breakthrough technology for very high quality factors in ... · l. d. cooley, a. grassellino, 2013...
TRANSCRIPT
Breakthrough Technology for Very
High Quality Factors in SRF Cavities
Alexander Romanenko
27th Linear Accelerator Conference (LINAC’2014)
2 Sep 2014
Outline
• Two major discoveries at Fermilab
– Nitrogen doping to lower the BCS surface resistance
• Lower than the previously perceived “theoretical limit”
• Example at 16 MV/m: Q >3x1010 by doping vs. Q~1.5x1010 with the
standard ILC/XFEL cavity processing
• Also lowers residual
– Effective magnetic flux expulsion by fast/high thermal gradient
cooldown to achieve record low residual resistances
• Example: Q = 2.7x1011 in 27 mG ambient field
• Immediate implications
– LCLS-II
• Future perspectives
2 Sep 2014 Alexander Romanenko | LINAC'2014 2
• First breakthrough at Fermilab: Nitrogen doping can
drastically lower “fundamental” BCS losses in cavity walls
and increase Q several times
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A. Grassellino et al, 2013 Supercond. Sci. Technol. 26 102001 (Rapid Communication) – selected for highlights of 2013
0 5 10 15 20 25 30 35 4010
9
1010
1011
Q0
Eacc
(MV/m)
T= 2K
Nitrogen doping: a breakthrough in Q
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Standard state-of-the art preparation
This was the highest Q possible up to last year
Record after nitrogen doping – up to 4 times higher Q!
A. Grassellino et al, 2013 Supercond. Sci. Technol. 26 102001 (Rapid Communication)
1.3 GHz
Breakthrough in quality factor: nitrogen doping
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• Injection of small nitrogen partial pressure at the end of 800C degassing followed by several ums of EP-> drastic increase in Q
• Reproduced on tens of 1- and 9-cell cavities at FNAL
T=2K
A. Grassellino et al, 2013 Supercond. Sci. Technol. 26 102001 (Rapid Communication)
Characteristic anti-Q-slope
Doping is easily reproducible process
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2.00E+010 4.00E+010 6.00E+010 8.00E+0100
1
2
3
4
Count
Q0 (Eacc=16 MV/m, T=2K)
LCLS-II spec
EP+120C
bake
Fermilab nitrogen/argon
doping technology
N total Mean Standard
Deviation
Minimum Median Maximum
Q0 20 4.3e10 1.2e10 3.2e10 4.0e10 7.4e10
Obtained Q several times above state-of-the-art
Reproduced at other labs in 1- and 9-cells
Cornell
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JLab
Cutouts from N doped cavities - SIMS
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Only about 40 ppm of N is enough
What does N treatment do? N depth profiles by SIMS
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Nitrides Interstitial nitrogen in Nb
Non-doped
Doped
Depth (um)
2 4 6 8 10 12 14 16 184
6
8
10
standard treatment
standard treatment
nitrogen treatment
nitrogen treatment
R2K
BC
S (
n
)
Eacc
(MV/m)
Physics – origin of the effect
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A. Grassellino et al, 2013 Supercond. Sci. Technol. 26 102001 (Rapid Communication) A. Romanenko and A. Grassellino, Appl. Phys. Lett. 102, 252603 (2013)
Anti-Q-slope emerges from the BCS surface resistance decreasing with field
This is what Mattis-Bardeen theory predicted to be the lowest possible surface resistance for Nb -> we breached it!
Rs(T) = RBCS(T) + Rresidual
Nanostructural studies provide first clues
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• Hydrides may be the cause of the medium and high field Q slopes [see A. Romanenko, F. Barkov, L. D. Cooley, A. Grassellino, 2013 Supercond. Sci. Technol. 26 035003]
• Nitrogen doping may fully trap hydrogen => only intrinsic Nb behavior is then manifested?
Y. Trenikhina (IIT/FNAL), A. Romanenko – to be published
Nb
Electron diffraction patterns from the penetration depth taken at 94K reveal the difference
Non-doped Nb
Doped Nb
TEM on FIB-prepared cutouts
Secondary diffraction peaks appear signalling the formation of lossy niobium hydrides
Nb lattice
Nb lattice
Alexander Romanenko | LINAC'2014
Some possible mechanisms for intrinsic Nb behavior leading to
increasing Q with field
• Momentum of Cooper pairs changes the DoS
– B. P. Xiao et al, Physica C 490 (2013) 26-31
• Quasiparticle energy distribution deviates from thermal
equilibrium
– P. J. de Visser et al, Phys. Rev. Lett. 112, 047004 (2014)
• Time-dependent DoS
– A. Gurevich, Phys. Rev. Lett. 113, 087001 (2014)
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• Second breakthrough at Fermilab: cooldown rate and thermal
gradients around Tc drastically affect the Meissner effect and
can be used to achieve ultra-low residual resistances even in
high ambient fields
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A. Romanenko, A. Grassellino, O. Melnychuk, D. A. Sergatskov, J. Appl. Phys. 115, 184903 (2014)
Magnetic probes reveal the new physics
Full expulsion of the magnetic field should increase the field at equator ~2 times when going superconducting
Fluxgate magnetometers
H
A. Romanenko, A. Grassellino, O. Melnychuk, D. A. Sergatskov, J. Appl. Phys. 115, 184903 (2014)
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Efficient flux expulsion
Poor flux expulsion
It turns out the expulsion efficiency can be controlled by the cooldown procedure through Tc=9.2K (fast/slow, uniform or not)
Same Meissner behavior for EP, EP+120C, N doping, fine/single grain, cooling is what matters
Bare N doped 9-cell in vertical test
Oct, 2013
Efficient flux expulsion
Poor flux expulsion
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0 5 10 15 20 251.0x10
10
1.2x1010
1.4x1010
1.6x1010
1.8x1010
2.0x1010
2.2x1010
2.4x1010
2.6x1010
2.8x1010
3.0x1010
3.2x1010
3.4x1010
3.6x1010
3.8x1010
4.0x1010
#1: First fast from 300K
#2: Slow from 15K
#3: Fast from 15K
Q0
Eacc
(MV/m)
Bare/dressed cavities behave identical
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Flux expelled efficiently
Flux mostly trapped
Thermal currents are present but have no effect
Dressed 9-cell – VTS measurements
17
No effect of thermal currents in VTS of dressed cavities
Large (125 mG) magnetic fields generated by thermal currents, no effect on Q
Cooldown from 300K
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18
Dressed cavities behave exactly the same as bare
-15
-10
-5
0
5
65000 70000 75000
0
5
10
15
20
25
Mag
ne
tic fie
ld (
mG
) Cell 5 axial
Cell 1 axial
Cell 1 tangential
Cell 9 axial
Tem
pe
ratu
re (
K)
Time (sec)
Cell 1
Cell 1
Cell 9
Cell 9 - Cell 1
Warmup after slow
Fast from 15K
Everything the same as in our J. Appl. Phys. 115, 184903 (2014)
2 Sep 2014 Alexander Romanenko | LINAC'2014
Confirmed at Cornell and Jlab – VTS and HTS
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Cornell HTS measurements of the Fermilab N-doped 9-cell [See MOPP018 ]
JLab VTS data on a 9-cell [See TUPP138 ]
Possible mechanism #1
• Thermal gradient at the superconducting/normal conducting
boundary is aiding the flux expulsion => the higher dT/dx the
better (fast and from higher temperature preferred)
• See [J. Appl. Phys. 115, 184903 (2014)] for details
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Example of thermal difference across the 1-cell cavity
Possible mechanism #2
• See [J. Appl. Phys. 115, 184903 (2014)] for details
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Fast Slow
For this mechanism uniformity is “bad” -> leads to islands
22
Optimal cooling conditions produce record high Qs even when
ambient fields are large
Q=2.7e11 in 23mG!
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Also means that magnetic shielding requirements may be relaxed
Implications
• CW operation becomes possible for short-pulse machines
• LCLS-II at SLAC
– Adopted N doping as a baseline
• 2x higher Q leads to a factor of 2 decrease in required refrigeration => 1
cryoplant less = savings of ~50-100M$ in capital costs + 10s of M$ in
operational costs
• PIP-II at FNAL
– Increase of duty factor from 5% to 30% desired by Mu2e becomes
possible with doubling the Q
• Q can be preserved/improved and the magnetic shielding requirements
relaxed if the optimal cooling is mastered in the cryomodule
– Looks straightforward, no show stoppers here
• High gradient/high Q becomes possible => future e+e- machines at
drastically reduced costs
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Impact of Increasing Q on optimal Gradient
• ILC requirements: Eacc > 35 MV/m/ Q0 > 8x109
• Improving Q0 from 8x109 to 3x1010 has a big impact on machine
cost
• Optimal gradient moves to ~ (70 - 80) MV/m
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Conclusions
• We have two new breakthroughs increasing the Q
– Nitrogen doping
• Doped cavities become even more efficient at higher fields
– Efficient flux expulsion
• Opens up the route to minimize the residual resistance even in
poorly shielded realistic environment
– May allow to relax the specs on magnetic shielding
• Exciting time in SRF
– We will follow the science of these discoveries further and see
where it leads us
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THANK YOU
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27
BACKUP: Thermal currents easily minimized/removed by
cooling fast from lower T – for example ~20K
Thermal gradient
No thermal currents
2 Sep 2014 Alexander Romanenko | LINAC'2014