Studies on high temporal resolution probe diagnostic methods for rapidly changing plasmas

Abstract

In this thesis, high temporal resolution probe diagnostic methods for rapidly changing plasmas, such as the tokamak plasma and pulsed plasma, were studied. At first, the floating harmonic technique was used to measure the edge region of tokamak plasma, so called the scrape-off layer (SOL). Studies on this region is very important for understanding the interaction between the plasma and plasma facing components including walls. In this study, due to the large-area of tokamak plasma, long measurement cable was needed, resulting in large amount of the stray current; the stray current affects the total measurement current, and this results in unreliable measurement of the plasma parameters. Thus, dual frequency harmonic method, also called intermodulation frequency method, is applied to solve the stray current problem. By using this method with time division technique of the floating harmonic method, time-resolved measurement of the plasma parameters at the far SOL region of the tokamak plasma were performed. Especially, the plasma densities were in good agreement with those of interferometer. In addition, our probe observed the well known intermittent events, called ELM activity. The floating harmonic method is also applied to the measurement of the pulsed plasma. Recently, the pulsed plasma is used in various semiconductor manufacturing processes due to its advantages such as the reduction of the plasma damage. Using the time division technique of the floating harmonic method, the plasma parameters of the pulsed plasmas at various conditions were measured. These were in good agreement with well-known characteristics of the pulsed plasma. However, as the pulse frequency is increased, the time-division technique gives limited number of plasma parameters during one period of the pulsed plasma due to its limited applied frequency and sampling rate of our measurement system. To overcome this problem, the phase delay technique was proposed. This method can measure the plasma parameters of the pulsed plasma of tens of kHz with the time resolution of 0.8 μsec. Lastly, the single Langmuir probe method is applied to the measurement of the pulsed plasma. This classical electrical method gives various plasma parameters including the electron energy distribution function (the harmonic method gives only the electron temperature and the plasma density). In order to measure the pulsed plasma using the Langmuir probe, the boxcar technique has been used. In this study, the boxcar technique was improved to reduce the required time for measurement from hours to within a minute. Using this technique, the diffusive cooling of electrons at the after glow of the pulsed plasma is well observed, as well as the variation of the plasma parameters at the active and after glow region. These measurement methods and related studies are expected to be used in various plasma studies including the pulsed plasmas and tokamak plasmas.|본 논문에서는 빠르게 변화하는 플라즈마의 특성 분석을 위하여 높은 시간 분해능의 플라즈마 진단법에 대한 연구를 하였다. 첫 번째로 부유 고조화 방법을 이용하여 핵융합 플라즈마 외각 부분의 플라즈마 변수를 측정하는 연구를 수행하였다. 핵융합을 위해서는 1억도 이상의 고에너지 플라즈마 발생이 필수적인데, 이 때 이 플라즈마를 가두는 용기 벽과 플라즈마 사이의 상호작용은 매우 중요한 연구 분야 중 하나이다. 본 연구에서는 측정 시스템에서 발생하는 기생 전류의 영향을 없애기 위하여 기존 고조화 방법에 이중 주파수를 인가하는 상호변조주파수 방법이 이용되었다. 측정 결과, 빠르게 변화하는 플라즈마 변수를 잘 측정할 수 있었고, 플라즈마 밀도의 경우는 간섭계로부터 얻은 측정 결과와 잘 일치하는 경향성을 확인할 수 있었다. 또한 핵융합 플라즈마의 높은 가둠 모드에서 발생하는 플라즈마 경계면 불안정 현상(Edge Localized Mode)을 관찰함으로써 본 측정 방법의 적용 가능성을 확인할 수 있었다. 두 번째 연구에서는 부유 고조화 방법을 이용하여 펄스 플라즈마를 측정하는 연구를 수행하였다. 최근 인가 전력을 연속적으로 주지 않고, 펄스 형태로 주는 것이 플라즈마에 의한 손상(Damage) 저감 등 여러 측면에서 유리함이 보고되어 반도체 식각 공정에서 널리 이용되고 있다. 부유 고조화 방법의 시간 분해 방법을 이용하여 다양한 조건의 펄스 플라즈마를 측정할 수 있었고, 이론적으로 알려져 있는 결과와 잘 일치하였다. 그러나 펄스 주파수가 증가함에 따라 인가 주파수의 상한 및 장치 샘플링 속도의 문제로 펄스 플라즈마의 변화 추이를 잘 보여주지 못하게 되었다. 이를 해결하기 위하여 박스카 측정 개념이 적용된 위상 지연 방법이 개발되었고, 본 방법을 통해 0.8 μsec의 시간 분해능으로 빠르게 변화하는 펄스 플라즈마를 측정할 수 있었다. 세 번째 연구에서는 단일 랑뮤어 탐침을 이용하여 펄스 플라즈마를 측정하는 연구를 수행하였다. 부유 고조화 방법에 비해 다양한 플라즈마 변수를 얻을 수 있는 단일 랑뮤어 탐침법에서는 고속 플라즈마 진단을 위해서 박스카 측정 방법을 이용한다. 기존의 박스카 측정 방법은 펄스 한 주기의 플라즈마를 측정하기 위해서 대략 수 시간이 필요한 반면, 본 연구에서는 측정 방식의 개선을 통해 그 시간을 1 분 이내로 감소시킬 수 있었다. 측정 결과, 펄스 플라즈마의 잔광(After glow) 영역에서 널리 알려진 전자의 확산에 의한 손실 현상을(Diffusive cooling) 관찰할 수 있었다. 본 연구는 최근 활발히 연구되고 있는 핵융합 플라즈마 및 펄스 플라즈마를 고속으로 측정하는 진단법에 대한 연구로써, 향후 이 진단법들이 다양한 연구에서 잘 활용되기를 기대한다.; the stray current affects the total measurement current, and this results in unreliable measurement of the plasma parameters. Thus, dual frequency harmonic method, also called intermodulation frequency method, is applied to solve the stray current problem. By using this method with time division technique of the floating harmonic method, time-resolved measurement of the plasma parameters at the far SOL region of the tokamak plasma were performed. Especially, the plasma densities were in good agreement with those of interferometer. In addition, our probe observed the well known intermittent events, called ELM activity. The floating harmonic method is also applied to the measurement of the pulsed plasma. Recently, the pulsed plasma is used in various semiconductor manufacturing processes due to its advantages such as the reduction of the plasma damage. Using the time division technique of the floating harmonic method, the plasma parameters of the pulsed plasmas at various conditions were measured. These were in good agreement with well-known characteristics of the pulsed plasma. However, as the pulse frequency is increased, the time-division technique gives limited number of plasma parameters during one period of the pulsed plasma due to its limited applied frequency and sampling rate of our measurement system. To overcome this problem, the phase delay technique was proposed. This method can measure the plasma parameters of the pulsed plasma of tens of kHz with the time resolution of 0.8 μsec. Lastly, the single Langmuir probe method is applied to the measurement of the pulsed plasma. This classical electrical method gives various plasma parameters including the electron energy distribution function (the harmonic method gives only the electron temperature and the plasma density). In order to measure the pulsed plasma using the Langmuir probe, the boxcar technique has been used. In this study, the boxcar technique was improved to reduce the required time for measurement from hours to within a minute. Using this technique, the diffusive cooling of electrons at the after glow of the pulsed plasma is well observed, as well as the variation of the plasma parameters at the active and after glow region. These measurement methods and related studies are expected to be used in various plasma studies including the pulsed plasmas and tokamak plasmas.