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Dependence of EC injection power (0.6 and 1.4 MW), EC injection width (0.1–1.0 s) and prefilled He pressure (2.8×10−5∼2.2×10−3 mbar) on D2 outgas efficiency is investigated at a toroidal field of 2.4 T in a poloidal trapped particle configuration with an EC wave frequency of 140 GHz (X-mode) and a second harmonic resonance layer near the machine center (1.73 m). With increasing EC injection power, D2 outgas increases clearly. The dependence of the EC power injection is the most pronounced between the scanned parameters. It is found that an optimum prefilled He pressure for D2 outgas exists under the condition of a constant EC injection power. It is observed that both high EC injection power and the optimum prefilled He pressure enhance the ion flux to the center stack, suggesting that the D2 outgas originates from the center stack.

Results from KSTAR powder injection experiments, in which tens of milligrams of boron nitride (BN) were dropped into low-power H-mode plasmas, show an improvement in wall conditions in subsequent discharges and, in some cases, a reduction or elimination of edge-localized modes (ELMs). Injected powder is distributed by the plasma flow and is deposited on the wall and, over the course of several discharges, was observed to gradually reduce recycling by 33%, and decrease both the ELM amplitude and frequency. This is the first demonstration of the use of BN for ELM mitigation. In all of these experiments, an Impurity Powder Dropper (IPD) was used to introduce precise, controllable amounts of the materials into ELMy H-mode KSTAR discharges. The plasma duration was between 10 s and 15 s, Ip=500 kA, BT=1.8 T, PNBI=1.6 MW, and PECH=0.6 MW. Plasma densities were between 2 and 3×1019 m−3. In all cases, the pre-fill and startup gas-fueling was kept constant, suggesting that the decrease in baseline Dαemission is in fact due to a reduction in recycling. The results presented herein highlight the viability of powder injection for intra-shot and between-shot wall conditioning.

A new infra-red (IR) thermography system with high spatial resolution has been installed on KSTAR and is now mainly applied to measure the outer divertor heat load profile. The first measurement results of the outer divertor heat load profiles between ELMs have been applied to characterize the inter-ELMs outer divertor heat loads in KSTAR H-mode plasmas. In particular, the power decay length (λq) of the divertor heat load profile has been determined by fitting the profile to a convolution of an exponential decay and a Gaussian function. The analysis on the power decay length shows a good agreement with the recent multi-machine λq scaling, which predicts λq of the inter-ELMs divertor heat load to be ∼1 mm under the standard H-mode scenario in ITER. The divertor IR thermography system has also successfully measured the strike point splitting of the outer divertor heat flux during the application of resonant magnetic perturbation (RMP) fields. In addition, it has provided a clear evidence that the strike point splitting pattern depends on the RMP fields configuration.

Heat loads deposited on the first wall by mitigated Type I ELMs are expected to be the dominant contributor to the total thermal plasma wall load of the International Thermonuclear Experimental Reactor (ITER), particularly in the upper main chamber regions during the baseline H-mode magnetic equilibrium, due to the fast radial convective heat propagation of ELM filaments before complete loss to the divertor. Specific Type I ELMing H-mode discharges have been performed with a lower single null magnetic geometry, where the outboard separatrix position is slowly (∼7s) scanned over a radial distance of 7cm, reducing the wall probe–separatrix distance to a minimum of ∼9cm, and allowing the ELM filament heat loss to the wall to be analyzed as a function of radial propagation distance. A fast reciprocating probe (FRP) head is separately held at fixed position toroidally close and 4.7cm radially in front of the wall probe. This FRP monitors the ELM ion fluxes, allowing an average filament radial propagation speed, found to be independent of ELM energy, of 80–100ms−1 to be extracted. Radial dependence of the peak filament wall parallel heat flux is observed to be exponential, with the decay length of λq, ELM ∼25 ± 4mm and with the heat flux of q∥, ELM= 0.05MWm−2 at the wall, corresponding to q∥ ∼ 7.5MWm−2 at the second separatrix. Along with the measured radial propagation speed and the calculated radial profile of the magnetic connection lengths across the SOL, these data could be utilized to analyze filament energy loss model for the future machines.

Scrape-off layer (SOL) widths λq are presented from the KSTAR tokamak using fast reciprocating Langmuir probe assembly (FRLPA) measurements at the outboard mid-plane (OMP) and the infra-Red (IR) thermography at inboard limiter tiles in moderately elongated (κ = 1.45 – 1.55) L-mode inner wall-limited (IWL) plasmas under experimental conditions such as BT = 2.0 T, PNBI = 1.4 – 1.5 MW, line averaged densities 2.5 – 5.1 × 1019 m−3 and plasma current Ip = 0.4 − 0.7 MA. There is clear evidence for a double exponential structure in q||(r) from the FRLPA such that, for example at Ip = 0.6 MA, a narrow feature, λq,near (=3.5 mm) is found close to the LFCS, followed by a broader width, λq,main (=57.0 mm). Double exponential profiles (λq,near = 1.5 – 2.8 mm, λq,main = 17.0 – 35.0 mm) can be also observed in the IR heat flux mapped to the OMP throughout the range of Ip investigated. In addition, analysis of SOL turbulence statistics obtained with the FRLPA shows high relative fluctuation levels and positively skewed distributions in electron temperature and ion particle flux across the SOL, with both properties increasing for longer distance from the LCFS, as often previously observed in the tokamaks. Interestingly, the fluctuation character expressed in terms of spectral distributions remains unchanged in passing from the narrow to the broad SOL heat flux channel. Keywords: KSTAR, Inner wall limiter plasma, SOL width, Narrow feature, Electric probe, IR thermal image