M. Liepe

Cornell University, Итак, New York, United States

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Publications (107)51.76 Total impact

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    ABSTRACT: Cornell University has finished building a 10 m long superconducting accelerator module as a prototype of the main linac of a proposed ERL facility. This module houses 6 superconducting cavities- operated at 1.8 K in continuous wave (CW) mode - with individual HOM absorbers and one magnet/ BPM section. In pushing the limits, a high quality factor of the cavities (2•1010) and high beam currents (100 mA accelerated plus 100 mA decelerated) were targeted. The design of the cryomodule and the results of components tested before assembly will be presented in this paper.
    No preview · Article · Dec 2015
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    ABSTRACT: Recent progress on vapour diffusion coated Nb 3 Sn SRF cavities makes this material a very promising alternative for CW medium field SRF applications. In this paper we report on several systematic studies to determine the sources currently limiting the performance of Nb 3 Sn cavities to determine improved coating parameters to overcome these limitations. These include a detailed study of the sensitivity of Nb 3 Sn to trapped ambient magnetic flux, a first measurement of the field dependence of the energy gap in Nb 3 Sn and detailed measurements of the stoichiometry of the obtained Nb 3 Sn coatings with synchrotron x-ray diffrac-tion and STEM. Initial results from a study on the impact of the coating process parameters on energy gap, Q-slope, and residual resistance, show clear dependencies, and thus directions for process optimization.
    Full-text · Conference Paper · Dec 2015
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    ABSTRACT: Studies of superconducting Nb 3 Sn cavities and samples at Cornell University and Argonne National Lab have shown that current state-of-the-art Nb 3 Sn cavities are limited by material properties and imperfections. In particular, the presence of regions within the Nb 3 Sn layer that are deficient in tin are suspected to be the cause of the lower than expected peak accelerating gradient. In this paper we present results from a material study of the Nb 3 Sn layer fabricated using the vapour deposition method, with data collected using AFM, SEM, TEM, EDX, and XRD methods as well as with pulsed RF testing.
    Full-text · Conference Paper · Dec 2015
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    ABSTRACT: Determining the optimal arrangement of superconducting layers to withstand large-amplitude ac magnetic fields is important for certain applications such as superconducting radio-frequency cavities. In this paper, we evaluate the shielding potential of the superconducting-film-insulating-film-superconductor (SIS') structure, a configuration that could provide benefits in screening large ac magnetic fields. After establishing that, for high-frequency magnetic fields, flux penetration must be avoided, the superheating field of the structure is calculated in the London limit both numerically and, for thin films, analytically. For intermediate film thicknesses and realistic material parameters, we also solve numerically the Ginzburg-Landau equations. It is shown that a small enhancement of the superheating field is possible, on the order of a few percent, for the SIS' structure relative to a bulk superconductor of the film material, if the materials and thicknesses are chosen appropriately.
    No preview · Article · Oct 2015 · Physical Review Applied
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    Dan Gonnella · Matthias Liepe
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    ABSTRACT: Future particle accelerators such as the the SLAC "Linac Coherent Light Source-II" (LCLS-II) and the proposed Cornell Energy Recovery Linac (ERL) require hundreds of superconducting RF (SRF) cavities operating in continuous wave (CW) mode. In order to achieve economic feasibility of projects such as these, the cavities must achieve a very high intrinsic quality factor (Q0). In order to reach these high Q0's in the case of LCLS-II, nitrogen-doping has been proposed as a cavity preparation technique. When dealing with Q0's greater than 1x10^10, the effects of ambient magnetic field on Q0 become significant. Here we show that the sensitivity that a cavity has to ambient magnetic field is highly dependent on the cavity preparation. Specifically, standard electropolished and 120C baked cavities show a sensitivity of ~0.8 and ~0.6 nOhm/mG trapped, respectively, while nitrogen-doped cavities show a sensitivity of ~2 to 5 nOhm/mG trapped. Less doping results in weaker sensitivity. This difference in sensitivities is directly related to the mean free path of the RF surface layer of the niobium: smaller mean free paths giving larger losses from trapped flux in the dirty limit.
    Preview · Article · Sep 2015
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    ABSTRACT: A Temperature mapping (T-map) system for Superconducting Radio Frequency (SRF) cavities consists of a thermometer array positioned precisely on an exterior cavity wall, capable of detecting small increases in temperature; therefore it is a powerful tool for research on the quality factor (Q0) of SRF cavities. A new multi-cell T-mapping system is has been developed at Cornell University. The system has nearly two thousand thermometers to cover 7-cell SRF cavities for Cornell ERL project. A new multiplexing scheme was adopted to reduce number of wires. A 1mK resolution of the temperature increase Delta T is achieved. A 9-cell cavity of TESLA geometry was tested with the T-map system. By converting Delta T to power loss and quality factor, it has been found that for this cavity, most surface losses were generated by the first cell when the accelerating gradient is increased above 15MV/m. The comparison of Q-value between with and without hotspots shows the heating on cavity wall degraded cavity Q0 about 1.65 times. The power loss on the hotspots is about 40% of the total power. Effective and intuitive ways of displaying surface properties of the cavity interior, e.g. the residual resistivity, will be shown.
    Preview · Article · Aug 2015
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    M. Ge · F. Furuta · M. Liepe · G. Hoffstaetter
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    ABSTRACT: Q-slope issue, which is caused by the field dependent surface resistance, puzzled people for a long time in SRF fields. In this paper, we related the Q-slope with surface treatments; and proposed a surface-impurity model to explain the field-dependent of surface resistance of SRF cavities. Eighteen cavity-test results have been analyzed to examine the model. These cavities were treated by different recipes: Nitrogen-doping; BCP and HF-rinsing; EP with 120{\deg}C baking; and EP without 120{\deg}C baking. The performance of these cavities, which is normally represented by cavity quality factor versus accelerating gradient or surface magnetic field curves (Q0 vs. Eacc or Q0 vs. B), has included all types of Q-slope, such as Low-field Q-slope, Medium-field Q-slope, and Anti-Q-slope. The data fittings are quite successful; the fitting results will be shown. The model can be used to evaluate the effectiveness of the surface treatments. At last, the paper discussed the way to build a high-Q high-gradient SRF cavity.
    Full-text · Article · Jul 2015
  • S Posen · N Valles · M Liepe
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    ABSTRACT: Superconducting radio frequency (srf) cavities, essential components of many large particle accelerators, rely on the metastable flux-free state of superconducting materials. In this Letter, we present results of experiments measuring the magnetic field limits of two srf materials, Nb and Nb_{3}Sn. Resonators made using these materials were probed using both high power rf pulses and dc magnetic fields. Nb, which is the current standard material for srf cavities in applications, was found to be limited by the superheating field H_{sh} when prepared using methods to avoid excessive rf dissipation at high fields. Nb_{3}Sn, which is a promising alternative material that is still in the early stages of development for srf purposes, was found to be limited between the onset field of metastability H_{c1} and H_{sh}. Analysis of the results shows that the limitation is consistent with nucleation of flux penetration at defects in the rf layer.
    No preview · Article · Jul 2015 · Physical Review Letters
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    ABSTRACT: Determining the optimal arrangement of superconducting layers to withstand large amplitude AC magnetic fields is important for certain applications such as superconducting radiofrequency cavities. In this paper, we evaluate the shielding potential of the superconducting film/insulating film/superconductor (SIS') structure, a configuration that could provide benefits in screening large AC magnetic fields. After establishing that for high frequency magnetic fields, flux penetration must be avoided, the superheating field of the structure is calculated in the London limit both numerically and, for thin films, analytically. For intermediate film thicknesses and realistic material parameters we also solve numerically the Ginzburg-Landau equations. It is shown that a small enhancement of the superheating field is possible, on the order of a few percent, for the SIS' structure relative to a bulk superconductor of the film material, if the materials and thicknesses are chosen appropriately.
    Full-text · Article · Jun 2015
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    S. Posen · M. Liepe · D. L. Hall
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    ABSTRACT: Many future particle accelerators require hundreds of superconducting radiofrequency (SRF) cavities operating with high duty factor. The large dynamic heat load of the cavities causes the cryogenic plant to make up a significant part of the overall cost of the facility. This contribution can be reduced by replacing standard niobium cavities with ones coated with a low-dissipation superconductor such as Nb 3Sn. In this paper, we present results for single cell cavities coated with Nb 3Sn at Cornell. Five coatings were carried out, showing that at 4.2 K, high Q 0 out to medium fields was reproducible, resulting in an average quench field of 14 MV/m and an average 4.2 K Q 0 at quench of 8 × 109. In each case, the peak surface magnetic field at quench was well above Hc 1, showing that it is not a limiting field in these cavities. The coating with the best performance had a quench field of 17 MV/m, exceeding gradient requirements for state-of-the-art high duty factor SRF accelerators. It is also shown that—taking into account the thermodynamic efficiency of the cryogenic plant—the 4.2 K Q 0 values obtained meet the AC power consumption requirements of state-of-the-art high duty factor accelerators, making this a proof-of-principle demonstration for Nb 3Sn cavities in future applications.
    Preview · Article · Feb 2015 · Applied Physics Letters

  • No preview · Article · Jan 2015
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    ABSTRACT: Over the past years it became evident that the quality factor of a superconducting cavity is not only determined by its surface preparation procedure, but is also influenced by the way the cavity is cooled down. Moreover, different data sets exists, some of them indicate that a slow cool-down through the critical temperature is favourable while other data states the exact opposite. Even so there where speculations and some models about the role of thermo-currents and flux-pinning, the difference in behaviour remained a mystery. In this paper we will for the first time present a consistent theoretical model which we confirmed by data that describes the role of thermo-currents, driven by temperature gradients and material transitions. We will clearly show how they impact the quality factor of a cavity, discuss our findings, relate it to findings at other labs and develop mitigation strategies which especially addresses the issue of achieving high quality factors of so-called nitrogen doped cavities in horizontal test.
    Full-text · Article · Nov 2014
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    ABSTRACT: The superconducting RF linac for LCLS-II calls for 1.3 GHz 9-cell cavities with an average intrinsic quality factor Q0 of 2.7x10^10 at 2 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 Q0 of 3x10^10. The sensitivity of the cavities' residual resistance to ambient magnetic field was determined to be 0.5 nOhm/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 cryomodule, which provide critical information for the LCLS-II project.
    Full-text · Article · Nov 2014 · Journal of Applied Physics
  • Sam Posen · Matthias Liepe
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    ABSTRACT: A 1.3 GHz ${\mathrm{Nb}}_{3}\mathrm{Sn}$ superconducting radio-frequency cavity prepared with a modified annealing step reached ${B}_{pk}>50\text{ }\text{ }\mathrm{mT}$, well above ${B}_{c1}=25\ifmmode\pm\else\textpm\fi{}7\text{ }\text{ }\mathrm{mT}$, without the strong $Q$-slope observed in previous ${\mathrm{Nb}}_{3}\mathrm{Sn}$ cavities. At 4.2 K, it has a ${Q}_{0}$ of approximately $1\ifmmode\times\else\texttimes\fi{}1{0}^{10}$ at $>10\text{ }\mathrm{MV}/\mathrm{m}$, far outperforming Nb at useable gradients. At 2 K, quench occurred at $$\sim${}55\text{ }\text{ }\mathrm{mT}$, apparently due to a defect, so additional treatment may increase the maximum gradient. Material parameters of the coating were extracted from $Q$ vs $T$ data, including a ${T}_{c}$ of $18.0\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{ }\mathrm{K}$, close to the maximum literature value. High power pulses were used to reach fields far higher than in CW measurements, and near ${T}_{c}$, quench fields close to the superheating field were observed. Based on a review of previous experience with ${\mathrm{Nb}}_{3}\mathrm{Sn}$ cavities, a speculative mechanism involving weak link grain boundaries is presented to explain how the modified annealing step could be the cause of the absence of strong $Q$-slope. Finally, an analysis of the progress to date provides hints that the path forward for ${\mathrm{Nb}}_{3}\mathrm{Sn}$ cavities should focus on minimizing defects.
    No preview · Article · Nov 2014 · Physical Review Special Topics - Accelerators and Beams
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    Full-text · Dataset · Sep 2014
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    ABSTRACT: The dependence of the Q-value on the RF field (Q-slope) is actively studied in various accelerator laboratories. Although remedies against this dependence have been found, the physical cause still remains obscure. A rather straightforward two-fluid model description of the Q-slope in the low and high field domains is presented with emphasis on the recently experimentally identified improvement of the Q-value by so-called "N-doping".
    Full-text · Article · Jul 2014
  • S. Posen · D. Gonnella · M. Liepe
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    ABSTRACT: Nb3Sn coatings on niobium SRF cavities have the potential to significantly reduce cryogenic costs due to their extremely small surface resistance (Rs). In this paper, we present new results showing the repeatability of Cornell's fabrication process, which produces high Q0 cavities that reach medium fields with minimal Q-slope. We also show the results of attempts to smooth RF surfaces and reduce defects via material removal. However, both HF rinsing and centrifugal barrel polishing resulted in strong performance degradation.
    No preview · Article · Jul 2014
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    ABSTRACT: The superconducting RF linac for LCLS-II calls for 1.3 GHz 9-cell cavities with an average intrinsic quality factor Q0 of 2.7 × 1010 at 2K and 16 MV/m accelerating gradient. A collaborative effort between Cornell University, FNAL, and JLab has been set up with the goal of developing and demonstrating a cavity treatment protocol for the LCLS-II cavities meeting these specifications. The high Q0 treatment protocol is based on nitrogen doping of the RF surface layer during a high temperature heat treatment. This novel SRF cavity preparation was recently developed at FNAL and shown to result in SRF cavities of very high Q0 at 2K with an increase in Q0 from low to medium fields. N-doped single cell cavities at Cornell, FNAL, and JLab routinely exceed LCLS-II specification. 9-cell N-doped cavities at FNAL achieveanaverage Q0(T=2K,16MV/m) of ∼ 3.4 × 1010 with an average quench field of ∼ 19 MV/m, meeting therefore overall with good margin the LCLS-II specification.
    No preview · Article · Jul 2014
  • D. Gonnella · M. Liepe
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    ABSTRACT: It is well known that external magnetic fields can cause higher residual resistance in superconducting RF cavities if the field is present during cooldown. However, the effect of cavity preparation and surface mean free path on the resulting residual resistance from magnetic field is less well studied. In this paper, we report on recent studies at Cornell in which two SRF cavities (one nitrogen-doped and one 120°C baked) were cooled through Tc in an applied uniform external magnetic field. Trapped flux and residual resistance were measured for a variety of cooldowns and applied magnetic fields. It was found that the residual resistance due to trapped flux in the nitrogen-doped cavity was three times larger than in the 120°C baked cavity.
    No preview · Article · Jul 2014
  • D. Gonnella · M. Liepe
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    ABSTRACT: Recent results from Cornell and FNAL have shown that superconducting RF cavities given a heat treatment in a nitrogen atmosphere of a few mTorr display an increase in Q0 with increasing accelerating field, opposite to the medium field Q slope usually observed. Three cavities was prepared at Cornell using this method and subsequently tested after different amounts of material removal. Cavity performance and material properties were extracted for each cavity and correlated with material removal. This has given new insights into how material properties and the anti-Q slope depend on cavity preparation.
    No preview · Article · Jul 2014

Publication Stats

1k Citations
51.76 Total Impact Points

Institutions

  • 2003-2015
    • Cornell University
      • • Cornell Laboratory for Accelerator-based ScienceS and Education (CLASSE)
      • • Laboratory for Elementary Particle Physics
      Итак, New York, United States
  • 2000-2014
    • Deutsches Elektronen-Synchrotron
      Hamburg, Hamburg, Germany
  • 2000-2002
    • Universität Hamburg
      • Institut für Experimentalphysik
      Hamburg, Hamburg, Germany