The “Q disease” in Superconducting Niobium RF Cavities

Knobloch, Jens

doi:10.1063/1.1597364

Cited by 41 publications

(47 citation statements)

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“…The concentration of impurities (hydrogen, oxygen, nitrogen, and carbon) within the rf penetration layer (≈40 nm for pure Nb) is believed to be the key factor regarding the performance of cavities, either by lowering the electron mean-free path [4] and energy gap [5] or by trapping potential hydrogen which can cluster and precipitate niobium hydrides upon cooldown [6,7]. Traditionally, Nb-based SRF cavities were subjected to several production steps, including different polishing cycles like buffered chemical polishing (BCP) and electrochemical polishing (EP), as well as heat treatments at 800°C and 120°C [1,8] in a high vacuum in order to lower the hydrogen content inherent to the material [9]. In recent years, controlled annealing of SRF cavities in nitrogen atmospheres, at either high or low temperatures for different time spans, showed a decrease in their surface resistance [2,3].…”

Section: Introductionmentioning

confidence: 99%

Niobium near-surface composition during nitrogen infusion relevant for superconducting radio-frequency cavities

Semione

Pandey

Tober

et al. 2019

Phys. Rev. Accel. Beams

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A detailed study of the near-surface structure and composition of Nb, the material of choice for superconducting radio-frequency accelerator (SRF) cavities, is of great importance in order to understand the effects of different treatments applied during cavity production. By means of surface-sensitive techniques such as grazing incidence diffuse x-ray scattering, x-ray reflectivity, and x-ray photoelectron spectroscopy, single-crystalline Nb(100) samples were investigated in and ex situ during annealing in an ultrahigh vacuum as well as in nitrogen atmospheres with temperatures and pressures similar to the ones employed in real Nb cavity treatments. Annealing of Nb specimens up to 800°C in a vacuum promotes a partial reduction of the natural surface oxides (Nb 2 O 5 , NbO 2 , and NbO) into NbO. Upon cooling to 120°C, no evidence of nitrogen-rich layers was detected after nitrogen exposure times of up to 48 h. An oxygen enrichment below the Nb-oxide interface and posterior diffusion of oxygen species towards the Nb matrix, along with a partial reduction of the natural surface oxides, was observed upon a stepwise annealing up to 250°C. Nitrogen introduction to the system at 250°C promotes neither N diffusion into the Nb matrix nor the formation of new surface layers. Upon further heating to 500°C in a nitrogen atmosphere, the growth of a new subsurface Nb x N y layer was detected. These results shed light on the composition of the near-surface region of Nb after low-temperature nitrogen treatments, which are reported to lead to a performance enhancement of SRF cavities.

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Section: Introductionmentioning

confidence: 99%

Niobium near-surface composition during nitrogen infusion relevant for superconducting radio-frequency cavities

Semione

Pandey

Tober

et al. 2019

Phys. Rev. Accel. Beams

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“…SRF cavities that are heavily loaded with hydrogen (>2 wt. ppm [38]) commonly exhibit a fall-off of Q immediately with the application of H RF (called Q sickness or Q disease [39]). This may be due to large hydride precipitates.…”

Section: Figurementioning

confidence: 99%

First-principles calculations of niobium hydride formation in superconducting radio-frequency cavities

Ford

Cooley

Seidman

2013

Supercond. Sci. Technol.

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Abstract. Niobium hydride is suspected to be a major contributor to degradation of the quality factor of niobium superconducting radio-frequency (SRF) cavities. In this study, we connect the fundamental properties of hydrogen in niobium to SRF cavity performance and processing. We modeled several of the niobium hydride phases relevant to SRF cavities and present their thermodynamic, electronic, and geometric properties determined from calculations based on density-functional theory. We find that the absorption of hydrogen from the gas phase into niobium is exothermic and hydrogen becomes somewhat anionic. The absorption of hydrogen by niobium lattice vacancies is strongly preferred over absorption into interstitial sites. A single vacancy can accommodate six hydrogen atoms in the symmetrically equivalent lowest-energy sites and additional hydrogen in the nearby interstitial sites affected by the strain field: this indicates that a vacancy can serve as a nucleation center for hydride phase formation. Small hydride precipitates may then occur near lattice vacancies upon cooling. Vacancy clusters and extended defects should also be enriched in hydrogen, potentially resulting in extended hydride phase regions upon cooling. We also assess the phase changes in the niobium-hydrogen system based on charge transfer between niobium and hydrogen, the strain field associated with interstitial hydrogen, and the geometry of the hydride phases. The results of this study stress the importance of not only the hydrogen content in niobium, but also the recovery state of niobium for the performance of SRF cavities.

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“…Hydrides have been identified in the early 1990s as a source of increased rf surface dissipation upon slow cooldowns [106][107][108][109][110][111][112]. Although the bulk concentration was less than 0.01 at.%, precipitation of hydrides was evidenced upon slow cooldown (rate < 1 K= min) or when high RRR cavities were allowed to stay between 75-150 K (with a saturation of the effect after 3 hours [112]).…”

Section: Hydridesmentioning

confidence: 99%

“…Although the bulk concentration was less than 0.01 at.%, precipitation of hydrides was evidenced upon slow cooldown (rate < 1 K= min) or when high RRR cavities were allowed to stay between 75-150 K (with a saturation of the effect after 3 hours [112]). The quality factor Q 0 value of slow cooled cavities can lose up to 3 orders of magnitude, and R BCS as well as the residual resistance R 0 are affected [108]. The incorporation of H happens principally during etching, but it is a collateral effect resulting from the absence of the oxide, which is an effective diffusion barrier to atomic H [110].…”

Section: Hydridesmentioning

confidence: 99%

Influence of crystalline structure on rf dissipation in superconducting niobium

Antoine

2019

Phys. Rev. Accel. Beams

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Bulk niobium is the most common material used in the fabrication of rf superconducting cavities for accelerators. Predicting and reducing the rf surface dissipation in these cavity structures is mandatory, since it has a tremendous cost impact on the large accelerator projects. In this paper the author hopes to demonstrate that sources of dissipation usually attributed to external causes (mainly flux trapping during cooldown and hydrides precipitates) are related to the same type of crystalline defects that affect the local superconducting properties and can be at the source of early vortex penetration at the surface. We also want to show how these types of defects can explain some of the discrepancies observed from other laboratories in niobium cavity doping experiments. Understanding the origin and the role of these defects could provide direction for improving material specifications as well as improving fabrication control from sheet material to completed cavity. In particular, we will demonstrate that dislocation entanglements, due to the fabrication damage layer, have the strongest impact for the pinning behavior of trapped flux, as well as hydrogen segregation in cavity niobium. The author wishes to present to the superconducting radio frequency (SRF) accelerator community the synthesis of experimental results scattered in the literature, completed with some personal results. The results of this effort provide a new perspective on recently published work in the domain of SRF cavity doping and sensitivity to trapped flux during cooldown. I will also try to draw whenever possible, some conclusions about other types of superconductors used for SRF applications including Nb=Cu thin films and to discuss their possible change of behavior with field or frequency. I will concentrate on surface and material science aspects since the experimental results on rf cavities have already been treated elsewhere.

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The “Q disease” in Superconducting Niobium RF Cavities

Cited by 41 publications

References 0 publications

Niobium near-surface composition during nitrogen infusion relevant for superconducting radio-frequency cavities

Niobium near-surface composition during nitrogen infusion relevant for superconducting radio-frequency cavities

First-principles calculations of niobium hydride formation in superconducting radio-frequency cavities

Influence of crystalline structure on rf dissipation in superconducting niobium

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