Professors
Matthias Liepe
Assistant Professor of Physics
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-present. Alfred P. Sloan Research Fellow, 2008-present.
Research Areas
Radio Frequency Superconductivity, Accelerator Physics and Particle Accelerators, ultra-fast Feedback Controls
Current Research
RF Superconductivity
Superconducting radio frequency (RF) cavities are feet-long structures, providing extremely high electric field gradients (tens of MV/m) for the acceleration of particle beams. The electric field inside these cavities oscillates at resonant microwave frequencies (GHz), 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 quality factors. 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 like the International Linear Collider, the X-ray Free Electron Laser at DESY, a muon accelerator, and the Energy Recovery Linac Light Source planned here at Cornell University all rely on the performance we hope to achieve in next generations of superconducting cavities.
Cornell's Superconducting Radio Frequency (SRF) group is a world leader in the application of superconductivity for accelerating cavities in high energy 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 behavior of superconductors in high fields at microwave frequencies to designing RF cavities and whole superconducting linear accelerators to studying the non-linear beam dynamics in superconducting linacs.
Current and Future Research Activities
Superconducting cavities involve research in extreme areas like superconductors of lowest surface resistance, ultra high vacuum and super fluid Helium cryogenics, highest magnetic and electric radio-frequency fields (tens of MV/m), oscillators with quality factors exceeding 1010 and vibration control of feet-long structures in the nm range. My current research concentrates on the following areas:
- Understanding of the behavior of superconducting surfaces at low temperatures when exposed to very high electric and magnetic fields at microwave frequencies. The present understanding of the physics of superconductors in high microwave fields has large holes. In the next years we hope to find answers to open questions like: Why does the RF surface resistance increase strongly at high RF fields? What is the critical magnetic field for a superconductor in microwave fields? Can new superconducting materials (niobium-3-tin, new high Tc superconductors) yield even higher fields and/or lower surface resistance? Answering these questions is of great importance for the future viability of superconductors to provide even higher fields for future frontier energy accelerators.
- Electron beam emittance preservation and beam dynamics in superconducting RF linacs. When a particle beam passes though 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 Cornell ERL injector prototype, which is just starting operation, 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 particle accelerators like the Cornell Energy Recovery Linear Accelerator (ERL) and the International Linear Collider. In addition to developing the cavities for these superconducting accelerators, we are developing related and technologically challenging components like RF input couplers, Higher-Order-Mode dampers and frequency tuners. For the ERL, we are designing, building and testing entire, complex SRF cryomodules. This work relates to a wide breath of scientific and engineering questions.
- Ultra-fast feedback systems for particle accelerators. Advanced feedback systems enable us to operate superconducting cavities at highest quality factors while providing particle beams of highest energy stability. One of the key challenges we are presently working on is active vibration compensation for the superconducting cavities in order to stabilize them on an nm scale.
Graduate Students
Nick Valles is studying the behavior of superconductors at highest magnetic RF fields. He is measuring the critical magnetic RF field of the superconductor niobium at low temperatures. He is also working on complex optimization routines for the design of superconducting RF cavities for particle accelerators like the Cornell ERL.
Yie Xi's research focuses on developing a sample test system to measure high field RF properties of higher temperature superconductors like niobium-3-tin and MgB2. In addition, his work includes surface analysis of material samples to find correlations between the RF performance of superconductors and their surface properties like roughness and crystal dislocations.
Sam Posen is working with us on testing high gradient SRF cavities to study the physics of thermal breakdown at high RF fields. He is also working on designing and optimizing the next generation cryomodules for the Cornell ERL.
Undergraduate Students
Quintin Stedman is investigating the effect of small cavity shape fluctuations from fabrication errors on the beam induced fields in the superconducting cavities for the Cornell ERL. Henry Hinnefield is working on optimizing a system to measure the properties of small superconductor samples at very high magnetic RF fields. Russell Wolf is simulating the electromagnetic microwave fields (so-called wakefields) left behind by a particle beam when it travels through SRF cavities.
There are many opportunities for motivated undergraduate and graduate student to get involved. Presently, we have openings for one new graduate student. Contact me if you are interested in our work and would like to try us out for a few weeks!