• Journal of the Korean Society for Nondestructive Testing
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• v.28 no.2
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• pp.211-216
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• 2008

In this study, we have developed a program which can calculate phase and group velocities, attenuation and wave structures of each mode in multi-layered plates. The wave structures of each mode are obtained, varying material properties and number of layers. The key in the success of guided wave NDE is how to optimize the mode selection scheme by minimizing energy loss when a structure is in contact with liquid. In this study, the normalized out-of-plane displacements at the surface of a free plate are used to predict the variation of modal attenuation and verily the correlation between attenuation and wave structure. It turns out that the guided wave attenuation can be efficiently obtain from the out-of-plane displacement variation of a free wave guide alleviating such mathematical difficulties in extracting complex roots for the eigenvalue problem of a liquid loaded wave guide. Through this study, the concert to optimize guided wave mode selection is accomplished to enhance sensitivity and efficiency in nondestructive evaluation for multi-layered structures.

• Proceedings of the Optical Society of Korea Conference
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• 2009.02a
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• pp.459-460
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• 2009

In this study, surface structure and near-field distribution of a surface acoustic wave filter were measured with both reflection mode and collection mode near-field scanning optical microscopes, and there results were compared with each other.

• Proceedings of the Acoustical Society of Korea Conference
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• spring
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• pp.403-408
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• 1999

In this paper, ultrasonic tomography by the Mode-Converted Rayleigh wave (MCRW) in the back-scattered direction is presented. When a beam with a short pulse and narrow beam width enters a reflector with smooth surface, in general, two major arrivals can be observed in the output waveform: the specular reflection and the radiation of the MCRW from the reflector surface. The time-delay between the two waves is relatively large and thus can be measured easily. This large time-delay is due to the fact that the MCRW is slower than incident wave. In our method, this large time- delay is used for ultrasonic image reconstruction. To effectively detect the MCRW, the arrayed-receiving transducers are circularly arranged around the transmitter. In addition, a deconvolution method is employed to remove specular echo signals for reconstructing the MCRW image.

• Geophysics and Geophysical Exploration
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• v.11 no.3
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• pp.230-241
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• 2008

Characteristics of transfer responses for arrays like triangle, hexagon and semicircle were investigated. To characterize the site near Izmit city with ambient noise measurement, dispersion curves of surface waves were derived with using array technique like F-K, High resolution F-K, MSPAC and H/V ratio was calculated. Also, MASW was surveyed to get the high frequency part of dispersion curves. The transition from fundamental mode to first high mode of surface waves for dispersion curve was observed. Dispersion curve of fundamental mode of ambient noise and first higher mode of MASW was used in inversion to get S-wave velocity structure of subsurface. None-unique problem of results of surface wave inversion was solved with comparison of result of refraction tomography performed with first arrivals of MASW data.

• 전기의세계
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• v.28 no.4
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• pp.47-52
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• 1979

Total number of resonant modes in a microwave oven cavity may be maximized for a given frequency bandwidth to obtain more uniform power distribution by choosing proper size of the cavity. The total number of modes is calculated for a dielectrically loaded rectangular cavity and its size is suggested here for which the change in the number of modes is less sensitive to the change of dielectric layer thickness and its total number of modes is maximized in a given range of cavity sizes. A prove coupled rectangular cavity is constructed and the total existing modes are measured to see the change of modes depending on the dielectric layer thickness and the cavity size. Surface wave mode existing in the dielectric layer is confirmed by measuring Q and the input impedance of the cavity for this mode, which closely compares with the calculation.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.50.3-50.3
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• 2019

The EUV wave is a disturbance that has been believed to be the fast-mode (shock) wave, which can propagate passing through magnetic field lines. After the passage of EUV waves, coronal streamers start to show kink-mode oscillations, and the footpoints, i.e., magnetic separatrices, of the oscillating streamers are observed as the so-called stationary front. We compare the stationary front observed by EUV imagers and coronal streamers observed in coronagraphic images. We analyze the successive events occurred in September 2011. We find that the stationary fronts are consistent with the coronal streamer boundaries, and they are located along the boundaries of coronal holes and active regions. Our results confirm that EUV waves are in fact fast-mode waves and demonstrate that the stationary front is a promising tool to probe into the source of slow solar wind that is the boundary of coronal streamers on the solar surface.

• Journal of the Korean Society for Nondestructive Testing
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• v.26 no.4
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• pp.239-245
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• 2006

Experimentally measured Lamb wave group velocities in composite materials with anisotropic characteristics are not accorded with the theoretical group velocities as calculated with the Lamb wave dispersion equation. This discrepancy arises from the fact that the angle between the group velocity direction and the phase velocity direction in anisotropic materials exists. Wave propagation in a composite material with anisotropic characteristics should be considered with respect to magnitude in addition to direction. In this study, $S_0$ mode phase velocity dispersion corves are depicted with the variation of degree with respect to the fiber direction using a Lamb wave dispersion relation in the unidirectional, bidirectional, and quasi-isotropic composite plates. Slowness surface is sketched by the reciprocal value of the phase velocity curves. The magnitude and direction of the group velocity are calculated from the slowness surface. The theoretically determined group velocity, which is calculated from the slowness surface, Is compared with experimentally measured group velocities. The proposed method shows good agreements with theoretical and experimental results.

• Journal of the Korean Geophysical Society
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• v.9 no.3
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• pp.231-240
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• 2006

The crustal structure of the Korean Peninsula was investigated by analyzing phase velocity dispersion data of Rayleigh waves. Earthquakes recorded by three component broad-band velocity seismographs during 1999-2004 in South Korea were used in this study. The fundamental mode Rayleigh waves were extracted from vertical components of seismograms by multiple filter technique and phase match filter method. Phase velocity dispersion curves of the fundamental mode signal pairs for 14 surface wave propagation paths on the great circle in the range 10 to 80 sec were computed by two-station method. Treating the shear velocity of each layer as an independent parameter, phase velocity data of Rayleigh wave were inverted. All the result models can be explained by a rather homogeneous crust of shear-wave velocity increasing from 2.8 to 3.25 km/sec from top to about 33 km depth without any distinctive crustal discontinuities and an uppermost mantle of shear-wave velocity between 4.55 and 4.67 km/sec. Our results turn out to agree well with recent study of Cho et al. (2006 b) based on the analysis of seismic background noises to recover short-period (0.5-20 sec) Rayleigh- and Love-wave group velocity dispersion characteristics.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.22 no.8
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• pp.771-776
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• 2012

A technique is presented that uses a circular waveguide for the measurement of the bulk shear(S-wave) velocities of unconsolidated, saturated media, with particular application to near surface soils. The technique requires the measurement of the attenuation characteristics of the fumdamental T(0,1) mode that propagates along an embedded pipe, from which the acoustic properties of the surrounding medium are inferred. From the dispersion curve analysis, the feasibility of using T(0,1) mode which is non-dispersive and have constant attenuation over all frequency range is discussed. The principles behind the technique are discussed and the results of an experimental laboratory validation are presented. The experimental data are best fitted for the different depths of wetted sand and the shear velocities as a function of depths are formulated using power law curves.

• Proceedings of the Korean Vacuum Society Conference
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• 2000.02a
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• pp.183-183
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• 2000

Aigrain에 의해 Helicon이라는 이름이 명명된 이후, helicon은 저온의 금속과 같은 높은 전도도(conductivity)를 갖는 매질이나 강한 자기장이 걸려있는 plasma를 전파해 나가는 저주파 전자기장을 지칭해왔다. 이온화된 개스에서 이러한 전자기장은 전자 공명 주파수(electron cyclotron frequency)와 이온 공명 주파수(ion cyclotron frequency) 사이의 주파수로 전파하며 전리층 (ionosphere)을 통과하며 발생하는 가청 주파수 영역대의 음조가 강하하는 현상에 의해 low-frequency whistler라고도 불린다. Helicon wave plasma는 Boswell에 의해 처음 발생된 후, 높은 이온화율(~100%)로 인해 많은 연구가 이루어져 왔다. 1985년에 Chen은 helicon plasma의 높은 이온화율을 설명하기 위해 Landaudamping을 제시하였다. 이러한 설명은 1997년에 Shamrai에 의해 TG mode가 도입되기 전까지 직접적인 실험결과 없이 helicon plasma 발생의 mechanism으로 받아들여졌다. shamrai의 이론에 의하면 정전기파(electrostatic wave)는 plasma의 표면(surface)에서 강하게 감쇄되어 energy를 전달하게 된다. Cho는 radial density 분포가 외각보다 중심이 높을 경우 TG wave의 power 전달이 중심에서 일어날 수 있음을 계산하였다. Helicon plasma의 특성은 높은 이온화율에 의한 높은 밀도($\geq$1012cm3), 1-2 kW의 rf power에서 상대적으로 낮은 전자 온도( 4eV), $\omega$ci $\omega$LH<$\omega$ $\omega$ce $\omega$pe 영역대의 주파수, 자기장 50-1200 Gauss, 압력 1-10 mTorr로 특정지을 수 있다. 이러한 외부분수들의 조건에 k라 helicon plasma는 여러 종류의 mode로 존재한다. Degeling은 이러한 mode의 변화를 capacitive mode, inductive mode, 그리고 helicon mode(wave mode)의 세가지 부분으로 구분하였다. Helicon plasma가 갖는 높은 이온화율은 여러 가지 응용으로의 가능성을 가지고 있다. 그 예로 plasma processing, plasma wave에 의한 입자 가속, 그리고 가스 레이저 활성 매질 발생 등이 있다. 특히 plasma processing의 경우 helicon plasma는 높은 밀도, 비교적 낮은 자기장, remote operation 등이 가능하다는 점에서 현재 연구가 활발히 진행되고 있다. 상업용으로도 PMT와 Lucas Signatone Corp.에 서 helicon source가 제작되었다. 또한 높은 해리율을 이용하여 저유전 물질인 SiOF의 증착에서 적용되고 있다. 이 외에도 다수의 연구결과들이 발표되었다.