Unravelling the mechanisms of rf field, frequency, and temperature dependencies of nonlinear surface resistance in superconducting cavities
Alex Gurevich, Old Dominion University, (Principal Investigator)
Jean Delayen, Old Dominion University, (Co-Principal Investigator)
Pashupati Dhakal, Jefferson Lab, (Co-principal Investigator)
Eric Lechner, Jefferson Lab, (Co-principal Investigator)
Superconducting Radio-Frequency (SRF) resonators are the key enabling components of modern particle accelerators used in many areas of fundamental sciences, medical and defense applications, and quantum information technology. Advances in the development of SRF niobium accelerating cavities have significantly increased their quality factors Q up to 1011 at operating temperatures 1.7-2 kelvin and frequencies 1-2 GHz. In turn, the maximum accelerating gradients in the high-performance cavities can reach ~ 50 MV/m which approaches the fundamental depairing limit of superconducting niobium. Furthermore, several research groups have discovered that high-temperature treatments and infusion of niobium with titanium, nitrogen or oxygen can give rise to a striking increase of the quality factors with the amplitude of rf field, commonly referred to as "Q-rise". These breakthroughs have shown that the mature niobium cavity technology still has a lot of unrealized potential, requiring the comprehensive understanding of many interconnected mechanisms determining the Q factors to establish their fundamental limits and the ways of further increasing Q by materials treatments. It is the goal of this project in which we propose integrated experimental and theoretical research to advance the SRF science and technology by providing a better understanding of fundamental mechanisms of rf losses and performance limits of Nb cavities and investigating the ways of pushing their limits by materials treatments.
We propose to infer the mechanisms of the Q-rise from combined measurements of Q factors as functions of rf field amplitude, frequency, and temperature using multi-technique characterization of SRF cavities to separate multiple contributions to the rf losses. A theory of nonlinear RF losses determined by a nonequilibrium superconducting electron liquid under strong electromagnetic field will be developed. We will use SRF tests of niobium elliptical and co-axial cavities combined with their temperature and magnetic field mapping and theory to separate interconnected contributions of superconducting quasiparticles, trapped magnetic vortices and materials defects to the field and frequency dependencies of Q factors. This will allow us to reveal the effect of nonequilibrium electrons driven by strong RF fields on the nonlinear surface resistance and extract fundamental kinetics characteristics of electrons interacting with lattice vibrations. To measure the frequency dependence of Q on the same cavity, we will use different resonant modes of co-axial cavities which have been co-developed at ODU and JLab. The experimental data will be analyzed using theoretical models of nonlinear SRF losses due to trapped vortices and nonequilibrium electrons which have developed in the group of PI and will be further advanced for this project.
Our exploratory research of mechanisms of rf losses in high-performance superconducting RF accelerating cavities will be performed in collaboration between research groups at Old Dominion University and Jefferson National Laboratory. In this project experimental work at JLab and ODU will be integrated with theoretical work at ODU to provide prompt interpretation of experimental data and guidance of further experiments. The project involves three main tasks: 1. Measurements of the field, frequency, and temperature dependencies of surface resistance in elliptical and co-axial cavities at temperatures from 1.5 K to 4.2 K and frequencies from 0.3 to 3 GHz. 2. Investigation of the effect of materials treatments, particularly the nitrogen and oxygen infusion on the dependencies of the surface resistance on rf field, frequency and temperature, and optimization of the SRF cavity performance. 3. Development of a theory of nonlinear surface resistance addressing the role of impurities and nonequilibrium effects at high RF field to understand the fundamental limits of Q factors.
If successful, this project will advance the fundamental understanding of SRF materials and train the next generation of young researchers for the accelerator science and technology. Results obtained during this project will be disseminated through scientific publications and conference presentations and transferred to the U.S. national labs, industrial partners, and academia. The stakeholders that will benefit from this project being a part of Stewardship Accelerator Program include future accelerator projects involving DOE offices of NP, HEP and BES.
