Divertor plasma detachment studies in KSTAR by experiments and modelling = KSTAR 디버터 플라즈마 분리 현상 실험 및 전산모사 해석

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Plasma-surface interactions within the tokamak are inevitable. The material separated from the wall flows into the plasmas, and the plasma facing components itself acts as a source and sink of particles and heat, affecting the plasma of the entire region of the tokamak. Particularly, in the case of a tokamak having a divertor structure, significant heat and particle flow along the magnetic field strike the target surface. As long as the solid material is used as a divertor target material, the target heat flux can exceed the sustainable limit of the material. Therefore, excessive divertor heat flux control is required to ensure sustainable operation of future devices such as ITER and K-DEMO. The detached divertor operation presented as a solution to this problem is very important because they are attracted attention as future operation scenarios in ITER and K-DEMO. Despite its importance, some of the physical mechanisms for the transition to detached divertor regime still remains elusive due to the difficulty of diagnosing and complexity of the modelling. Characteristics of the divertor detachment depend on the device, as it depends on the divertor geometry, the wall material, and the magnetic configuration. It is required to figure out the key mechanism by comparing the results from various devices. In this thesis, the experiment and modelling of KSTAR plasmas were conducted under various divertor conditions to understand the characteristics of edge and divertor plasma, focusing on divertor detachment phenomena. In the 2017 KSTAR campaign, both attached and detached plasma conditions were obtained by scanning the upstream density levels through D2 gas puffs. Asymmetric detachment between inner and outer targets was observed by Langmuir probe measurements. The experimental results were qualitatively consistent with the predictions of the SOLPS-ITER modelling in which the outer (low-field side) target is detached at lower upstream densities than the inner (high-field side) target. Density scans were performed using the SOLPS-ITER code to investigate the main mechanism leading to asymmetric detachment. SOLPS-ITER code results were post-processed according to the two-point formatting method. It demonstrates that the main cause of the target particle flux roll-over, which indicates the beginning of the divertor detachment, is the volumetric loss of the momentum and power inside the flux tubes. The source and sink of the momentum and power in the balance equation are decomposed based on the physical mechanism. The main causes of momentum and power loss are interactions between plasma and neutrals (neutral atoms and molecules). The trajectory of the kinetic neutral particles calculated from the EIRENE code shows the asymmetric distribution of neutral particles between the inner and outer targets. The asymmetry of neutral particles is attributed to the specific divertor geometry of KSTAR. Correlation analysis demonstrates that deuterium molecules are primarily responsible for the rapid momentum loss where electron temperature below 5 eV. This revealed that the asymmetric distribution of recycled neutral particles due to the divertor geometry caused the loss of volumetric momentum and power asymmetrically and that the outer target detaches at lower upstream density than the inner target. In the density ramp experiments performed in the 2017 and 2018 KSTAR campaigns, the line-averaged electron density increased monotonically during discharges. Rapid transition of divertor conditions was found within tens of milliseconds. The transition to the detached state occurred simultaneously on both targets and occurred regardless of the radial distance from the LCFS. As the upstream density reaches a particular level, a transition occurs. The critical upstream density appears to be independent of the modulation of the fuelling rate. A sudden transition of divertor condition means that the intermediate state between the attached and detached regimes is unstable, resulting in a bifurcation within tens of milliseconds. The mechanisms driving the intermediate state unstable are explained by various physical hypothesis, such as upward movement of the ionization front near the X-point and core penetration of the fuel atom and sputtered carbon impurity, but it still needs to be clarified further because of the limited KSTAR diagnostics. As a future study, the divertor geometry scans will be conducted to clarify the effect of divertor geometry on asymmetric detachment. The density ramp experiment of the H-mode plasmas will is planned in 2018 to investigate the bifurcation characteristics of the H-mode discharges. Once the cause of the divertor bifurcation is revealed, the key mechanism leading to a detached regime will become clear. Based on this understanding, it can contribute to specifying the operation space for the detached plasmas that are compatible with promising operation scenarios.
Min, Kyoung Wookresearcher민경욱researcher
한국과학기술원 :물리학과,
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학위논문(박사) - 한국과학기술원 : 물리학과, 2019.2,[xvii, 213 p. :]


KSTAR▼ascrape-off layer▼adivertor▼aSOLPS simulation▼adivertor detachment▼adivertor condition bifurcation▼adivertor asymmetry; KSTAR▼ascrape-off layer▼a디버터▼aSOLPS 전산모사▼a디버터 분리 현상▼a디버터 조건 분기▼a디버터 분리 비대칭성

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