Performance degradation induced by different forms of carbon contamination in superconducting rf cavities and their recovery strategies

The performance degradation caused by carbon contaminants has become one of the significant obstacles to the long-term, high-gradient, stable operation of the SRF cavities. This paper investigates the sources and effects of different forms of carbon contamination and recovery strategies through experiments on niobium samples and a 162.5 MHz half-wave SRF cavity. Our findings reveal that adventitious carbon and oxides are the primary chemical compounds on the niobium surface, with the work function decreasing linearly as carbon content increases. Additionally, it was observed that carbides can form through the chemical deposition of residual carbon-containing gas in a vacuum under the action of argon ions. The adsorption and deposition of carbon-containing gas molecules reduce surface work function, thereby lowering the field emission onset and quench point of the cavity. However, the combination of thermal cycling and rf conditioning facilitates the desorption of adsorbed gas molecules, leading to a full recovery of cavity performance. In contrast, carbon deposition forms strong chemical bonds with the cavity surface, necessitating more intensive removal techniques. We demonstrate that plasma cleaning effectively eliminates carbon contamination and restores cavity performance. Inspired by the formation of NbC in niobium sample experiments, we propose that, during long-term SRF cavity operation, carbides primarily originate from the chemical deposition of carbon-containing gas molecules adsorbed on the surface in the presence of energy-carrying particles. These insights deepen our understanding of carbon contaminant-induced cavity performance degradation and recovery, providing a basis for selecting suitable online performance recovery methods to maintain the cavity’s stable, high-gradient operation.