Abstract
In niobium superconducting radio frequency (SRF) cavities for particle acceleration, a decrease of the quality factor at lower fields-a so-called low field Q slope or LFQS-has been a long-standing unexplained effect. By extending the high Q measurement techniques to ultralow fields, we discover two previously unknown features of the effect: (i)saturation at rf fields lower than E_{acc}∼0.1 MV/m; (ii)strong degradation enhancement by growing thicker niobium pentoxide. Our findings suggest that the LFQS may be caused by the two level systems in the natural niobium oxide on the inner cavity surface, thereby identifying a new source of residual resistance and providing guidance for potential nonaccelerator low-field applications of SRF cavities.
Highlights
In niobium superconducting radio frequency (SRF) cavities for particle acceleration, a decrease of the quality factor at lower fields—a so-called low field Q slope or low field Q slope” (LFQS)—has been a long-standing unexplained effect
Our findings suggest that the LFQS may be caused by the two level systems in the natural niobium oxide on the inner cavity surface, thereby identifying a new source of residual resistance and providing guidance for potential nonaccelerator low-field applications of SRF cavities
In addition to the physics of residual resistance, understanding of the LFQS has recently acquired strong practical cross-discipline interest as a range of potential nonaccelerating applications of high Q SRF cavities emerged in particle physics [12], quantum computing [13,14,15], astrophysics [16], superconducting parametric conversion [17], and gravitational wave detection [18,19], for which operation in the limit of very low rf fields and/or temperatures (T ≲ 25 mK) is of interest
Summary
In niobium superconducting radio frequency (SRF) cavities for particle acceleration, a decrease of the quality factor at lower fields—a so-called low field Q slope or LFQS—has been a long-standing unexplained effect. Our findings suggest that the LFQS may be caused by the two level systems in the natural niobium oxide on the inner cavity surface, thereby identifying a new source of residual resistance and providing guidance for potential nonaccelerator low-field applications of SRF cavities.
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