Measurements of reduction cross sections of metal-oxides by atomic hydrogen

本研究的目的是测量原子氢辐照下金属氧化物的还原截面。本研究结果有助于定量预测和控制氧杂质在等离子体-壁相互作用下的行为。本研究分两步完成。第一步是利用电子回旋共振（ECR）等离子体开发小型原子氢源。本阶段的重点是为ECR新开发一个局部磁场，因为本研究中使用的俄歇表面分析仪（AES）对AES材料本身的磁化非常敏感。为此，新开发了一种小型永磁系统，取代了大型电磁线圈，并对特高压和冷却等许多要求进行了优化。第二步是通过测量等离子体温度和密度来计算原子氢的φ_H通量。然后，利用原子氢通量，用AES测量了金属氧化物还原的时间常数τ_d。根据此时间常数，计算出还原截面为σ_d = 1/(τ_d φ_H)。实验结果表明，以Ni为例，吸附的氧在氢原子的照射下明显还原。还原截面σ_d约为7 × 10^<-18> cm^2，与等离子体壁面调节放电清洗实验结果吻合较好。另一方面，在Cr的情况下，没有观察到氧的明显还原。这一结果与氧化铬对氢原子的化学平衡并无太大不同。提高φ_H的流量是今后发展的主要技术问题。这些结果主要在等离子体研究会议上发表。

The purpose of this research is to measure the reduction cross sections of metal oxides due to irradiation of atomic hydrogen. The results obtained in this study contribute to quantitative prediction and control of behaviors of oxygen impurities due to plasma-wall interactions.This research has been accomplished with two steps. The first step is to develop a small size atomic-hydrogen source using electron-cyclotron resonance (ECR) plasma. The point in this stage is to newly develop a localized magnetic field for ECR, because the Auger surface analyzer (AES) used in this study is very sensitive to magnetization of the AES material itself. For this purpose, instead of large solenoid coils, a small-size permanent-magnet system has been newly developed, in which many requirements such as UHV and cooling are also optimized.The second step is to evaluate the flux φ_H of atomic hydrogen by measuring plasma temperature and density. Then, using this atomic hydrogen flux, the time constant τ_d of metal-oxide reduction is measured with AES. From this time constant, the reduction cross section is evaluated as σ_d = 1/(τ_d φ_H).Experimental results have shown that, in case of Ni, adsorbed oxygen is clearly reduced under the irradiation of atomic hydrogen. The reduction cross section σ_d is evaluated to be about 7x10^<-18> cm^2, which is in good agreement with the values so far estimated in discharge cleaning experiments for plasma-wall conditioning. On the other hand, in case of Cr, no clear reduction of oxygen has been observed. This result is not so different as expected from chemical equilibrium of chromium-oxide against atomic hydrogen. Increase of the flux φ_H is the technical main issue to be developed in future. These results have been mainly presented in plasma research conferences.