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Matthias Liepe

Associate Professor

Newman, Floyd R. Laboratory, Room 328
mul2@cornell.edu
607-255-4951

Educational Background

Diplom, 1998, University of Hamburg, Germany. Visiting Scientist, Cornell University, 1998-1999. Ph.D., 2001, University of Hamburg, Germany. Research Assistant, Deutsches Elektronen Synchrotron, DESY, Germany, 1998-2001. Research Associate, Cornell University 2001-2006. Assistant Professor, Cornell University, 2006-2013. Alfred P. Sloan Research Fellow, 2008-2012. Associate Professor, Cornell University, 2013-present. Graduate and Professional Student Assembly (GPSA) Faculty Award, 2015. Cornell Superconducting RF Group Leader, 2016-present.

Website(s)

Overview

Radio Frequency Superconductivity, Accelerator Physics and Particle Accelerators

My current research focusses on studying the science and technology of bulk and thin-film superconductors in high electromagnetic fields at ultra-high frequencies and the application of these superconductors, e.g. in superconducting RF cavities for particle accelerators.

Departments/Programs

  • Physics

Graduate Fields

  • Physics

Affiliations

  • Cornell Laboratory for Accelerator-based Sciences and Education (CLASSE)
  • Laboratory for Elementary-Particle Physics (LEPP)
  • Superconducting Radio-Frequency (SRF)

Research

Superconducting radio frequency (RF) cavities are feet-long structures, providing extremely high electric field gradients (tens of MV/m) for the acceleration of charged particle beams. The electric field inside these cavities oscillates at GHz frequencies, with exceptional high quality factors of 1E10 to 1E11. By using superconducting materials operated at temperatures between 1.5K and 4K for the walls of the cavities, we can achieve such high efficiency. The evolution in the performance of superconducting cavities has revolutionized the performance and scientific reach of particle accelerators for a variety of science applications, including high-energy physics, nuclear physics, synchrotron radiation based research, and high power lasers. Future particle accelerators for science, industry, energy generation, and medical applications all rely on the performance we hope to achieve in next-generation superconducting RF cavities.

I am the head of Cornell’s Superconducting Radio Frequency (SRF) group, which is a world leader in the science of microwave superconductivity and its application to accelerating cavities for particle accelerators. We have an extensive, state of the art infrastructure for the design, fabrication, preparation and test of superconducting cavities. Our research program is multi-faceted and interdisciplinary, and therefore ideal suited for graduate research. It ranges from studying the fundamental behavior of superconductors in high GHz fields to complex multi-parameter optimizations of RF cavities to studying the non-linear beam dynamics in superconducting linacs.

SRF cavities not only enable accelerator-based sciences, but they also allow the measurement of superconducting response under extreme conditions with very high sensitivity. They allow the study of surface resistance, critical fields, superconducting magnetic microwave shielding, and the metastability of the superheating-field barrier. They are a testing ground for the science of disorder and defects, coupling superconductivity and high fields to grain boundaries, surface anisotropy, surface oxides, and crystal orientation. My current research concentrates on the following areas:

Synthesis, characterization, and microwave surface resistance of new superconducting compound materials for SRF cavity applications: Current SRF cavities exclusively use niobium as superconductor, and are approaching theoretical limits. However, new potentially game-changing materials (e.g. Nb3Sn, MgB2, NbN) have the potential for fields and cavity quality factors far above the niobium limit. Moving to higher-kappa, compound superconductors brings new questions: What alternative superconductors with critical temperatures higher than that of niobium can open the path towards a new generation of SRF cavities with even lower RF surface resistance and higher accelerating fields? Does the small coherence length of these superconductors limit their usefulness due to grain boundary losses or defects? How can these more complex compound superconductors be synthesized with ideal stoichiometry and defect free? Our research program on Nb3Sn is world leading, and has resulted in the first ever alternative material SRF cavity to clearly outperform traditional niobium cavities.

Superheating fields in superconductors: The highest gradient niobium SRF cavities are operated with peak magnetic fields beyond Hc1, the field where vortices (that would cause massive losses) would penetrate in equilibrium. Operation is possible until a higher superheating field Hsh because of a surface barrier to flux penetration. Our group has done a first measurement of the full temperature dependence of the superheating field of niobium using SRF cavities, and has shown that it depends strongly on the preparation of the niobium surface. Much remains unknown. How does the superheating field depend on the Fermi surface anisotropy, i.e. could optimally oriented superconducting surfaces offer higher fields in SRF cavities? How do strong-coupling effects, as present in many of the higher-temperature traditional superconductors, impact Hsh? Can defects bypass the metastable superheating-field barrier for large-kappa materials?

Processing, characterization and microwave surface resistance of the RF surface penetration layer of superconductors:  The surface resistance of a superconductor in microwave fields is determined by a highly complex surface layer of a few 100 nm thickness (roughly the penetration depth of the field), with oxides, grain boundaries, impurities, and defects present. This surface resistance, and is observed strong field dependence, strikingly depend on the surface treatment protocol (etching, polishing, annealing). Open questions include: What is the physics underlying the residual surface resistance at the lowest temperatures? What are the effects of surface oxides on the surface electronic structure of materials and their impact on RF cavity performance characteristics? What surface morphology and (likely) mixed metallic phases arise from electrochemical polishing of niobium? What are the sources of the observed strong field dependence of the microwave surface resistance?

Electron beam emittance preservation and beam dynamics in superconducting RF linacs: When a particle beam passes through a superconducting linac, it interacts with the cavity environment.  This can lead to excessive fields (Higher-Order-Modes) excited by the beam in the cavities, degradation of the beam quality (emittance growth) and beam instability. Our CBETA ERL prototype currently under construction at Cornell will give us a unique tool for studying questions like: What is the spectrum of electromagnetic fields excited by the beam? Where is the excited high frequency (10 GHz – 100 GHz) power absorbed? What effects contribute to emittance growth in an SRF linac, and do measurements agree with numerical simulations of these various effects?

Developing the superconducting linac technology for future high-power, high energy-efficiency particle accelerators: In addition to designing and optimizing the cavities for future superconducting accelerators, we are developing related and technologically challenging components like RF input couplers, Higher-Order-Mode dampers and frequency tuners. We are designing, building and testing entire, complex SRF cryomodules. This work relates to a wide breath of scientific and engineering questions. A particular focus of our work is on making future particle accelerators much more energy efficient.

Current Graduate Students
Daniel Hall is exploring the fabrication, physics and performance of high-kappa superconductors like niobium-3-tin in microwave fields. He is coating very high quality Nb3Sn surfaces in an ultra-high vacuum furnace, is measuring their RF surface impedance, and is using advanced surface analysis (e.g. FIB/TEM, XRD) to analyze Nb3Sn films to determine the sources of current performance limitations. His studies include vortex dynamics and grain boundary effects in superconductors, high field effects in superconductors, and thermodynamic Nb3Sn synthesis. Daniel’s next-generation Nb3Sn cavities are reaching world-record performances, with cryogenic efficiencies 30 times higher than the best conventional niobium cavities at 4.2K.

James Maniscalco is investigating the field dependence of the BSC surface resistance and is also studying metastability and critical fields in superconductors. He is using unique experimental setups to measure critical fields of different superconductors and the temperature and field dependence of the RF surface resistance at high RF and DC surface magnetic fields.

Pete Koufalis is working on ultra-high efficient SRF cavities, exploring impurity doping of the RF penetration layer. Impurity doping has been shown to strongly impact the RF surface resistance and critical fields of the superconductor, and we are exploring the physics behind these observed effects to determine optimal dopants and doping levels for SRF applications.

Ryan Porter is currently working on surface chemistry of superconductors and the impact of surface roughness on the performance of thin-film superconductors.

Alumni Graduate Students
Yi Xie and Sam Posen are now scientists at Fermilab, Dan Gonnella is working on the LCLS-II project at SLAC, and Nick Valles is working at Raytheon.

There are many opportunities for motivated undergraduate and graduate students to get involved. Contact me if you are interested in our work and would like to know more!

Group meetings: Mondays 11:00am, general SRF group meeting, 311 Newman Lab

Courses

Publications

  1. Impact of nitrogen doping of niobium superconducting cavities on the sensitivity of surface resistance to trapped magnetic flux, Dan Gonnella, John Kaufman and Matthias Liepe, J. Appl. Phys. 119, 073904 (2016). http://dx.doi.org/10.1063/1.4941944

  2. Shielding Superconductors with Thin Films as Applied to rf Cavities for Particle Accelerators, S. Posen, M. K. Transtrum, G. Catelani, M. Liepe, and J. P. Sethna, Phys. Rev. Applied 4, 044019 (2015). http://dx.doi.org/10.1103/PhysRevApplied.4.044019

  3. Radio Frequency Magnetic Field Limits of Nb and Nb3Sn, S. Posen, N. Valles, and M. Liepe, Phys. Rev. Lett. 115, 047001 (2015). http://dx.doi.org/10.1103/PhysRevLett.115.047001

  4. Proof-of-principle demonstration of Nb3Sn superconducting RF cavities for high Q0 applications, S. Posen, M. Liepe. D. Hall, Applied Physics Letters 106, Issue 8 (2015). http://dx.doi.org/10.1063/1.4913247

  5. Analysis of Nb3Sn surface layers for superconducting RF cavity applications, C. Becker, S. Posen, N. Groll, R. Cook, C. M. Schlepuetz, D. Hall, M. Liepe, M. Pellin, J. Zasadzsinski, and T. Proslier, Applied Physics Letters 106, Issue 8 (2015). http://dx.doi.org/10.1063/1.4913617

  6. Nitrogen-doped 9-cell cavity performance in a test cryomodule for LCLS-II, D. Gonnella, R. Eichhorn, F. Furuta, M. Ge, D. Hall, V. Ho, G. Hoffstaetter, M. Liepe, T. O'Connell, S. Posen, P. Quigley, J. Sears, V. Veshcherevich, A. Grassellino, A. Romanenko and D. A. Sergatskov, J. Appl. Phys. 117 , 023908 (2015). http://dx.doi.org/10.1063/1.4905681

  7. Advances in development of Nb3Sn superconducting radio-frequency cavities, S. Posen and M. Liepe, Phys. Rev. ST Accel. Beams 15, 112001 (2014). http://journals.aps.org/prstab/abstract/10.1103/PhysRevSTAB.17.112001

  8. The main linac cavity for Cornell's energy recovery linac: Cavity design through horizontal cryomodule prototype test, N. Valles, M. Liepe, F. Furuta, M. Gi, D. Gonnella, Y. He, K. Ho, G. Hoffstaetter, D.S. Klein, T. O'Connell, S. Posen, P. Quigley, J. Sears, G.Q. Stedman, M. Tigner, V. Veshcherevich, Nuclear Instruments & Methods in Physics Research A (2013), http://dx.doi.org/10.1016/j.nima.2013.07.021 

  9. Record high-average current from a high-brightness photoinjector, B. Dunham, J. Barley, A. Bartnik, I. Bazarov, L. Cultrera, J. Dobbins, G. Hoffstaetter, B. Johnson, R. Kaplan, S. Karkare, V. Kostroun, Y. Li, M. Liepe, X. Liu, F. Loehl, J. Maxson, P. Quigley, J. Reilly, D. Rice, D. Sabol, E. Smith, K. Smolenski, M. Tigner, V. Vesherevich, D. Widger, and Z. Zhao, Appl. Phys. Lett. 102, 034105 (2013). http://link.aip.org/link/doi/10.1063/1.4789395

  10. Mechanical optimization of superconducting cavities in continuous wave operation, S. Posen and M. Liepe, Phys. Rev. ST Accel. Beams (2012). http://prst-ab.aps.org/abstract/PRSTAB/v15/i2/e022002