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

本研究的目的是测量金属氧化物在氢原子辐照下的还原截面。本研究的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中的行为。第一步是利用电子回旋共振(ECR)等离子体研制小型原子氢源。这一阶段的重点是为ECR新开发一种局域磁场，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，取代了大型螺线管线圈，并对超高压和冷却等许多要求进行了优化。第二步，通过测量等离子体温度和密度来评估原子氢的通量φ_H。然后，利用该原子氢通量，用俄歇能谱测定了金属氧化物还原反应的时间常数τ_d。根据这个时间常数，计算出还原截面为σ_d=1/(τ_dφ_H)。实验结果表明，在Ni的情况下，在氢原子的照射下，吸附的氧被明显地还原。还原截面σ_d约为7×10~(-1)~(-18)cm~(2)，与放电清洗实验中所估算的等离子体壁调节值符合得很好。另一方面，在铬的情况下，没有观察到明显的氧还原。这一结果与氧化铬对氢原子的化学平衡并不像预期的那样不同。提高助熔剂φ_H是今后发展的主要技术问题。这些结果主要发表在等离子体研究会议上。

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.