Quench-Limited SRF Cavities: Failure at the Heat-Affected Zone

Champion, M.; Cooley, L. D.; Ginsburg, C. M.; Sergatskov, Dmitri; Geng, Rongli; Hayano, H.; Iwashita, Yoshihisa; Tajima, Y.

doi:10.1109/tasc.2009.2019204

Cited by 16 publications

(18 citation statements)

References 7 publications

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“…Pits associated with the heat-affected zone (HAZ) of equatorial welds on superconducting radio-frequency (SRF) cavities continue to pose limitations on the achievement of high accelerating gradient [1]. Pits are observed on about 3% of cavity equator electron-beam welds [2][3][4], and even though some are (curiously) benign, roughly one cavity in every five is limited by quench at a pit location.…”

Section: Introductionmentioning

confidence: 99%

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Annealing to mitigate pitting in electropolished niobium coupons and SRF cavities

Cooley¹,

Hahn²,

Hicks³

et al. 2012

AIP Conference Proceedings

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Ongoing studies at Fermilab investigate whether dislocations and other factors instigate pitting during cavity electropolishing (EP), despite careful processing controls and the inherent leveling mechanism of EP itself. Here, cold-worked niobium coupons, which exhibited increased tendencies for pitting in our past study, were annealed in a high vacuum furnace and subsequently processed by EP. Laser confocal scanning microscopy and special defect counting algorithms were used to assess the population of pits formed. Hardness measurements indicated that annealing for 2 hours at 800 °C produced recovery, whereas annealing for 12 hours at 600 °C did not, as is consistent with known changes for cavities annealed in a similar way. The 800 °C anneal was effective in some cases but not others, and we discuss reasons why tendencies for pitting remain. We discuss implications for cavities and continued work to understand pitting.

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Section: Introductionmentioning

confidence: 99%

“…Bubbles of trapped gas might occur when small defects are vaporized by the electron beam [1], or possibly when hydrogen incorporated during pre-weld etching becomes accumulated. Such bubbles would be consistent with the spherical shape noted for many pits [4].…”

Section: Introductionmentioning

confidence: 99%

Annealing to mitigate pitting in electropolished niobium coupons and SRF cavities

Cooley¹,

Hahn²,

Hicks³

et al. 2012

AIP Conference Proceedings

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“…Sub-par and, especially, poor cavities often exhibit quenches at particular locations, as revealed by varying the resonating mode to isolate bad cavity cells and by locating the quench with external thermometer arrays [2], [8]. When the quench location is subsequently examined by new cavity optical inspection tools [9], large pits, bumps, and other features are found [8], [10].…”

Section: Introductionmentioning

confidence: 99%

“…When the quench location is subsequently examined by new cavity optical inspection tools [9], large pits, bumps, and other features are found [8], [10]. Typical sizes are 0.1 to 1.0 mm width and 10 to 100 m depth [10].…”

Section: Introductionmentioning

confidence: 99%

Impact of Forming, Welding, and Electropolishing on Pitting and the Surface Finish of SRF Cavity Niobium

Cooley

Burk

Cooper

et al. 2011

IEEE Trans. Appl. Supercond.

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Abstract-A broad range of coupon electropolishing experiments are described to ascertain the mechanism(s) by which large defects are formed near superconducting radiofrequency (SRF) cavity welds. Cold-worked vs. annealed metal, the presence of a weld, and several variations of electropolishing (EP) parameters were considered. Pitting is strongly promoted by cold work and agitation of the EP solution. Welding also promotes pitting, but less so compared with the other factors above. Temperature increase during EP did not strongly affect glossiness or pitting, but the reduced viscosity made the electrolyte more susceptible to agitation. The experiments suggest that several factors that are rather benign alone are combined by the cavity forming, welding, and processing sequence to promote the formation of defects such as pits. Process changes to mitigate these risks are discussed.

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“…Preparation of coupon samples is difficult to make relevant to actual cavity processing conditions since the thermal gradients, flow patterns, and mechanical treatments such as deep drawing are difficult to simulate exactly. While topographical data can be obtained directly from cavities by replicas [16,17], and inferred from images obtained by new optical inspection systems [9], so far the primary use of cavity inspections has been to debug gross processing defects [8,18], and discussion of fine-scale information is only just beginning.…”

mentioning

confidence: 99%

Model for initiation of quality factor degradation at high accelerating fields in superconducting radio-frequency cavities

Dzyuba¹,

Romanenko²,

Cooley³

2010

Supercond. Sci. Technol.

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A model for the onset of the reduction in SRF cavity quality factor, the so-called Q-drop, at high accelerating electric fields is presented. Since magnetic fields at the cavity equator are tied to accelerating electric fields by a simple geometric factor, the onset of magnetic flux penetration determines the onset of Q-drop. We consider breakdown of the surface barrier at triangular grooves to predict the magnetic field of first flux penetration H pen . Such defects were argued to be the worst case by Buzdin and Daumens, [1998 Physica C 294 257], whose approach, moreover, incorporates both the geometry of the groove and local contamination via the Ginzburg-Landau parameter . Since previous Q-drop models focused on either topography or contamination alone, the proposed model allows new comparisons of one effect in relation to the other. The model predicts equivalent reduction of H pen when either roughness or contamination were varied alone, so smooth but dirty surfaces limit cavity performance about as much as rough but clean surfaces do. Still lower H pen was predicted when both effects were combined, i.e. contamination should exacerbate the negative effects of roughness and vice-versa. To test the model with actual data, coupons were prepared by buffered chemical polishing and electropolishing, and stylus profilometry was used to obtain distributions of angles. From these data, curves for surface resistance generated by simple flux flow as a function of magnetic field were generated by integrating over the distribution of angles for reasonable values of . This showed that combined effects of roughness and contamination indeed reduce the Q-drop onset field by ~20%, and that that contamination contributes to Q-drop as much as roughness. The latter point may be overlooked by SRF cavity research, since access to the cavity interior by spectroscopy tools is very difficult, whereas optical images have become commonplace. The model was extended to fit cavity test data, which indicated that reduction of the superconducting gap by contaminants may also play a role in Q-drop.

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Quench-Limited SRF Cavities: Failure at the Heat-Affected Zone

Cited by 16 publications

References 7 publications

Annealing to mitigate pitting in electropolished niobium coupons and SRF cavities

Annealing to mitigate pitting in electropolished niobium coupons and SRF cavities

Impact of Forming, Welding, and Electropolishing on Pitting and the Surface Finish of SRF Cavity Niobium

Model for initiation of quality factor degradation at high accelerating fields in superconducting radio-frequency cavities

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