Joonwon Kim

Development of KSTAR ECE imaging system for measurement of temperature fluctuations and edge density fluctuations.

The ECE imaging (ECEI) diagnostic tested on the TEXTOR tokamak revealed the sawtooth reconnection physics in unprecedented detail, including the first observation of high-field-side crash and collective heat transport [H. K. Park, N. C. Luhmann, Jr., A. J. H. Donné et al., Phys. Rev. Lett. 96, 195003 (2006)]. An improved ECEI system capable of visualizing both high- and low-field sides simultaneously with considerably better spatial coverage has been developed for the KSTAR tokamak in order to capture the full picture of core MHD dynamics. Direct 2D imaging of other MHD phenomena such as tearing modes, edge localized modes, and even Alfvén eigenmodes is expected to be feasible. Use of ECE images of the optically thin edge region to recover 2D electron density changes during L/H mode transitions is also envisioned, providing powerful information about the underlying physics. The influence of density fluctuations on optically thin ECE is discussed.

3ω method to measure thermal properties of electrically conducting small-volume liquid

This work presents a method to measure the thermal conductivity and heat capacity of electrically conducting small-volume liquid samples using the 3omega technique. A mathematical model of heat transfer is derived to determine the thermal properties from the 3omega signal considering the device geometry. In order to validate the model, an experimental apparatus has been designed and set up to measure the thermal properties (thermal conductivity and heat capacity) of seven different liquid samples. The results show good agreement with other literature values, demonstrating that the suggested method is effective for measuring the thermal properties of electrically conducting liquids. More importantly, the result with a sample volume of 1 microl demonstrates the resolution of the thermal conductivity as precise as 0.01% which corresponds to a thermal-conductivity change of 10(-4) Wm K in the case of water-based solutions.

Cost-effective laser interference lithography using a 405 nm AlInGaN semiconductor laser

This paper presents a cost-effective interference lithography system that uses a 405 nm AlInGaN semiconductor laser. This method is cost-effective because the AlInGaN semiconductor laser has a long coherence length (~20 m) and low price (e.g. only 1/3 that of the HeCd laser). This system successfully fabricated uniform nano-periodic patterns (line, dot and hole) in a photoresist (PR) over a 2 × 2 cm sample area. The PR patterns agreed well with simulations. Tall silicon nano-structures were fabricated by deep reactive ion etching (DRIE) using a PR pattern as a direct etch mask layer. Aspect ratios of 25 with smooth and vertical sidewalls were achieved after 32 DRIE cycles.

A superhydrophobic dual-scale engineered lotus leaf

A surface was created with the same superhydrophobic property as the lotus leaf (lotus effect) by dipping of sandblasted porous alumina into polytetrafluoroethylene (PTFE, Teflon®: DuPont™) solution. The fabricated engineered lotus leaf had PTFE dual-scale structures. This fabrication process has several advantages, including low fabrication cost, simplicity and easy coverage of a large area. The sandblasted porous alumina template was fabricated by sandblasting of an aluminum sheet and anodization in oxalic acid. To obtain PTFE dual-scale structures, PTFE replication based on the dipping method was used, with a 0.3 w% PTFE solution. To remove the aluminum and alumina layers, wet etching by chromic and phosphoric acid mixed solution and liquid HgCl2 solution was used. The fabricated surface has a superhydrophobic property whose apparent contact angle of the PTFE dual-scale structures was approximately 165° and sliding angle is less than 2°.

Wettability of dual-scaled surfaces fabricated by the combination of a conventional silicon wet-etching and a ZnO solution method

We introduce effective dual-scaled surfaces using the combination of the conventional silicon wet-etching technique (for microstructures) and a solution method for ZnO nanorod formation (for nanostructures). The microstructures were sloped to facilitate the overall deposition of a ZnO seed layer as well as the growth of nanostructures over the entire surface area. The ZnO nanorods were formed using growth solutions of various pHs. The fabricated dual-scaled surfaces were also coated with hydrophobic self-assembled monolayers (SAMs) and compared with the surfaces without the SAM coating to examine the structural effects on both hydrophilic and hydrophobic regions. A total of 16 different samples were examined and analyzed systematically by comparing the static (i.e. the contact angle) and dynamic (i.e. spreadability and contact angle hysteresis) wettability.

Development and characterization of a cartridge-type pneumatic dispenser with an integrated backflow stopper

This paper presents the development and characterization of a simple yet effective cartridge-type (i.e. an integrated liquid reservoir) pneumatic dispensing system. The unique design was a backflow stopper inside the dispenser to function as a flow restrictor. A thin flexible membrane with computer-controlled pneumatic actuation is used to produce various droplet volumes. The detailed fabrication is explained, and systematic characterization verified the performance of our device. The current device configuration can dispense droplet volumes from 100 nL to 400 nL by controlling the pressure applied to the flexible membrane.

Comparative study between the reflective optics and lens based system for microwave imaging system on KSTAR

Recently, two-dimensional microwave imaging diagnostics such as the electron cyclotron emission imaging (ECEI) system and microwave imaging reflectometry (MIR) have been developed to study magnetohydrodynamics instabilities and turbulence in magnetically confined plasmas. These imaging systems utilize large optics to collect passive emission or reflected radiation. The design of this optics can be classified into two different types: reflective or refractive optical systems. For instance, an ECEI/MIR system on the TEXTOR tokamak [Park et al., Rev. Sci. Instrum. 75, 3787 (2004)] employed the reflective optics which consisted of two large mirrors, while the TEXTOR ECEI upgrade [B. Tobias et al., Rev. Sci. Instrum. 80, 093502 (2009)] and systems on DIII-D, ASDEX-U, and KSTAR adopted refractive systems. Each system has advantages and disadvantages in the standing wave problem and optical aberrations. In this paper, a comparative study between the two optical systems has been performed in order to design a MIR system for KSTAR.

Data acquisition and processing system of the electron cyclotron emission imaging system of the KSTAR tokamak.

A new innovative electron cyclotron emission imaging (ECEI) diagnostic system for the Korean Superconducting Tokamak Advanced Research (KSTAR) produces a large amount of data. The design of the data acquisition and processing system of the ECEI diagnostic system should consider covering the large data production and flow. The system design is based on the layered structure scalable to the future extension to accommodate increasing data demands. Software architecture that allows a web-based monitoring of the operation status, remote experiment, and data analysis is discussed. The operating software will help machine operators and users validate the acquired data promptly, prepare next discharge, and enhance the experiment performance and data analysis in a distributed environment.