MICROSCOPIC INVESTIGATION OF HIGH-GRADIENT SUPERCONDUCTING CAVITIES AFTER REDUCTION OF FIELD-EMISSION

In the previous companion paper we showed that high power RF processing (HPP) is an effective technique to reduce field emission in superconducting cavities, so higher accelerating gradients can be reached. In this work we show improved understanding of the mechanisms at work when field emitters process. Thermometry measurements of the outer wall of single-cell cavities reveal the field emission from localized sites and also the reduction in field emission by processing. Subsequent scanning electron microscope (SEM) examination of the RF surface at the emission/processed sites reveals 5-10 mum sized molten craters, micron sized molten particles of foreign elements, and sub-mm sized spots shaped like starbursts. These features indicate that processing occurs through a violent melting/vaporization phenomenon. A ''model'' for RF processing is presented based upon the experimental evidence, both from this study and from others.