Negative hydrogen ion sources for NBI systems at fusion devices rely on the surface conversion of hydrogen atoms and positive ions to negative hydrogen ions. In these sources the surface work function is decreased by adsorption of caesium (work function of 2.1 eV), enhancing consequently the negative ion yield. However, the performance of the ion source decreases during plasma pulses up to one hour, suggesting a deterioration of the work function. Fundamental investigations are performed in a laboratory experiment in order to study the impact of the plasma on the work function of a freshly caesiated stainless steel surface. A work function of 2.1 eV is achieved in the first 10 s of plasma, while further plasma exposure leads to the removal of Cs from the surface and to the change of the work function: a value of around 1.8–1.9 eV is measured after 10–15 min of plasma exposure and then the work function increases, approaching the work function of the substrate (≥4.2 eV) after 5 h...

中文翻译：

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鉴于ITER，负离子源长时间暴露于等离子体中时，Cs通量与转换器表面功函数的相关性

用于聚变设备的NBI系统的负氢离子源依赖于氢原子和正离子到负氢离子的表面转化。在这些离子源中，铯的吸附会降低表面功函数（2.1 eV的功函数），从而提高负离子产率。但是，离子源的性能在长达一小时的等离子脉冲期间会降低，这表明功函数会下降。在实验室实验中进行了基础研究，以研究等离子体对刚切过的不锈钢表面的功函数的影响。在等离子体的前10 s达到2.1 eV的功函数，而进一步的等离子体暴露导致表面上Cs的去除和功函数的变化：约为1.8–1。