Additional Losses in High Purity Niobium Cavities related to Slow Cooldown and Hydrogen Segregation

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Several years ago the SRF--community was unpleasantly surprised by the discovery that superconducting RF-cavities made from high purity niobium showed significant degradations of the Q-values when kept for longer periods of time at intermediate temperatures around 100 K. The first temperature map taken on such a degraded cavity showed a rather uniform distribution of the additional losses. This fact and the roughly 100 K holding temperature resulted in the hypothesis of precipitation of hydride phases in niobium. A large number of investigations in several laboratories followed this discovery and the results supported the initial explanation of hydride precipitation. It was ... continued below

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495 Kilobytes pages

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Halbritter, J.; Kneisel, Peter & Saito, Kenji October 1, 1993.

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Several years ago the SRF--community was unpleasantly surprised by the discovery that superconducting RF-cavities made from high purity niobium showed significant degradations of the Q-values when kept for longer periods of time at intermediate temperatures around 100 K. The first temperature map taken on such a degraded cavity showed a rather uniform distribution of the additional losses. This fact and the roughly 100 K holding temperature resulted in the hypothesis of precipitation of hydride phases in niobium. A large number of investigations in several laboratories followed this discovery and the results supported the initial explanation of hydride precipitation. It was experimentally verified that the Q-degradation could be avoided, if the cavities were quickly cooled down through the dangerous temperature region; hydrogen degassing at elevated temperatures eliminated the cavity deterioration, but subsequent extensive chemical surface treatment seemed to reverse the process. A summary of the recent experimental observations has been given, but the detrimental effect of hydrogen precipitation in niobium cavities has been known for many years. For large scale accelerator projects like CEBAF the cryogenic system might prefer certain cooldown cycles and it is important to know the cooling conditions under which the cavity performance is not effected. Such investigations were done in the past and have extended to other temperature regimes. The results and the analysis of these experiments are reported in the following based on a model of weak links between hydrogen segregates and the niobium matrix, which has been developed by one of the authors (JH) for high T{sub c} and classical superconductors.

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495 Kilobytes pages

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  • 6th RF Superconductivity Workshop, Newport News, VA (US), 10/1993

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  • Report No.: CEBAF-PR-93-067
  • Report No.: DOE/ER/40150-2017
  • Grant Number: AC05-84ER40150
  • Office of Scientific & Technical Information Report Number: 791943
  • Archival Resource Key: ark:/67531/metadc742185

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  • October 1, 1993

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  • Oct. 19, 2015, 7:39 p.m.

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  • Feb. 5, 2016, 7:50 p.m.

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Halbritter, J.; Kneisel, Peter & Saito, Kenji. Additional Losses in High Purity Niobium Cavities related to Slow Cooldown and Hydrogen Segregation, article, October 1, 1993; Newport News, Virginia. (digital.library.unt.edu/ark:/67531/metadc742185/: accessed September 21, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.