Abstract
The influence of heat treatment (HT) above 1100°C on field emission (FE) in superconducting radiofrequency (1.5 GHz) cavities was investigated. The experiments show higher average achievable fields with HT than with only a chemical treatment (CT); i.e. the average maximum surface field E pk improved to 29 MV/m from 18 MV/m without taking advantage of the benefits of He processing. Using He processing, HT raised the average E pk reached by cavities to 38 MV/m from the 22 MV/m achieved by a combination of CT and He processing. Surface magnetic fields greater than 1000 Oe were achieved in three out of the eight heat treatments, in contrast to one out of fifteen chemical treatments. The highest surface electric and magnetic fields achieved were 50.5 MV/m and 1260 Oe respectively. If these surface electric (magnetic) fields were reached in a 5-cell accelerating structure of the same cell geometry, the accelerating field would be 20.5 (27) MeV/m at a Q of ∼ 2 × 10 9. Most of the HT tests (including the record) were still limited by FE. We find that FE can be progressively reduced by He processing with increased rf power. Up to 160 W of rf power have been used during processing. A high speed/superfluid FE temperature mapping system was used to measure the power deposited by the impact of electrons emanating from field emitters. FE and defect associated heating are characterized through detailed analysis of temperature distribution maps over cavity surfaces. The maps show a greater abundance of emitters present on CT cavity surfaces than on HT surfaces.
Published Version
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