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

Tongtong Zhu1,2,*, Andong Wu1,3,*,†, Yuan He1,3,‡, Shichun Huang1,3, Zongheng Xue1, Chunlong Li1, Hao Guo1, Pingran Xiong1, Tiancai Jiang1,3 et al.

Qingwei Chu1,3, Didi Luo1,3, Ziqin Yang1,3, Teng Tan1,3, Mengxin Xu1,3, Zhijun Wang1,3, Shenghu Zhang1,3, Kun Zhang2, and Hongwei Zhao1,3

  • *These authors contributed equally to this work.
  • †Contact author: antonwoo@impcas.ac.cn
  • ‡Contact author: hey@impcas.ac.cn
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Phys. Rev. Accel. Beams 28, 123101 – Published 23 December, 2025

DOI: https://doi.org/10.1103/k7bz-sf3f

Abstract

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.

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Phys. Rev. Accel. Beams 28, 123101– Published 23 December, 2025
  • Received 8 February 2025
  • Accepted 28 October 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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