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Wolfgang Weingarten

Publications and source records attributed to Wolfgang Weingarten.

9 recordsLinked to original sources
arXiv2023

Field-dependent surface resistance for superconducting niobium accelerating cavities -- condensed overview of weak superconducting defect model

Small (compared to coherence length) weak superconducting defects when located at the surface, combined with the proximity and percolation effects, are claimed responsible for various observations with superconducting rf accelerating cavities with non-constant Q-value, such as "Q-slope" and "Q-drop", the role of temperature, the result of "nitrogen doping" and its relation to the free path, and the influence of the static external magnetic field. The Ginzburg-Landau equations are used to confirm the results.

physics.acc-ph↗
arXiv2020

Fluxon-induced losses in niobium thin-film cavities revisited

Long standing data from niobium thin film accelerating cavities will be revisited and analysed by a modified London model of RF superconductivity. Firstly, the applicability of this model is explored using data of the BCS surface resistance and its dependence on the RF magnetic field, temperature and mean free path. Secondly, the RF losses from trapped magnetic flux are analysed with regard to their dependence on these same parameters.

physics.acc-ph↗
arXiv2018

N-doped surfaces of superconducting niobium cavities as a disordered composite

The Q-factor of superconducting accelerating cavities can be substantially improved by a special heat treatment under N2 atmosphere (N-doping). Recent experiments at Fermi National Laboratory investigated the dependence of Q on the RF frequency and showed, unexpectedly, both an increase and a decrease with the RF field amplitude. This paper shall explain this finding by extending a previously proposed model founded on the two fluid model of RF losses, percolation and the proximity effect in a disordered composite.

physics.acc-ph↗
arXiv2017

Experimental evidence for electric surface resistance in niobium

Identifying the loss mechanisms of niobium cavities enables an accurate determination of applications for future accelerator projects and points to research topics required to mitigate current limitations. For several cavities an increasing surface resistance above a threshold field, saturating at higher field has been observed. Measurements on samples give evidence that this effect is caused by the surface electric field. The measured temperature and frequency dependence is consistent with a model that accounts for these losses by interface tunnel exchange between localized states in oxides formed along grain boundaries and the adjacent superconductor.

cond-mat.supr-con↗
arXiv2015

On superconducting niobium accelerating cavities fired under N2-gas exposure

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".

physics.acc-ph↗
arXiv2013

Losses in superconducting Niobium Films caused by Interface Tunnel Exchange

Identifying the loss mechanisms of niobium film cavities enables an accurate determination of applications for future accelerator projects and points to research topics required to mitigate their limitations. Measurements on samples show that the electric field is a dominant loss mechanism for niobium films, acting through interface tunneling between localized states in surface oxides and delocalized states in the superconducting niobium.

physics.acc-ph↗
arXiv2012

Extension of the Measurement Capabilities of the Quadrupole Resonator

The Quadrupole Resonator, designed to measure the surface resistance of superconducting samples at 400 MHz has been refurbished. The accuracy of its RF-DC compensation measurement technique is tested by an independent method. It is shown that the device enables also measurements at 800 and 1200 MHz and is capable to probe the critical RF magnetic field. The electric and magnetic field configuration of the Quadrupole Resonator are dependent on the excited mode. It is shown how this can be used to distinguish between electric and magnetic losses.

physics.acc-ph↗