1016-1017 molecules/sec. It is shown that inter-shot ICWC is a powerful tool for superconducting tokamaks like KSTAR and ITER.

Developing in-situ ellipsometry for tokamak discharges in KSTAR

An in-situ ellipsometer based on four-detector polarimeter(FDP) is under development at KSTAR. In-situ ellipsometer for tokamak discharges will measure the characteristics of thin films deposited onto a quartz window near the edge region in real time. These characteristics contain local deposition/erosion rates as well as hydrogen to carbon ratio, which have to be measured in-situ, for more clear insight view of plasma-wall interaction in tokamak edge plasmas. This paper reports the status of in-situ ellipsometer development for tokamak discharges at KSTAR, to study plasma-wall interaction and fuel retention. Basic concept, design and construction of the ellipsometer are described.

Plasma-surface interaction activities in KSTAR

Selected topics of Plasma-Surface Interaction (PSI) activities in KSTAR are briefly introduced. SOL parameter measurements, particle balance and fuel retention, in-vessel dust research, and finally tungsten R & D are discussed. Some quantitative numbers from the initial phase of the operation are given for comparison with that of other machines.