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Title: Large Grain Niobium Cavity R and D In Asia and the Future

Abstract

The status of the large grain niobium cavity R and D in Asia and the future scope are presented. Recently KEK has received CBMM and NingXia large grain niobium sheets through collaborations. KEK has fabricated 1.3 GHz single cell cavities using these materials and measured the cavity performance. Those results are presented in this paper.

Authors:
; ; ;  [1]; ;  [2];  [3]; ; ;  [4]; ;  [5]
  1. KEK, Tsukuba Ibaraki-ken (Japan)
  2. Pusan University, Pusan (Korea, Republic of)
  3. Kyungpook University, Daegu (Korea, Republic of)
  4. IHEP, Beijing (China)
  5. Jefferson Lab, 12000 Jefferson Ave., Newport News, VA 23606 (United States)
Publication Date:
OSTI Identifier:
21067375
Resource Type:
Journal Article
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 927; Journal Issue: 1; Conference: International niobium workshop on single crystal - large grain niobium technology, Araxa (Brazil), 30 Oct - 1 Nov 2006; Other Information: DOI: 10.1063/1.2770688; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; GHZ RANGE; GRAIN SIZE; JAPANESE ORGANIZATIONS; MONOCRYSTALS; NIOBIUM; PERFORMANCE; SHEETS; SUPERCONDUCTING CAVITY RESONATORS; SUPERCONDUCTORS

Citation Formats

Saito, K., Furuta, F., Saeki, T., Inoue, H., Shim, J., Ahn, J., Kim, E. S., Xu, Q., Zong, Z., Gao, J., Kneisel, P., and Myneni, G. R.. Large Grain Niobium Cavity R and D In Asia and the Future. United States: N. p., 2007. Web. doi:10.1063/1.2770688.
Saito, K., Furuta, F., Saeki, T., Inoue, H., Shim, J., Ahn, J., Kim, E. S., Xu, Q., Zong, Z., Gao, J., Kneisel, P., & Myneni, G. R.. Large Grain Niobium Cavity R and D In Asia and the Future. United States. doi:10.1063/1.2770688.
Saito, K., Furuta, F., Saeki, T., Inoue, H., Shim, J., Ahn, J., Kim, E. S., Xu, Q., Zong, Z., Gao, J., Kneisel, P., and Myneni, G. R.. 2007. "Large Grain Niobium Cavity R and D In Asia and the Future". United States. doi:10.1063/1.2770688.
@article{osti_21067375,
title = {Large Grain Niobium Cavity R and D In Asia and the Future},
author = {Saito, K. and Furuta, F. and Saeki, T. and Inoue, H. and Shim, J. and Ahn, J. and Kim, E. S. and Xu, Q. and Zong, Z. and Gao, J. and Kneisel, P. and Myneni, G. R.},
abstractNote = {The status of the large grain niobium cavity R and D in Asia and the future scope are presented. Recently KEK has received CBMM and NingXia large grain niobium sheets through collaborations. KEK has fabricated 1.3 GHz single cell cavities using these materials and measured the cavity performance. Those results are presented in this paper.},
doi = {10.1063/1.2770688},
journal = {AIP Conference Proceedings},
number = 1,
volume = 927,
place = {United States},
year = 2007,
month = 8
}
  • The status of the large grain niobium cavity R&D in Asia and the future scope are presented. Recently KEK has received CBMM and NingXia large grain niobium sheets through collaborations. KEK has fabricated 1.3 GHz single cell cavities using these materials and measured the cavity performance. Those results are presented in this paper.
  • The performance of superconducting radio-frequency (SRF) resonant cavities made of bulk niobium is limited by nonlinear localized effects. Surface analysis of regions of higher power dissipation is thus of intense interest. Such areas (referred to as “hotspots”) were identified in a large-grain single-cell cavity that had been buffered-chemical polished and dissected for examination by high resolution electron microscopy, electron backscattered diffraction microscopy (EBSD), and optical microscopy. Pits with clearly discernible crystal facets were observed in both “hotspot” and “coldspot” specimens. The pits were found in-grain, at bicrystal boundaries, and on tricrystal junctions. They are interpreted as etch pits induced bymore » crystal defects (e.g. dislocations). All coldspots examined had a qualitatively lower density of etch pits or relatively smooth tricrystal boundary junctions. EBSD mapping revealed the crystal orientation surrounding the pits. Locations with high pit density are correlated with higher mean values of the local average misorientation angle distributions, indicating a higher geometrically necessary dislocation content. In addition, a survey of the samples by energy dispersive x-ray analysis did not show any significant contamination of the samples’ surface. In conclusion, the local magnetic field enhancement produced by the sharp-edge features observed on the samples is not sufficient to explain the observed degradation of the cavity quality factor, which starts at peak surface magnetic field as low as 20 mT.« less
  • The performance of a superconducting radio-frequency (RF) cavity made of residual resistivity ratio (RRR) > 200 large-grain niobium has been investigated as a function of material removal, between 70 and 240 ?m, by buffered chemical polishing (BCP). Temperature maps of the cavity surface at 1.7 and 2.0 K were taken for each step of chemical etching and revealed localized losses (''hot-spots''), which contribute to the degradation of the cavity quality factor as a function of the RF surface field. It was found that the number of ''hot-spots'' decreased for larger material removal. Interestingly, the losses at the ''hot-spots'' at differentmore » locations evolved differently for successive material removal. The cavity achieved peak surface magnetic fields of about of 130 mT and was limited mostly by thermal quench. By measuring the temperature dependence of the surface resistance (Rs) at low field between 4.2 K and 1.7 K, the variation of material parameters such as the energy gap at 0 K, the residual resistance and the mean free path as a function of material removal could also be investigated. This contribution presents the results of the RF tests along with the temperature maps and the analysis of the losses caused by the ''hot-spots''.« less
  • The most challenging issue for understanding the performance of superconducting radio-frequency (rf) cavities made of high-purity (residual resistivity ratio > 200) niobium is due to a sharp degradation (“Q-drop”) of the cavity quality factor Q0(Bp) as the peak surface magnetic field (Bp) exceeds about 90 mT, in the absence of field emission. In addition, a low-temperature (100 – 140 C) “in-situ” baking of the cavity was found to be beneficial in reducing the Q-drop. In this contribution, we present the results from a series of rf tests at 1.7 K and 2.0 K on a single-cell cavity made of high-puritymore » large (with area of the order of few cm2) grain niobium which underwent various oxidation processes, after initial buffered chemical polishing, such as anodization, baking in pure oxygen atmosphere and baking in air up to 180 °C, with the objective of clearly identifying the role of oxygen and the oxide layer on the Q-drop. During each rf test a temperature mapping system allows measuring the local temperature rise of the cavity outer surface due to rf losses, which gives information about the losses location, their field dependence and space distribution. The results confirmed that the depth affected by baking is about 20 – 30 nm from the surface and showed that the Q-drop did not re-appear in a previously baked cavity by further baking at 120 °C in pure oxygen atmosphere or in air up to 180 °C. These treatments increased the oxide thickness and oxygen concentration, measured on niobium samples which were processed with the cavity and were analyzed with Transmission Electron Microscope (TEM) and Secondary Ion Mass Spectroscopy (SIMS). Nevertheless, the performance of the cavity after air baking at 180 °C degraded significantly and the temperature maps showed high losses, uniformly distributed on the surface, which could be completely recovered only by a post-purification treatment at 1250 °C. A statistic of the position of the “hot-spots” on the cavity surface showed that grain-boundaries are not the preferred location. An interesting correlation was found between the Q-drop onset, the quench field and the low-field energy gap, which supports the hypothesis of thermo-magnetic instability governing the Q-drop and the baking effect.« less