[Eng-div] Fwd: [Accelerator_staff] Accelerator Seminar: 10/30/14 - Alex Gurevich

[Gayle Sundeen]

Reminder! 

----- Forwarded Message -----

From: "Audrey Nichols" <anichols at jlab.org> 
To: "acc staff" <acc_staff at jlab.org> 
Cc: gurevich at odu.edu 
Sent: Monday, October 27, 2014 4:17:04 PM 
Subject: [Accelerator_staff] Accelerator Seminar: 10/30/14 - Alex Gurevich 


ACCELERATOR SEMINAR 

Nonlinear Surface Resistance and the Extended Q(H) Rise in Alloyed Nb Cavities 

Alex Gurevich 
Old Dominion University 


A theory of dissipative nonlinear surface resistance R s (H) of superconductors under strong electromagnetic fields at low temperatures is proposed. Closed-form expressions for the surface resistance R s (f,H) have been obtained by solving equations of nonequilibrium BCS theory in the dirty limit for which R s (H) has a significant minimum as a function of a low-frequency (hf << kT) rf magnetic field H a . The calculated microwave suppression of R s (H a ) is in good agreement with recent experiments on Ti-alloyed Nb cavities from JLab. The Q(H) rise can extend up to the fields ≈ (0.5-0.7)H c and can be strongly sample-dependent due to the subgap states in the density of states caused by materials defects and impurities. The latter suggests new opportunities for tuning the SRF performance by optimizing the density of states and producing appropriate dirty layers at the cavity surface.   It is shown that superimposed dc and ac fields, H(t)=H 0 + H a sin( ω t) can be used to reduce rf dissipation by tuning R s (f,H 0 ) with the parallel dc field H 0 . 
Thursday, October 30, 2014 
11:00 a.m. 
CEBAF Center, Room F113 



Seminar   archives are found here:    http://wwwold.jlab.org/accel/seminars.html 


------ 
Audrey N. Barron 
Administrative Assistant 
Center for Advanced Studies of Accelerators 
Jefferson Lab 
757-269-7327 
anichols at jlab.org / www.jlab.org 




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