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REMINDER!<br>
<br>
On 5/14/2012 9:25 AM, Audrey N. Barron wrote:
<blockquote cite="mid:4FB107B9.9070700@jlab.org" type="cite">
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<font face="Arial">ACCELERATOR SEMINAR<br>
<br>
<span style="font-family:
"Arial","sans-serif";">"Surface Impedance
of Superconducting Radio Frequency (SRF) Materials"<br>
<br>
Binping Xiao<br>
College of William Mary and Jefferson Lab<br>
<br>
</span><span
style="font-size:10.0pt;font-family:"Arial","sans-serif";mso-no-proof:yes">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<sub>011</sub> 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.<o:p></o:p></span></font>
<p class="MsoNormal"
style="text-align:justify;text-justify:inter-ideograph"><font
face="Arial"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif";mso-no-proof:yes">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
μΩ has been observed at 2.2 K.<o:p></o:p></span></font></p>
<font face="Arial"><span
style="font-size:10.0pt;font-family:"Arial","sans-serif&
quot;;mso-fareast-font-family:
宋体;mso-ansi-language:EN-US;mso-fareast-language:ZH-CN;mso-bidi-
language:AR-SA; mso-no-proof:yes">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.</span></font>
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<font face="Arial"><br>
</font><span
style="font-family:"Arial","sans-serif""><font
face="Arial">Thursday, May 17, 2012<br>
11:00 a.m.<br>
CEBAF Center, Room F326/327 </font><br>
<br>
<o:p></o:p></span>Â <span style="font-size: 10pt; font-family:
"Arial","sans-serif";"></span> </blockquote>
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