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Title: Nonlinear Dynamics of Vortices in Different Types of Grain Boundaries

Abstract

As a major component of linear particle accelerators, superconducting radio-frequency (SRF) resonator cavities are required to operate with lowest energy dissipation and highest accelerating gradient. SRF cavities are made of polycrystalline materials in which grain boundaries can limit maximum RF currents and produce additional power dissipation sources due to local penetration of Josephson vortices. The essential physics of vortex penetration and mechanisms of dissipation of vortices driven by strong RF currents along networks of grain boundaries and their contribution to the residual surface resistance have not been well understood. To evaluate how GBs can limit the performance of SRF materials, particularly Nb and Nb3Sn, we performed extensive numerical simulations of nonlinear dynamics of Josephson vortices in grain boundaries under strong dc and RF fields. The RF power due to penetration of vortices both in weakly-coupled and strongly-coupled grain boundaries was calculated as functions of the RF field and frequency. The result of this calculation manifested a quadratic dependence of power to field amplitude at strong RF currents, an illustration of resistive behavior of grain boundaries. Our calculations also showed that the surface resistance is a complicated function of field controlled by penetration and annihilation of vortices and antivortices in strongmore » RF fields which ultimately saturates to normal resistivity of grain boundary. We found that Cherenkov radiation of rapidly moving vortices in grain boundaries can produce a new instability causing generation of expanding vortex-antivortex pair which ultimately drives the entire GB in a resistive state. This effect is more pronounced in polycrystalline thin film and multilayer coating structures in which it can cause significant increase in power dissipation and results in hysteresis effects in I-V characteristics, particularly at low temperatures.« less

Authors:
 [1]
  1. Old Dominion Univ., Norfolk, VA (United States)
Publication Date:
Research Org.:
Thomas Jefferson National Accelerator Facility, Newport News, VA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Nuclear Physics (NP) (SC-26)
OSTI Identifier:
1422724
Report Number(s):
JLAB-ACC-17-2641; DOE/OR/23177-4351
DOE Contract Number:
AC05-06OR23177
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English

Citation Formats

Sheikhzada, Ahmad. Nonlinear Dynamics of Vortices in Different Types of Grain Boundaries. United States: N. p., 2017. Web. doi:10.2172/1422724.
Sheikhzada, Ahmad. Nonlinear Dynamics of Vortices in Different Types of Grain Boundaries. United States. doi:10.2172/1422724.
Sheikhzada, Ahmad. Mon . "Nonlinear Dynamics of Vortices in Different Types of Grain Boundaries". United States. doi:10.2172/1422724. https://www.osti.gov/servlets/purl/1422724.
@article{osti_1422724,
title = {Nonlinear Dynamics of Vortices in Different Types of Grain Boundaries},
author = {Sheikhzada, Ahmad},
abstractNote = {As a major component of linear particle accelerators, superconducting radio-frequency (SRF) resonator cavities are required to operate with lowest energy dissipation and highest accelerating gradient. SRF cavities are made of polycrystalline materials in which grain boundaries can limit maximum RF currents and produce additional power dissipation sources due to local penetration of Josephson vortices. The essential physics of vortex penetration and mechanisms of dissipation of vortices driven by strong RF currents along networks of grain boundaries and their contribution to the residual surface resistance have not been well understood. To evaluate how GBs can limit the performance of SRF materials, particularly Nb and Nb3Sn, we performed extensive numerical simulations of nonlinear dynamics of Josephson vortices in grain boundaries under strong dc and RF fields. The RF power due to penetration of vortices both in weakly-coupled and strongly-coupled grain boundaries was calculated as functions of the RF field and frequency. The result of this calculation manifested a quadratic dependence of power to field amplitude at strong RF currents, an illustration of resistive behavior of grain boundaries. Our calculations also showed that the surface resistance is a complicated function of field controlled by penetration and annihilation of vortices and antivortices in strong RF fields which ultimately saturates to normal resistivity of grain boundary. We found that Cherenkov radiation of rapidly moving vortices in grain boundaries can produce a new instability causing generation of expanding vortex-antivortex pair which ultimately drives the entire GB in a resistive state. This effect is more pronounced in polycrystalline thin film and multilayer coating structures in which it can cause significant increase in power dissipation and results in hysteresis effects in I-V characteristics, particularly at low temperatures.},
doi = {10.2172/1422724},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon May 01 00:00:00 EDT 2017},
month = {Mon May 01 00:00:00 EDT 2017}
}

Thesis/Dissertation:
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  • An analytic model for minority-carrier transport in polysilicon devices was developed, and experimental corroboration for the model is provided. The model is used to facilitate the development of experimental techniques, compatible with conventional device processing, to control the effects of grain boundaries. Techniques are investigated to reduce the bulk recombination current of the dark current by gettering intragrain impurities in polysilicon. Key assumptions are made, with justification, to simplify the three-dimensional, nonlinear boundary-value problem that defines minority-carrier transport, including recombination, in polysilicon devices. These assumptions enable the separation of the grain-boundary recombination analysis, which is based on quasi-equilibrium, from themore » intragrain transport analysis, which is done by partitioning the grain into subregions in which the minority-carrier flow is predominantly one-dimensional. The analyses are coupled through the effective minority-carrier recombination velocity at the grain boundary, which generally is dependent on the minority-carrier density in the adjacent quasi-neutral grain. Experimental results are presented that imply potential improvements afforded by aluminum diffusion in both bulk and thin-film polysilicon solar cells.« less
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