thin films: a mandatory path for the future of srf ?
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Thin films: a mandatory path for the future of SRF ?. What kind of thin films ?. Niobium on copper (µm) After ~ 20 years stagnation : new revolutionary deposition techniques Great expectations in cost reduction No improved performances/ bulk Nb Higher Tc material (µm) - PowerPoint PPT PresentationTRANSCRIPT
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 1
Thin films: a mandatory path for the future of SRF
?
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 2
What kind of thin films ?
Niobium on copper (µm) After ~ 20 years stagnation : new revolutionary
deposition techniques
Great expectations in cost reduction
No improved performances/ bulk Nb
Higher Tc material (µm) Based on superheating model.
Higher field and lower Q0 expected
Superheating model questioned for type II SC
Higher Tc material (nm), multilayer Based on trapped vortices model (Gurevich)
Higher field and lower Q0 expected
Recent experimental evidences
Specific characterization tools needed
Better understanding of SRF physics needed
Subtask
2
Subtask 3
Subtask
1
Subtask
4
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 3
Subtask
2
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011
Classical theory BCS + RF :
Magnetic RF field limits Eacc : Eacc HRF
Phase transition when magnetic HRF ~> HSH (superheating field)
For Nb HSH ~1,2.HC (thermodynamic)
Higher Tc => higher Hc => higher Eacc
But… BCS valid only near TC , clean limit we work at 2 K + rather dirty limit.
BCS model needs to be completed
4
Limits in a RF cavity
Meissner state
GL parameter k (= l/x)
Mixed state
C
C
H
H 1
0.6 1.2 1.6 2.0
Type II
CH
H
0 0.4
0.5
1.0
1.5
Type I
22 C
C
H
HC
sh
H
HNormal state
2.0
2.5Pb Nb
Superheated state
MaterialOperating
temperature (K)Eacc : theoric limit (MV/m)
For Hpic/Eacc ~40 Oe/(MV/m)
Nb 2 55
Nb 4 49
Nb3Sn 4 95
MgB2 4 80
MgB2 4 52
Hypothesis :
HSH ~1,2.HC for k ~1
HSH ~0,75.HC for k>>1
TC ↑and HC ↑
Note : increase of Q0 also very
important
Subtask
1
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 5
Nb3Sn development at CERN
Development of a thermal deposition set up.
Comparison between :
thermal diffusion of tin into niobium
co-deposition by sputtering of Nb and Sn
followed by a thermal reaction
Subtask
3
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011
Theoretical Work from Gurevich : temperature correction
Non linear BCS resistance at high field : quadratic variation of RBCS
Vortices : normal area ~ some nm can cause “hot spots” ~ 1 cm (comparable to what is observed on cavities)
At high field vortices => thermal dissipation => Quench
Nb is the best for SRF because it has the highest HC1, (prevents
vortex penetration)
HC1 Nb (0.17 T)
1E+08
1E+09
1E+10
1E+11
1E+12
0 5 10 15 20 25 30 35
Epk (MV/m)
Qo
QUENCH
10 20 30 40 50 60 70
HC1 Nb3Sn (0.05 T)
Nb3Sn
Nb
1.5 GHz Nb3Sn cavity (Wuppertal, 1985)
1.3 GHz Nb cavity (Saclay, 1999)
6
High field dissipations : due to vortices ?
A. Gurevich, "Multiscale mechanisms of SRF breakdown". Physica C, 2006.
441(1-2): p. 38-43
A. Gurevich, "Enhancement of RF breakdown field of SC by multilayer
coating". Appl. Phys.Lett., 2006. 88: p. 12511.
P. Bauer, et al., "Evidence for non-linear BCS resistance in SRF cavities ".
Physica C, 2006. 441: p. 51–56
Nb is close to its ultimate limits
(normal state transition)
avoiding vortex penetration => keep below
HC1
increasing the field => increase HC1
“invent” new superconductors with HC1> HC1Nb
Subtask
1
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 7
Overcoming niobium limits (A.Gurevich, 2006) :
Keep niobium but shield its surface from RF field to prevent vortex penetration
Use nanometric films (w. d < l) of higher Tc SC : => HC1 enhancement
Example :
NbN , x = 5 nm, l = 200 nm
HC1 = 0,02 T
20 nm film => H’C1 = 4,2 T
Breaking Niobium monopoly
x 200
Nd
eHH applNb
Cavity's internal surface →Outside wall
Happlied
HNb
(similar improvement with MgB2 or Nb3Sn)
high HC1 => no transition, no vortex in the layer
applied field is damped by each layer
insulating layer prevents Josephson coupling
between layers
applied field, i.e. accelerating field can be
increased without high field dissipation
thin film w. high Tc => low RBCS at low field =>
higher Q0
Subtask
3
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 8
High Tc nanometric SC films : low RS, high HC1
1E+09
1E+10
1E+11
1E+12
0 20 40 60
16080
Accelerating Field Eacc (MV/m)
Magnetic field B (mT)
Qu
alit
y co
effi
cie
nt
Q0
In summary : take a Nb cavity…
deposit composite nanometric SC (multilayers) insideNb / insulator/ superconductor / insulator /superconductor…
Increasing of Eacc AND Q0 !!!
Best candidatesNb3SnMgB2
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 9
SC structure optimization Deposition techniques optimization
Magnetron sputtering Inac (Grenoble), Atomic Layer Deposition INP (Grenoble)
Nb
NbN
Al2O3
MgO
Cu
Metallic substrates more realistic):
Multilayers optimization
From samples to cavities : ALD involves the use of a pair of reagents
Application of this AB Scheme
Reforms a new surface
Adds precisely 1 monolayer
Viscous flow (~1 Torr) allows rapid growth
No line of site requirements
=> uniform layers, larges surfaces, well adapted to complex shapes : cavities!
up grade of existing cavities ?
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011
Development (optimization) of thin films easier on flat samples
allows to scan multiple parameters with standard fabrication devices => good fabrication parameter
then optimization of the deposition technique to cavity geometry
Need to develop specific RF cavities for flat samples
usually higher frequency
gives only low field behavior (Rs, then Q0 estimation)
In between we need a tool to evaluate maximum achievable field
HC1DC ≤ HC1
RF ≤ HC ~ HSH : measure of HC1
SQUID or Local magnetometry
10
Need for specific characterization
Subtask
1
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011
CERN Thin film test stand • Quadrupole resonator: focus magnetic
fields on sample• operation in higher order mode• Calorimetric measurement• B-field focused on sample by resonator
loops operated in quadrupole mode
Plans at HZB• Build complementary apparatus at HZB
with sub-nanoohm resolution• Reduce sample size to 1-2 cm diameter(build insert for CERN apparatus)
11
Sample RF cavity
Quadrupole resonator at CERN
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011
increasing I=
increasing B
12
3rd harmonic measurement, local measurement : sample size >> excitation/measuring coil
b0cos (wt) applied in the coil
temperature ramp
third harmonic signal appears @ BC1
Excitation/Detection coil (small/sample)
Differential Locking Amplifier
= T/TcSample SL : third harmonic signal for various b0
Local magnetometry (1)
TCi at Bi
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 13
Local magnetometry (3)
SL sample : 250 nm Nb + 14 nm MgO + 25 nm NbN
8.90K < Tp° < 16K : behavior ~ NbN alone
Tp°< 8.90K, i.e. when Nb substrate is SC , => BC1SL >> BC1
Nb
4 6 8 10 12 14 16 180
2
4
6
8
10
12
14
16
18
ref
SL
T (K)
B (m
T)
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011
2 4 6 8 10 12 14 16 180
10
20
30
40
50
60
70
80
90
100
ref
SL
T (K)
B (m
T)
SQUID resultsHP
14
Local magnetometry + SQUID measurement @ 4,5K
SL sample : 250 nm Nb + 14 nm MgO + 25 nm NbN
At 4.5 K HPSL ~ 96 mT (+ 78 mT /Nb alone) !!! (~ 20 MV/m)
Need to extend measure @ higher field and lower temperature
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 15
Structure of the task
Sub task Infrastructure Upgrade
HBZ CERN CEA-Irfu INPG CEA-Inac Collaboration outside Eucard2 MP Invest MP Invest MP Invest MP Invest MP Invest
1) Physics of superconductivity Predictions for task 2 and 3 Understanding the results of task 4
0.2 MY 1 MY 0.5 MY INFM (It)
2) Performance improvements / cost reduction of Nb coatings
HIPIMS
Cavity coating facility
20 k€
Sheffield Hallam University
3) Development of New Superconductors
Nb3Sn
Multilayers
Thermal diffusion facility @ CERN
80 k€
R&D conformal deposition set up @ INPG Cavity conformal deposition set up @ CEA
1 MY. 80 k€ 0.8 MY 95 k€ 0.5 MY
Jlab (USA), LNE (Fr) Multilayers deposition, various techniques
4) Characterization tools (measurement of samples prepared in task 2 & 3) Development of new
techniques
Sample characterization (RF performances)
Sample characterization
(material properties)
Cavity @ HZB
1.8 MY 50 1 MY
Magnetometer @ Saclay 10 k€
IPNO, LKB : other RF characterizations
10 k€ 0.2 MY 0.5 MY 10k€
Consumables: Helium, substrates, targets….
10 k€ 10 k€ 20 k€ 10k€
Mission - 10 k€ - 10 k€ - 15 k€ 5 k€ 5 k€ Total 2 70 k€ 0 3 135 k€ 1 100k€ 1.5 25 k€ 5.5 MY + 430 k€ 250 k€ 120 k€ 405 k€ 190 k€ 160 k€ 945 k€
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 16
Conclusions & perspectives:
New techniques combined with theoretical (serious!)
work open new technological pathways for SRF
performances
Improvement expected for Eacc AND Q0 : all accelerator
applications can benefit from these approaches !
It is a truly innovative topic with promising exploratory
results : opens the way to Eacc > 100 MV/m and Q0 x 10…
It is difficult to have it granted by one particular
machine project or laboratory.
It is right in the scope of the EC support : basic R&D for
a whole scientific community
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 17
Compléments
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 18
Adjustment of coil distance
glass bead wedge : 60 µm coil
glue glue + copper powder
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 19
First exp. results on high quality model samples
Collaboration with J.C. Villégier, CEA-Inac / Grenoble
Reference sample RMonocrystalline sapphire
250 nm Nb “bulk”
~ 15 nm insulator (MgO)
~ 25 nm NbN
Test sample SL
Monocrystalline sapphire
250 nm Nb “bulk”
14 nm insulator (MgO)
~ 12 nm NbN
Test sample ML
x 4
Choice of NbN: ML structure = close to Josephson junction preparation (SC/insulator compatibility) Use of asserted techniques for superconducting electronics circuits preparation:
Magnetron sputtering Flat monocrystalline substrates
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011
5 5.5 6 6.5 7 7.5 8 8.5 90
0.01
0.02
0.03
0.04
20
Local magnetometry (4) Sample SL : small Nb signal @ ~TcNb : Nb is sensed through the NbN layer !
Since the Nb layer feels a field attenuated by the NbN layer, the apparent transition field is higher.
This curve provides a direct measurement of the attenuation of the field due to the NbN layer
8 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.90
0.5
1
1.5
2
Nb Ref
Nb/SL
T (K)
B (
mT
)
BC1 curves for Niobium in the reference (direct
measurement) and in SL (under the NbN layer).
Nd
eHH applNb
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 22
Bulk Nb ultimate limits : not far from here !
Cavité 1DE3 : EP @ Saclay
T- map @ DESYFilm : courtoisieA. Gössel +
D. Reschke(DESY,
Début 2008)
The hot spot is not localized : the material is ~ equivalent at each location => cavity not limited /local defect, but by material properties ?
CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 23
Rappels sur les principaux supras
Matériau TC (K) rn (µWcm)
HC
(Tesla)*HC1
(Tesla)*HC2
(Tesla)*lL (nm)* Type
Pb 7,1 0,08 n.a. n.a. 48 I
Nb 9,22 2 0,2 0,17 0,4 40 II
Mo3Re 15 0,43 0,03 3,5 140 II
NbN 17 70 0,23 0,02 15 200 II
V3Si 17 II
NbTiN 17,5 35 0,03 151 II
Nb3Sn 18,3 20 0,54 0,05 30 85 II
Mg2B2 40 0,43 0,03 3,5 140 II- 2gaps
YBCO 93 1,4 0,01 100 150 d-wave