M. Liepe

Cornell University, Итак, New York, United States

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Publications (89)52.07 Total impact

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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.
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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.
  • 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.
    Physical Review Letters 07/2015; 115(4):047001. · 7.51 Impact Factor
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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.
  • 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.
    Applied Physics Letters 02/2015; 106(8):082601. DOI:10.1063/1.4913247 · 3.52 Impact Factor
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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.
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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.
    Journal of Applied Physics 11/2014; 117(2). DOI:10.1063/1.4905681 · 2.19 Impact Factor
  • 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.
    Physical Review Special Topics - Accelerators and Beams 11/2014; 17(11). DOI:10.1103/PhysRevSTAB.17.112001 · 1.52 Impact Factor
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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".
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    ABSTRACT: Future particle accelerators will require continuous wave operation of SRF cavities capable of supporting high beam currents. An example of this is the Energy Recovery Linac (ERL) at Cornell University, a next generation light source designed to run high currents (100 mA) with a high bunch repetition rate (1.3 GHz). Obtaining the beam emittance necessary to meet design specification requires strong damping of higher-order modes that can lead to beam breakup. We discuss the optimization and verification of the accelerating cavity. Next we show that an ERL constructed from the optimized cavity geometry - including realistic shape errors - can support beam currents in excess of 300 mA while still maintaining beam stability. A niobium prototype 7-cell cavity was fabricated and tested in a horizontal cryomodule. We show that the prototype cavity exceeds quality factor and gradient specifications of 2×1010 at 16.2 MV/m at 1.8 K by 50%, reaching Q=(3.0±0.3)×1010. The prototype cavity also satisfies all design constraints and has a higher order mode spectrum consistent with the optimized shape geometry. At 1.6 K, the cavity set a record for quality factor of a multicell cavity installed in a horizontal cryomodule reaching Q=(6.1±0.6)×1010.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2014; 734:23-31. DOI:10.1016/j.nima.2013.07.021 · 1.32 Impact Factor
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    ABSTRACT: Cornell University has been designing and building superconducting accelerators for various applications for more than 50 years. Currently, an energy-recovery linac (ERL) based synchrotron-light facility is proposed making use of the existing CESR facility. As part of the phase 1 R&D; program funded by the NSF, critical challenges in the design were addressed, one of them being a full linac cryo-module. It houses 6 superconducting cavities- operated at 1.8 K in continuous wave (CW) mode - with individual HOM absorbers and one magnet/ BPM section. Pushing the limits, a high quality factor of the cavities (2⋅1010) and high beam currents (100 mA accelerated plus 100 mA decelerated) are targeted. We will present the design of the main linac cryo-module (MLC) being finalized recently, its cryogenic features and report on the status of the fabrication which started in late 2012.
    12/2013; 1573(1). DOI:10.1063/1.4860931
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    ABSTRACT: A comment to the authors' SRF Conference pre-print [1] was submitted by A. Gurevich to the arXiv [2]. In this response, we show that the arguments used in the comment are not valid. [1] arXiv:1309.3239 [2] arXiv:1309.5626
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    ABSTRACT: The SIS structure---a thin superconducting film on a bulk superconductor separated by a thin insulating film---was propsed as a method to protect alternative SRF materials from flux penetration by enhancing the first critical field $B_{c1}$. In this work, we show that in fact $B_{c1}$ = 0 for a SIS structure. We calculate the superheating field $B_{sh}$, and we show that it can be enhanced slightly using the SIS structure, but only for a small range of film thicknesses and only if the film and the bulk are different materials. We also show that using a multilayer instead of a single thick layer is detrimental, as this decreases $B_{sh}$ of the film. We calculate the dissipation due to vortex penetration above the $B_{sh}$ of the film, and find that it is unmanageable for SRF applications. However, we find that if a gradient in the phase of the order parameter is introduced, SIS structures may be able to shield large DC and low frequency fields. We argue that the SIS structure is not beneficial for SRF cavities, but due to recent experiments showing low-surface-resistance performance above $B_{c1}$ in cavities made of superconductors with small coherence lengths, we argue that enhancement of $B_{c1}$ is not necessary, and that bulk films of alternative materials show great promise.
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    ABSTRACT: High-power, high-brightness electron beams are of interest for many applications, especially as drivers for free electron lasers and energy recovery linac light sources. For these particular applications, photoemission injectors are used in most cases, and the initial beam brightness from the injector sets a limit on the quality of the light generated at the end of the accelerator. At Cornell University, we have built such a high-power injector using a DC photoemission gun followed by a superconducting accelerating module. Recent results will be presented demonstrating record setting performance up to 65 mA average current with beam energies of 4–5 MeV.
    Applied Physics Letters 01/2013; 102(3). DOI:10.1063/1.4789395 · 3.52 Impact Factor
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    ABSTRACT: The development of high current, high brightness electron guns is critically important for free-electron laser and energy recovery linear-accelerator light source facilities. In this paper, we will review the technical requirements of such projects and the status of on-going research throughout the community including results of emittance measurements and high current beam performance.
    Proceedings of International Particle Accelerator Conference, Kyoto, Japan; 05/2010
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    E. Chojnacki · E. Smith · R. Ehrlich · M. Liepe · J. Sears
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    ABSTRACT: Several of the typical RF absorbing materials utilized in particle accelerator environments have DC electrical conductivities that decrease significantly when cooled to cryogenic temperatures. If such RF absorbers are in close proximity to a beamline, they are prone to collecting static charge and may deflect the particle beam. The DC electrical conductivities of two types of ferrite and a ceramic are measured at various temperatures, showing that the electrical conductivities indeed decrease, often to unacceptably low levels in regard to static charge accumulation.
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    ABSTRACT: Cornell University has developed and fabricated a SRF injector cryomodule for the acceleration of the high current (100 mA), low emittance beam in the Cornell ERL injec- tor prototype. This cryomodule is based on superconduct- ing rf technology with five 2-cell rf cavities operated in the cw mode. To support the acceleration of a low energy, ul- tra low emittance, high current beam, the beam tubes on one side of the cavities have been enlarged to propagate Higher-Order-Mode power from the cavities to broadband RF absorbers located at 80 K between the cavities. Each cavity is surrounded by a LHe vessel and equipped with a frequency tuner including piezo-driven tuners for fast fre- quency control. The cryomodule provides the support and precise alignment for the cavity string, the 80 K cooling of the HOM loads, and the 2 K LHe cryogenic system for the high cw heat load of the cavities. In this paper results of the commissioning phase of this cryomodule will be reported.
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    ABSTRACT: Beam-breakup calculation algorithms have been devel-oped in the general framework of the Cornell X-ray ERL design software, enabling their extension to multi-pass op-tics design for ERLs. A status report of this work is pre-sented, together with initial results comparing the insta-bility thresholds calculated for single-and two-turn optics with recently developed RF cavity designs.
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    ABSTRACT: A single-cavity horizontal test cryomodule (HTC) has been designed and fabricated recently at Cornell University for ERL project. This cryomodule is a shortened version of the full injector cryomodule, housing five superconducting cavities. It serves as a test bench for new design features and for testing fully dressed two-cell ERL injector cavities. The cryostat design has been optimized for precise cavity alignment, good magnetic shielding, and high cryogenic loads from the RF cavities, input couplers, and HOM loads. The HTC was made long enough so in the future it can accommodate longer, multicell cavities of the ERL main linac. In this paper we report on results from the first full test of the HTC, including RF system and superconducting cavity performance, cryomodule studies and operation of a new 2 K cryogenic system.

Publication Stats

881 Citations
52.07 Total Impact Points

Institutions

  • 2003–2014
    • Cornell University
      • Laboratory for Elementary Particle Physics
      Итак, New York, United States
  • 2000–2014
    • Deutsches Elektronen-Synchrotron
      Hamburg, Hamburg, Germany
  • 2002
    • University of Hamburg
      • Institut für Experimentalphysik
      Hamburg, Hamburg, Germany