Nonequilibrium hydrogen plasma jets have been widely used in material processing such as thin film deposition, surface cleaning, and passivation. Such thermal nonequilibrium plasma exhibits high electron temperature and low heavy-particle temperatures. This is because thermal nonequilibrium plasma can be used for the processing of the materials which cannot endure high heat flux. Another important requirement is high degree of dissociation, which increases the efficiency of such processing.

Unlike the active utilization of plasmas seen in material processing, there are situations where plasmas cause serious problems. One of them is seen in aerospace engineering, and it is aerodynamic heating of spacecrafts entering planetary atmospheres. Considering the atmospheric entry flights at high altitude around the outer planets, the spacecrafts are surrounded by nonequilibrium hydrogen plasma flows behind the strong shock waves. This is because the smaller number density at high altitude leads to insufficient collisions between molecules to reach their equilibrium state. The accurate prediction of the aerodynamic heating during such entries requires the understanding of the relaxation processes of hydrogen molecules. However, the relaxation models of hydrogen molecules have not been established yet. One of the reasons is that experimental data are very scarce. Steady nonequilibrium plasma jets are appropriate for this investigation. Direct measurement of the plasma temperatures along the jet enables the experimental determinations of the relaxation collision numbers, which can contribute to the improvement of the models.

Based on the backgrounds described above, the plasma jets with the following characteristics can meet the needs both for hydrogen plasma processing and the relaxation study of hydrogen molecules: 1) Steady operation is possible. 2) Plasma flow is in strong thermal nonequilibrium. Electron temperature is very high, and heavy-particle temperatures are not high. 3) Plasma jet length, which can be determined by the emission of excited atomic hydrogen, is long. 4) Degree of dissociation is high. 5) Relaxations between translational and other internal modes occur. Especially in the present work, the translational-rotational relaxation was focused. The objective of this work is to produce the plasma jets to meet these requirements.

As a method to generate such hydrogen plasma flows, a new type of an experimental facility using the combination of applied magnetic field and an orifice was proposed in this study. This concept is the hybrid of two conventional devices of a solid nozzle and applied magnetic-field. A solid nozzle enabling aerodynamic acceleration of the flows has disadvantage of large heat loss. Applied magnetic field enabling the smaller heat loss has disadvantage of insufficient acceleration of the neutral particles. A solid nozzle covers the disadvantage of applied magnetic field, and applied magnetic field covers the disadvantage of a solid nozzle. Therefore, the combination of both was expected to be advantageous. Aiming for the decrease in the heat loss, and the generation of nonequilibrium jet and flow acceleration by strong free expansion, an orifice was used instead of a solid nozzle. The schematic picture of the experimental facility is shown in Fig. 1. This facility design already met the first requirement, 1) steady operation.

The experiments were done using helium or using hydrogen admixed with a small amount of argon. Helium was used to investigate the basic dynamics of the produced jets because helium is a monatomic gas and molecular weight of helium is similar to that of hydrogen. For both gases, emission spectroscopic methods to diagnose the plasma jets were developed in this work. In helium operation, the electron temperature and the degree of ionization were determined by the intensity fitting of the selected helium emission lines, based on the master equations. In hydrogen operation, the rotational, vibrational, and electron temperatures were determined by the line intensity fitting of Fulcher-a band of hydrogen molecule, the translational temperature was determined by the profile fitting of Balmer H6 line, and the degree of dissociation was determined by actinometry. These emission spectroscopic methods developed in this work were used to investigate the characteristics of the produced jets.

The usefulness of the combination of applied magnetic field and an orifice was shown in Fig. 2. There are four types of the produced plasma jets: 1) applied magnetic field and an orifice, 2) only applied magnetic field, 3) only an orifice, and 4) no applied magnetic field and no orifices. The mass flow rate of hydrogen is 0.6 SLM, and the input power is 1 kW. The orifice diameter is 6 mm. The applied DC current is 50 A, which corresponds to 42 mT of axial magnetic flux density at the center of the coil. As shown in Fig. 2, the hydrogen plasma jets produced by the combination of applied magnetic field and an orifice exhibited the longest jet length and the strongest emission. The axial distribution of the degree of dissociation obtained by the spectroscopic measurement along the centerline is shown in Fig. 3. The remarkable difference was found downstream of the orifice at x<400 mm. Though only applied magnetic field or only an orifice could increase the degree of dissociation of the produced jets, the combination of both increased the degree of dissociation much more remarkably. This combination successfully exhibited the largest degree of dissociation. The degree of dissociation in this combination was about 2 times of that in only applied magnetic field, about 5 times of that in only an orifice, and about 9 times of that in no applied magnetic field and no orifices. Higher value of the degree of dissociation is considered to be due to the increase in the electron number density, which is caused by both the increase in the static pressure by the orifice installment and the decrease in the heat loss by the applied magnetic field. Therefore, two of the requirements, 3) long jet length and 4) high degree of dissociation were met by the combination of applied magnetic field and an orifice.

Emission spectroscopic measurements revealed that the produced plasma jets have strong thermal nonequilibrium. Only the electron temperature was of the order of 1 eV, and other temperatures are low. Nonequilibrium between translational, rotational, vibrational, and electron temperatures were successfully produced, too. Therefore, the second requirement, 2) strong thermal nonequilibrium was met.

Regarding the fifth requirement, 5) relaxed flow, the translational-rotational relaxation could be spectroscopically observed by selecting the appropriate experimental condition, as shown in Fig. 4. This is due to the strong free expansion at the orifice, and ,the longer jet length realized by this combination. The rotational relaxation collision numbers were estimated using their temperature profiles, which supported two of the latest theoretical results.

As conclusions, the achievement of this work is that the combination of applied magnetic field and an orifice was much superior to any of only applied magnetic field, only an orifice, or no applied magnetic field and no orifices, for the production of nonequilibrium plasma jets desirable both in hydrogen plasma processing and in the relaxation study of hydrogen molecules.

As the contents of this thesis, chapter 1 gives the introduction. Chapter 2 gives the description of the developed experimental facility. Chapter 3 describes the experiments using helium. Chapter 4 describes the experiments using hydrogen. Finally in chapter 5, conclusions are stated.

Fig. 1 Schematic picture of a new experimental facility developed in this work

Fig. 2 Usefulness of the combination of applied magnetic field and an orifice

Fig. 3 Axial distribution of degree of dissociation

Fig. 4 Spectroscopic observation of translational-rotational relaxation

本論文は、「Characteristics of Rarefied Plasma Jet with Applied Magnetic Field and an Orifice(印加磁場とオリフィスを有する希薄プラズマジェットの特性)」と題し、本文5章および付録5項から成っている。

第1章は序論であり、研究の背景と目的を述べている。希薄なプラズマジェットは、惑星大気圏に突入する飛行体まわりの高エンタルピー流体現象解明だけでなく、プラズマプロセッシングなどの工業的応用においても重要な対象である。特に、木星などの外惑星探査やプラズマプロセッシングでの利用価値を考慮すると、水素の希薄プラズマジェットが重要であり、本研究においても水素プラズマを主な実験対象としている。希薄プラズマジェットでは、十分な分子間衝突が起こらないため内部エネルギーモードの励起が遅れ、熱非平衡状態となっている。筆者は、大気圏突入気流の研究とプラズマプロセッシングに要求される気流条件を整理し、両者に望まれる気流生成装置の条件として、定常運転、熱非平衡流、大きなジェット長、高い解離度、気流中で非平衡緩和現象が起こり、しかもそれが発光分光法によって観測可能であること、を挙げている。それらを実現するものとして、印加磁場による電磁気的作用とオリフィスによる空力的作用を組み合わせた新しいプラズマジェット生成装置を提案している。

第2章は、本研究で開発した希薄プラズマジェット生成装置の詳細である。誘導結合型プラズマ源、印加磁場用DCコイル、オリフィス等の各構成要素や光学観測装置について説明されている。

本研究で提案するプラズマジェット生成装置は、主に水素への適用を目的としているが、その前段階としてヘリウムプラズマでの実験が行われ、その結果が第3章で説明されている。ヘリウムは分子量が水素に近く、かつ単原子分子であるため反応過程が簡単であり、印加磁場とオリフィスによるプラズマジェットのダイナミクスを調べるのに適している。分子間衝突と輻射を考慮したマスター方程式に基づく発光分光法による電離度等の計測法を新たに開発して適用した結果、印加磁場とオリフィスの組み合わせは、ジェットの発光部分が長くなるだけでなく、電離度を上昇させるのに有効であることを実験的に示している。

第4章では、本装置によって生成された希薄水素プラズマジェットに関する特性の詳細が述べられている。はじめに、気流診断のため新たに開発された発光分光法の説明とその妥当性および精度の検討がなされている。ここで、並進温度は水素原子バルマー線に対するスペクトルフィッティングにより、回転、振動、電子の各温度はFulcher-α帯線強度フィッティングにより、解離度はアクチノメトリ法によって求められている。本研究で提案する印加磁場とオリフィスの組み合わせは、印加磁場のみ、オリフィスのみ、両方なしのいずれと比較しても気流の解離度において顕著な上昇をもたらすことを明らかにしている。また、並進温度と回転温度は、プラズマ生成部で平衡になっているが、オリフィス下流での自由膨張により、強い非平衡性を示した後、それが徐々に緩和していく過程の観察にも成功している。得られるジェットのプラズマ温度、解離度、ジェット長は、印加磁場を強く、オリフィス径を小さく、流量を小さく、投入パワーを上げるほど大きくなっている。これらはいずれも、高エンタルピー流研究やプラズマプロセッシング用のプラズマジェット生成装置の性能を向上させるのに有用な知見である。

第5章は結論であり、本研究で得られた成果をまとめている。

付録は5項から成り、分光法におけるアーベル変換による処理法、水素バルマー線の発光強度の理論計算法、超音速希薄水素プラズマ流における衝撃層の観測、アクチノメトリ法におけるアルゴン混合が水素気流に与える影響の評価、希薄流解析における可変剛体球モデル、について述べられている。

以上要するに、本論文は印加磁場とオリフィスを組み合わせた新しいプラズマジェット生成装置を開発し、新たに考案した発光分光による気流診断を行うことで、生成された水素プラズマジェットにおいて、解離度やジェット長さが顕著に増大することを実験的に実証し、高エンタルピー気流における非平衡緩和過程の研究やプラズマプロセッシングへの応用に有効であることを示した点で、先端エネルギー工学、特に高エンタルピー気体力学に貢献するところが大きい。

なお、本論文の第2章から第4章は鈴木宏二郎氏との共同研究であるが、論文提出者が主体となって実験および解析を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(科学)の学位を授与できると認める。