[Eng-div] Fwd: [Accelerator_staff] Accelerator Seminar: Binping Xiao - 5/17/12

[Gayle Coleman]

-------- Original Message --------
Subject: 	[Accelerator_staff] Accelerator Seminar: Binping Xiao - 5/17/12
Date: 	Mon, 14 May 2012 09:25:13 -0400
From: 	Audrey N. Barron <anichols at jlab.org>
Reply-To: 	anichols at jlab.org
Organization: 	Jefferson Laboratory
To: 	fel_team at jlab.org, acc_staff at jlab.org



ACCELERATOR SEMINAR

"Surface Impedance of Superconducting Radio Frequency (SRF) Materials"

Binping Xiao
College of William Mary and Jefferson Lab

Superconducting radio frequency (SRF) technology is widely adopted in 
particle accelerators. There remain many open questions, however, in 
developing a systematic understanding of the fundamental behavior of SRF 
materials, including niobium treated in different ways and various other 
bulk/thin film materials that are fabricated with different methods 
under assorted conditions. A facility that can measure the SRF 
properties of small samples in a range of 2~40 K temperature is needed 
in order to fully answer these questions. The Jefferson Lab surface 
impedance characterization (SIC) system has been designed to attempt to 
meet this requirement. It consists of a sapphire-loaded cylindrical Nb 
TE_011 cavity at 7.4 GHz with a 50 mm diameter flat sample placed on a 
non-contacting end plate and uses a calorimetric technique to measure 
the radio frequency (RF) induced heat on the sample. Driving the 
resonance to a known field on this surface enables one to derive the 
surface resistance of a relatively small localized area. Tests with 
polycrystalline and large grain bulk Nb samples have been done at <15 mT 
magnetic field. Based on BCS surface impedance, least-squares fittings 
have been done using SuperFit2.0, a code developed by G. Ciovati and the 
author.

Microstructure analyses and SRF measurements of large scale epitaxial 
MgB2 films have been reported. MgB2 films on 5 cm dia. sapphire disks 
were fabricated by a Hybrid Physical Chemical Vapor Deposition (HPCVD) 
technique. The electron-beam backscattering diffraction (EBSD) results 
suggest that the film is a single crystal complying with a 
MgB2(0001)//Al2O3(0001) epitaxial relationship. The SRF properties of 
different film thicknesses (200 nm and 350 nm) were evaluated using SIC 
system under different temperatures and applied fields at 7.4 GHz. A 
surface resistance of 9±2 ?? has been observed at 2.2 K.

Based on BCS theory with moving Cooper pairs, the electron states 
distribution at 0K and the probability of electron occupation with 
finite temperature have been derived and applied to anomalous skin 
effect theory to obtain the surface impedance of a superconductor with 
moving Cooper pairs. We present the numerical results for Nb.

Thursday, May 17, 2012
11:00 a.m.
CEBAF Center, Room F326/327

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