Microwave-excited surface wave plasma (SWP) is one of the most important discharging methods to generate uniform and dense plasma in a large-area. On the contrary to this high applicable possibility, the design of microwave-excited SWP apparatus is not facile because of the strong coupling between the plasma parameters and microwave propagation characteristics. This characteristic makes it difficult to design a SWP apparatus and optimize it for achieving expected plasma parameters.

A simulation technique is expected as one of the solutions for overcoming the difficulties on designing a SWP apparatus. In many conventional simulations to analyze the microwave propagation modes in a SWP apparatus, the plasma region is usually assumed to have uniform parameters. This method is very useful to evaluate the dependences of plasma parameters on the structural characteristics of a SWP apparatus. However, it is impossible to calculate the parameter distributions of discharged plasma in the apparatus. In order to calculate the distributions of plasma parameter in a microwave-excited SWP apparatus, it needs to calculate the microwave propagation mode and the plasma parameters simultaneously in a self-consistent method.

In this study, a three-dimensional self-consistent simulation tool for a microwave-excited plasma apparatus with considering the chemical reactions for metastable atoms has been developed. Using the simulation tool, the propagation characteristics of electromagnetic wave and the distribution of plasma parameters such as electron density and electron temperature have been calculated in ring dielectric line typed SWP (RDL-SWP) apparatus. And the results obtained by the self-consistent simulation have been compared to the experimental results to confirm the validity of the simulation tool.

In chapter 2, the self-consistent simulation tool which consists of the microwave model and the plasma model was explained. The governing equations and their discretized equations for each calculation model were presented and the boundary conditions and the stabilization condition were explained. The chemical reactions which were taken into account in the simulation and the rate coefficients for each reaction were indicated. In the last section of this chapter, the 3D cell arrangement for combining the microwave model and the plasma model was explained.

In chapter 3, the propagation characteristics of the electromagnetic wave were investigated in RDL-SWP apparatus with assuming uniform and time-independent plasma parameters. From the simulation results, it was confirmed that the propagation characteristics of the electromagnetic wave is influenced strongly by the electron density than the electron temperature or the Ar gas pressure in plasma. And the limitation of the electron density which could be generated in the apparatus was evaluated. From the simulation results, the limitation of generated plasma's electron density was thought to be around 2.8×1017 m-3 in RDL-SWP apparatus. On the condition over that electron density, the electromagnetic wave did not propagate sufficiently into the center of the apparatus and the incident power was not absorbed efficiently by plasma. This discharging characteristic was explained with comparing to the experimental results.

In chapter 4, a self-consistent simulation was performed in an imaginary simple model, which consists of a waveguide and a plasma discharging box, to evaluate the validity of the simulation tool qualitatively. From the simulation results, it was confirmed that the evolutions on the distributions of the plasma parameters such as electron density and electron temperature were well simulated by introducing the microwave into the plasma discharging chamber. Furthermore, it was also confirmed that the evolutions on the propagation characteristics of the electromagnetic wave were well updated by the evolution of the plasma parameters. Although these simulation results could not compare to the experimental results directly, the validity of this simulation tool have been confirmed conceptually and quantitatively in this chapter.

In chapter 5, using the self-consistent simulation tool developed in this study, the propagation characteristics of electromagnetic wave and the distributions of electron density and electron temperature in RDL-SWP apparatus were calculated at the Ar gas pressures of 10 and 30 m Torr with the incident microwave power of 1.5 kW. And the validity of this simulation tool was confirmed by comparing the vertical profiles of electron density and electron temperature obtained by simulation to those of the experimental results. The discharging characteristics of surface wave plasma were well described in the simulation results but the electron temperature at the Ar gas pressure of 10 mTorr was ~0.5 eV higher in all vertical space than the experimental results. The distributions of electric field intensity, power absorption, electron density and electron temperature which were obtained through the self-consistent simulation were indicated and the reasons of the mismatches on the electron temperature with the experimental results were also discussed in this chapter. And a designing direction to achieve high and uniform electron density with low electron temperature in a large-area was proposed on the basis of the simulation results

In chapter 6, the conclusion of this thesis was given.

本論文は「Numerical Analysis on Discharging Characteristics in Microwave-excited Surface Wave Plasma Apparatus with Self-consistent Simulation Method(セルフコンシステントシミュレーションによるマイクロ波励起表面波プラズマ装置の放電特性解析)」と題し、準安定原子を含むプラズマ化学反応を考慮し、マイクロ波励起表面波プラズマ装置におけるマイクロ波の伝搬特性とプラズマパラメータの空間分布を解析できるシミュレーションツールを開発し、実験データとの比較によりその有効性を検証したもので、6章から構成される。

第1章は「Introduction」であり、マイクロ波励起プラズマ、特にマイクロ波励起表面波プラズマと発生装置、およびその数値解析法の現状を整理した上で、本研究の目的を述べている。

第2章は「Simulation Method」と題し、マイクロ波伝搬を解析する電磁界解析部とプラズマ化学反応を含むプラズマ解析部から構成されるシミュレーションツールについて詳述している。各解析部について、支配方程式と離散化法、境界条件を示し、解析の安定化条件を説明している。本解析ツールで重要な意味を持つ化学反応と各反応の速度定数を示し、さらに、電磁界解析部とプラズマ解析部をつなげるための三次元セル構造に関しても説明している。

第3章は「Electromagnetic Wave Propagation in Uniform Plasma」と題し、定常状態にある均一なプラズマを仮定して、円形誘電体表面波プラズマ装置におけるマイクロ波伝搬特性を解析した結果について述べている。電子温度やアルゴンガス圧力より、電子密度が電磁波の伝搬特性に強く影響していることを確認し、そのプラズマ装置において発生可能なプラズマの電子密度限界も評価している。解析結果より、プラズマの電子密度限界が約2.8×1017 m-3であると予想され、それ以上の電子密度では電磁波が装置中心部まで充分に伝搬できなくなり、プラズマによる電力吸収が効果的に行われないことをシミュレーションにより示した。

第4章は「Self-consistent Simulation in Simple Model」と題し、導波管と放電容器から成る単純な理想モデルで解析を行った結果について述べている。解析結果から、マイクロ波によってプラズマの電子密度や電子温度が変化し、またこれらのプラズマパラメータの変化により電磁波の伝搬特性が変わるという結果が得られ、セルフコンシステントなシミュレーションが開発したツールによって可能であることを示し、ツールの有効性を定性的に確認している。

第5章は「Self-consistent Simulation in RDL-SWP Apparatus」と題し、開発したシミュレーションツールを用いて、10 mTorrと30 mTorrの2種類のアルゴンガス圧力条件で、1.5 kWのマイクロ波入力のもと、円形誘電体表面波プラズマ装置における電磁波の伝搬特性と電子温度や電子密度の空間分布を解析した結果について述べている。さらに実験で測定された電子密度と電子温度の分布と解析結果を比較することで、シミュレーションの妥当性の評価を行い、その結果、表面波プラズマの放電特性がシミュレーションによってよく再現できていることを示した。ただし、10 mTorrの低圧力条件では電子温度が約0.5 eV高い解析結果となっている。電子温度分布における実験結果と解析結果との相違の原因について、電界強度やパワー吸収の分布、電子密度や電子温度の分布から分析している。

第6章は「Conclusions」であり、本研究の成果を総括している。

以上これを要するに、本論文は、様々な応用が展開されているプラズマ発生源の一つであるマイクロ波励起表面波プラズマ装置の放電特性を解析するために、準安定原子を含むプラズマ化学反応を考慮した詳細な物理・化学モデルに基づくシミュレーションツールを開発し、実験結果との比較からその高い有用性を検証したものであり、先端エネルギー工学、特にプラズマ工学に貢献するところが少なくない。

したがって、博士(科学)の学位を授与できると認める。