Improvement of accelerating gradients in niobium quarter wave resonators

• Published in: Superconductor science & technology 2019-09, Vol.32 (9), p.95003
• Main Authors: Rai, A, Potukuchi, P N, Patra, P, Chaudhari, G K, Sonti, S S K, Karmakar, J, Karmakar, B, Sharma, A, Mathuria, D S, Pandey, A, Dutt, R N, Sahu, B K, Ghosh, S, Kanjilal, D
• Format: Article
• Language: English
• Subjects: accelerating gradient; field emission; heat treatment; helium processing; high pressure rinsing; quality factor; quarter wave resonator
• ISSN: 0953-2048; 1361-6668
• DOI: 10.1088/1361-6668/ab2794

Systematic studies have been performed on the effect of the surface processing techniques for improving accelerating gradients in superconducting niobium quarter wave resonators (QWR). These include high pressure rinsing (HPR), high temperature heat treatment of cavities and helium pulse processing. Tests done after HPR have not only shown a reduction in field emission in the cavities at high accelerating gradients but also an improvement in the low field quality factor (Q). The effect of the high temperature (650 °C) heat treatment of jacketed QWRs (QWR with the outer helium vessel) on the cavity gradients has also been investigated. This was performed for two different QWR designs and a substantial improvement in performance has been observed in both the cases. The increase in gradients is beyond that due to hydrogen degassing alone. Helium pulse processing during 4 K tests has been tried out on several cavities and its effect on the quality factor at both high and low gradients has been observed. This technique has been found to be useful for those resonators which have a high Q at lower fields but are limited due to the field emission at higher gradients. They have exhibited a marked improvement in the high field Q-slope over and above that obtained with conventional pulse processing under high vacuum. A comprehensive overview of all these developments carried out over the past few years has been reported.