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Romanenko, A.

Publications and source records attributed to Romanenko, A..

53 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Performance-defining properties of Nb 3 Sn coating in SRF cavities

Nb 3 Sn has potential to become the material of choice for fabrication of SRF cavities. The higher critical temperature of Nb 3 Sn potentially allows for an increased operational temperature in SRF cavities, which could enable a simplification of the cryogenic system, leading to significant cost reduction. We present extended characterization of a Nb 3 Sn coated Nb cavity prepared at Cornell University. Using a combination of thermometry during cavity RF measurements, and structural and analytical characterization of the cavity cutouts, we discovered Nb 3 Sn coating flaws in regions that exhibit significant heating during the RF testing. Results of extended comparison of cavity cutouts with different dissipation profiles not only indicate the cause of significant Q degradation but also establish figures of merit for material qualities in relation to the quality of SRF performance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Effect of interstitial impurities on the field dependent microwave surface resistance of niobium

Previous work has demonstrated that the radio frequency surface resistance of niobium resonators is dramatically reduced when nitrogen impurities are dissolved as interstitial in the material. This effect is attributed to the lowering of the Mattis-Bardeen surface resistance with increasing accelerating field; however, the microscopic origin of this phenomenon is poorly understood. Meanwhile, an enhancement of the sensitivity to trapped magnetic field is typically observed for such cavities. In this paper, we conduct a systematic study on these different components contributing to the total surface resistance as a function of different levels of dissolved nitrogen, in comparison with standard surface treatments for niobium resonators. Adding these results together, we are able to show which is the optimum surface treatment that maximizes the Q-factor of superconducting niobium resonators as a function of expected trapped magnetic field in the cavity walls. These results also provide insights on the physics behind the change in the field dependence of the Mattis-Bardeen surface resistance, and of the trapped magnetic vortex induced losses in superconducting niobium resonators.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Efficient expulsion of magnetic flux in superconducting radiofrequency cavities for high Q{sub 0} applications

Even when cooled through its transition temperature in the presence of an external magnetic field, a superconductor can expel nearly all external magnetic flux. This paper presents an experimental study to identify the parameters that most strongly influence flux trapping in high purity niobium during cooldown. This is critical to the operation of superconducting radiofrequency cavities, in which trapped flux degrades the quality factor and therefore cryogenic efficiency. Flux expulsion was measured on a large survey of 1.3 GHz cavities prepared in various ways. It is shown that both spatial thermal gradient and high temperature treatment are critical to expelling external magnetic fields, while surface treatment has minimal effect. For the first time, it is shown that a cavity can be converted from poor expulsion behavior to strong expulsion behavior after furnace treatment, resulting in a substantial improvement in quality factor. Microscopic investigations are performed to study the relevant changes in the material from this treatment. Future plans are described to build on this result in order to optimize treatment for future cavities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

LCLS-II SRF Cavity Processing Protocol Development and Baseline Cavity Performance Demonstration

The ”Linac Coherent Light Source-II” Project will construct a 4 GeV CW superconducting RF linac in the first kilometer of the existing SLAC linac tunnel. The baseline design calls for 280 1.3 GHz nine-cell cavities with an average intrinsic quality factor Q0 of 2.7·1010 at 2K and 16 MV/m accelerating gradient. The LCLS-II high Q0 cavity treatment protocol utilizes the reduction in BCS surface resistance by nitrogen doping of the RF surface layer, which was discovered originally at FNAL. Cornell University, FNAL, and TJNAF conducted a joint high Q0 R&D program with the goal of (a) exploring the robustness of the N-doping technique and establishing the LCLS-II cavity high Q0 processing protocol suitable for production use, and (b) demonstrating that this process can reliably achieve LCLS-II cavity specification in a production acceptance testing setting. In this paper we describe the LCLS-II cavity protocol and analyze combined cavity performance data from both vertical and horizontal testing at the three partner labs, which clearly shows that LCLS-II specifications were met, and thus demonstrates readiness for LCLS-II cavity production.

Liepe, M.↗
DOE OSTI2015

Magnetic flux studies in horizontally cooled elliptical superconducting cavities

Previous studies on magnetic flux expulsion as a function of cooldown procedures for elliptical superconducting radio frequency (SRF) niobium cavities showed that when the cavity beam axis is placed parallel to the helium cooling flow and sufficiently large thermal gradients are achieved, all magnetic flux could be expelled and very low residual resistance could be achieved. In this paper, we investigate flux trapping for the case of resonators positioned perpendicularly to the helium cooling flow, which is more representative of how SRF cavities are cooled in accelerators and for different directions of the applied magnetic field surrounding the resonator. We show that different field components have a different impact on the surface resistance, and several parameters have to be considered to fully understand the flux dynamics. A newly discovered phenomenon of concentration of flux lines at the cavity top leading to temperature rise at the cavity equator is presented.

43 PARTICLE ACCELERATORS↗
DOE OSTI2015

Nanostructural features degrading the performance of superconducting radio frequency niobium cavities revealed by transmission electron microscopy and electron energy loss spectroscopy

Nanoscale defect structure within the magnetic penetration depth of ∼100 nm is key to the performance limitations of niobium superconducting radio frequency cavities. Using a unique combination of advanced thermometry during cavity RF measurements, and TEM structural and compositional characterization of the samples extracted from cavity walls, we discover the existence of nanoscale hydrides in electropolished cavities limited by the high field Q slope, and show the decreased hydride formation in the electropolished cavity after 120 °C baking. Furthermore, we demonstrate that adding 800 °C hydrogen degassing followed by light buffered chemical polishing restores the hydride formation to the pre-120 °C bake level. We also show absence of niobium oxides along the grain boundaries and the modifications of the surface oxide upon 120 °C bake.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Cooling Dynamics Through Transition Temperature of Niobium SRF Cavities Captured by Temperature Mapping

Cool-down dynamics of superconducting accelerating cavities became particularly important for obtaining very high quality factors in SRF cavities. Previous studies proved that when cavity is cooled fast, the quality factor is higher than when cavity is cooled slowly. This has been discovered to derive from the fact that a fast cool-down allows better magnetic field expulsion during the superconducting transition. In this paper we describe the first experiment where the temperature all around the cavity was mapped during the cavity cool-down through transition temperature, proving the existence of two different transition dynamics: a sharp superconducting-normal conducting transition during fast cool-down which favors flux expulsion and nucleation phase transition during slow cool-down, which leads to full flux trapping.

43 PARTICLE ACCELERATORS↗
DOE OSTI2015

Nitrogen-doped 9-cell cavity performance in a test cryomodule for LCLS-II

The superconducting RF linac for Linac Coherent Light Source-II calls for 1.3 GHz 9-cell cavities with an average intrinsic quality factor Q{sub 0} of 2.7 × 10{sup 10} at 2.0 K and 16 MV/m accelerating gradient. Two niobium 9 cell cavities, prepared with nitrogen-doping at Fermilab, were assembled into the Cornell Horizontal Test Cryomodule (HTC) to test cavity performance in a cryomodule that is very similar to a full LCLS-II cryomodule. The cavities met LCLS-II specifications with an average quench field of 17 MV/m and an average Q{sub 0} of 3 × 10{sup 10}. The sensitivity of the cavities' residual resistance to ambient magnetic field was determined to be 0.5 nΩ/mG during fast cool down. In two cool downs, a heater attached to one of the cavity beam tubes was used to induce large horizontal temperature gradients. Here, we report on the results of these first tests of nitrogen-doped cavities in a cryomodule, which provide critical information for the LCLS-II project.

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

Ultra-high quality factors in superconducting niobium cavities in ambient magnetic fields up to 190 mG

Ambient magnetic field, if trapped in the penetration depth, leads to the residual resistance and therefore sets the limit for the achievable quality factors in superconducting niobium resonators for particle accelerators. Here, we show that a complete expulsion of the magnetic flux can be performed and leads to: (1) record quality factors Q > 2 × 10{sup 11} up to accelerating gradient of 22 MV/m; (2) Q ∼ 3 × 10{sup 10} at 2 K and 16 MV/m in up to 190 mG magnetic fields. This is achieved by large thermal gradients at the normal/superconducting phase front during the cooldown. Our findings open up a way to ultra-high quality factors at low temperatures and show an alternative to the sophisticated magnetic shielding implemented in modern superconducting accelerators.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Dependence of the residual surface resistance of superconducting radio frequency cavities on the cooling dynamics around T{sub c}

We report a strong effect of the cooling dynamics through T{sub c} on the amount of trapped external magnetic flux in superconducting niobium cavities. The effect is similar for fine grain and single crystal niobium and all surface treatments including electropolishing with and without 120 °C baking and nitrogen doping. Direct magnetic field measurements on the cavity walls show that the effect stems from changes in the flux trapping efficiency: slow cooling leads to almost complete flux trapping and higher residual resistance, while fast cooling leads to the much more efficient flux expulsion and lower residual resistance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗