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

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