Analysis of Critical Power Loss in a Superconductor

Rabinowitz, Mario

doi:10.1063/1.1659662

Cited by 40 publications

(25 citation statements)

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“…A discussion of the trapping of flux due to an incomplete MeissnerOchsenfeld effect, and the related power dissipation in an rf superconductor, is given by Rabinowitz. [3] Assuming that the only nonsuperconducting loss is due to trapped flux, the total average power loss for a magnetic field H p coswt at the cavity surface is giving an effective surface resistance for the cavity …”

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Frequency Dependence of Superconducting Cavity Q and Magnetic Breakdown Field

Rabinowitz

1971

Applied Physics Letters

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A theoretical explanation is given to account for the unexpected observation that L-and Sband Nb superconducting cavities were found to have lower Q and lower magnetic breakdown field than those of the higher X-band frequencies. According to established theories [1, 2] of rf superconductivity, one would expect the superconducting surface resistance R s to be approximately proportional to the square of the cavity angular frequency ω. Hence, in going to the lower frequency L-and S -band Nb cavities, it was expected that the Q's would be as high or higher than with comparably processed X-band cavities. However, as we shall see, this may not be the case when the surface resistance is dominated by trapped flux at the operating temperature. A discussion of the trapping of flux due to an incomplete MeissnerOchsenfeld effect, and the related power dissipation in an rf superconductor, is given by Rabinowitz. [3] Assuming that the only nonsuperconducting loss is due to trapped flux, the total average power loss for a magnetic field H p coswt at the cavity surface is giving an effective surface resistance for the cavity

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Frequency Dependence of Superconducting Cavity Q and Magnetic Breakdown Field

Rabinowitz

1971

Applied Physics Letters

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“…We assume that flux will penetrate first at sites where the surface barrier is reduced, which then causes an increase in surface resistance due to simple oscillatory fluxoid motion driven by the Lorentz force of the RF screening current, as considered in [26]. We then consider a worst case in which the entire surface profile is composed of triangular grooves to estimate the surface resistance.…”

Section: Model For Q-drop At High Fieldsmentioning

confidence: 99%

“…where  is a dissipation of 10 -14 -m determined by thermal stability conditions [26]. We then integrate contributions to surface resistance over the angle distribution:…”

Section: Model For Q-drop At High Fieldsmentioning

confidence: 99%

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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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“…Additionally, the normal resistivity is an important parameter in determining magnetic breakdown. [4] Not only have resistivity measurements not been reported for a cavity or a representative sample, but neither have thermal-conductivity measurements. The normal thermal conductivity is derivable from the Wiedemann-Franz law, at least for the less pure samples.…”

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Resistivity Ratio of Niobium Superconducting Cavities

Garwin

Rabinowitz

1972

Applied Physics Letters

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Resistivity measurements have been made on Nb cavities, as well as on Pb and Cu, at 296, 77, and 4.2 o K by means of a contactless induced-current method. For superconductors, a constant magnetic field drives the material normal below the transition temperature. These measurements provide a simple means for initial material evaluation as well as a direct means of monitoring the effects of material parameters (purity, heat treatment, gas incorporation, etc.) on the electron mean free path.Approximate determinations of H c , H c1 , and H c2 can also be derived from these measurements. Normal-state thermal conductivity and the Ginzburg-Landau parameter κ are calculated from the resistivity measurements.Measurement of the resistivity ratio (between 296 and 4.2 o K ) of Nb rf superconducting cavities is desirable for many reasons. Resistivity-ratio measurements before and after vacuum firing can indicate the decrease in impurity content and other crystalline defects, as well as measure the increase in the electron mean free path. The latter parameter is important in designing superconducting accelerators and in analyzing the results from superconducting cavities. The effects on the electron mean free path due to irradiation, incorporation of gases, etc., can also be determined. The difficulty in measuring resistivity by ordinary means lies in the size and unwieldy

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Analysis of Critical Power Loss in a Superconductor

Abstract: A critical power dissipation resulting from an oscillating magnetic field, H p cos ωt, can produce a magnetic breakdown field, H p '

Cited by 40 publications

References 13 publications

Frequency Dependence of Superconducting Cavity Q and Magnetic Breakdown Field

Frequency Dependence of Superconducting Cavity Q and Magnetic Breakdown Field

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

Resistivity Ratio of Niobium Superconducting Cavities

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