Thin films: a mandatory path for the future of SRF ?

CEA/DSM/Irfu/SACM/Lesar Claire Antoine – Eucard 2 preparation – July 2011 1

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) 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


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


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


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


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


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


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 ?


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


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


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


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)


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


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€


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


Compléments


Adjustment of coil distance

glass bead wedge : 60 µm coil

glue glue + copper powder


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


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


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 ?


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

Thin films: a mandatory path for the future of SRF ?

Documents

Transcript of Thin films: a mandatory path for the future of SRF ?