SRF Niobium cavity processing, fabrication technology

Thursday, 30 Jan. 2020 , 09:30 - 10:00

KEK: HIGH ENERGY ACCELERATOR RESEARCH ORGANIZATION, Japan

Taro Konomi

Indo-Japan Accelerator School cum Workshop (IJAS-2020), 28-31 January 2020January 29, 2020 IJAS2020 1

contents

January 29, 2020 IJAS2020 2

• Cavity measurement method• KEK activities.

• Standard recipe• Nitrogen doping• Nitrogen infusion

ILC Cost down R&D

3

e- Source

e+ Main Linac

e+ Sourcee- Main Linac

Damping Ring

• Main Linac• Length 11km x 2 (500 GeV and 250GeV ILC)

• Number of cavities 16000 (500GeV), 8000(250GeV)

• Cost 2/3 of total ILC cost (include tunnel)• Acc. Gradient as received (vertical test )

• Acc. Gradient 35MV/m ⇒ 40MV/m• Operation gradient

• Acc. Gradient 31.5MV/m ⇒ 35MV/ｍ

N-infusion：The mechanism is still not clear

【Effect】Gradient：10% improve

⇒ # of cavities and modules will be reducedQuality factor ：200% improve

⇒ cryogenic cost will be reduced

Low

loss

High gradient

S. Aderhold / A. Grassellino (TTC@Saclay)Target is 10% cost reduction

ILC superconducting cavity

4

1.3GHz single cell cavity

• ILC cavity is 1.3 GHz 9 cell.• We can use single cell cavity for researching surface treatment

beamRF Input

Cavity RF parameters

5

• ILC cavity is standing wave cavity.• Surface Electric peak located on Iris. Surface Magnetic peak located on Equator.

cell IrisEquator

1.3GHz 9cell cavity (EUV cavity)

0

20

40

60

80

100

120

-600 -400 -200 0 200 400 600

Cavity Shape

Z (mm)

-2

-1

0

1

2

0

1000

2000

3000

0

5 105

1 106

1.5 106

2 106

2.5 106

Axis Elec.

Shape

Surf. Magn.

Surf. Elec.

Parameter Value

Frequency 1.3 GHz

Transit time factor 0.73

R/Q 1009 Ohm

Ep/Eacc 2.03

Hp/Eacc 4.23 mT/(MV/m)

BCS and residual resistance

6

1

10

100

1000

0.2 0.3 0.4 0.5 0.6 0.7 0.8

Meas.R

BCS

Rres

Rs

(nO

hm

)

1/T (1/K)

The RF loss of superconducting cavities is determined by top surface (100 nm). RF loss can be described as follows:

𝑅𝑆 = 𝑅𝐵𝐶𝑆 + 𝑅𝑟𝑒𝑠

𝑅𝐵𝐶𝑆 𝑇, 𝐸𝑎𝑐𝑐 = 𝐴(𝐸𝑎𝑐𝑐)𝜔2

𝑇𝑒𝑥𝑝 −

∆(𝐸𝑎𝑐𝑐)

𝑘𝑇𝑐・

𝑇𝐶

𝑇

RBCS (BCS resistance)• Exponentially dependent on temperatureRres (Residual resistance)• magnetic flux pinned in cavity• thermal conductivity (plate thickness) of material• large dust and defects on the surface• Field Emission is also included.

• Niobium superconducting cavities show characteristic changes in surface resistance around 2K.• It can be divided into BCS resistance which appears due to superconductivity and residual resistance

which does not depend on temperature.

Vertical high gradient test

7

• Various sensors (temperature, DC magnetic, X-ray) are mounted around the cavity to detect the quench phenomena.• Heater and solenoid coil are installed to control the external magnetic field during the superconducting transition.

Heater

Flux gate sensor

X-ray sensor

temperature sensor

Solenoid Coil

Flux trapping

January 29, 2020 IJAS2020 8

𝑅res =2𝑃𝑣𝐻RF

∝ 𝑁trap

Residual resistance

Normal conducting supercondcuting

Residual resistance (Flux trapping)

January 29, 2020 IJAS2020 9

0

20

40

60

80

100

0 0.5 1 1.5 2 2.5 3 3.5

750 oC (R8a_1st)

900 oC (R8b_1st)

800 oC (R8c_1st)

Ex

pu

lsio

n (

%)

delta Temperature (K)

10-9

10-8

10-7

10-6

0.2 0.3 0.4 0.5 0.6 0.7 0.8

750 oC (R8_1st)

900 oC (R8b_1st)

800 oC (R8c_1st)

Rs

(Oh

m)

1/T (1/K)

• Increasing the annealing temperature makes it easier to expel magnetic field.• Outer solenoid coil add the DC magnetic field.• the magnetic field was positively expelled by temperature gradient in the cavity by

the heater on beam pipe.⇒Various measurements are possible without significant difference in residual resistance.

High gradient test

10

109

1010

1011

1012

0 5 10 15 20 25 30 35 40

1.5K1.6K1.8K2.0K

Qo

Eacc [MV/m]

Quench

No Field Emission

R2 14th VT (standard recipe)

0

5

10

15

20

0 5 10 15 20 25 30 35 40

Surface Resistance

RBCS

@2K

Rres

Rre

s, R

bcs

(n

Oh

m)

Eacc (MV/m)

• It is possible to divide into BCS resistance and residual resistance by measuring temperature dependence.• Temperature can be controlled by liquid helium pressure.

Sensitivity to magnetic flux

11

• Nitrogen treatment is more sensitive to DC magnetic flux than standard recipe.

Effect of Nitrogen treatment

12A. Grassellino et al., Supercondutor Science and Technology Vol.30Num.9 (2017)

ILC Standard recipe

standard N-dope N-infusion

Fabrication

Anneal (800℃)

Electric polishing (100um)

Final electric polishing (10~20um)

Vertical test

Assembly in cleanroom

baking(120℃)

N-process (N2@800℃) N-process(N2@120℃)

Surface treatment

13

KEK activities

Vacuum furnace

14

KEK small furnace

• Diffusion pump 1unitULVAC PFL-22 10000L/sec

• Mechanical booster pump 1unitULVAC PMB024CM 33300L/min

• Rotary pump unitULVAC PKS-070 7000L/min

• Diffusion pump with LN2 trap 1unit• Mechanical booster pump 1unit• Roots pump 1unint

Osaka Vac. RD600 500m3/h

KEK large furnace

• Turbo pump 3unintsSIMADZU TMP3202M 3000L/sec

• Scroll pump 3unintsANEST IWATA ISP500 500L/min

• Cryopump 1unint

ANELVA CAP220 10000L/sec

J-PARC furnace

14

• CRYO pump 1unintULVAC CRYO U20H 10000L/s

• Roots pump 1unint

ULVAC CR300B 5000L/min

COI furnace

2015-2016

2018-Present2016-2018

2015-2016

Standard recipe

15

10-6

10-5

10-4

10-3

10-2

10-1

0

200

400

600

800

1000

0 5 10 15 20 25 30

KEK small

Vac.

Temp.

Va

cuu

m (

Pa

)

Tem

p(o

C)

Time (hour)

10-6

10-5

10-4

10-3

10-2

10-1

0

200

400

600

800

1000

0 5 10 15 20 25 30

KEK Large

Vac.

Temp.

Va

cuu

m (

Pa

)

Tem

p(o

C)

Time (hour)

10-6

10-5

10-4

10-3

10-2

10-1

0

200

400

600

800

1000

0 5 10 15 20 25 30

J-PARC

Vac.

Temp.

Va

cuu

m (

Pa

)

Tem

p(o

C)

Time (hour)

10-6

10-5

10-4

10-3

10-2

10-1

0

200

400

600

800

1000

0 5 10 15 20 25 30

COI

Vac.

Temp.

Vacu

um

(P

a)

Tem

p(o

C)

Time (hour)

Performance of each furnace

16

Diffusion pump (10000L/sec)Diffusion pump with LN2 trap

Turbo pump (3000 L/sec x3)Cryopump (10000 L/sec)

Cryopump (10000 L/sec)

Oil free furnace can reach 1order lower

Residual Gas analysis

17

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

0 5 10 15 20 25

2 H218 H2O28 N232 O244 CO2

Total preussure

Ion

cu

rren

t (A

)

Tota

l pressu

re (Pa)

Time (hour)

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

0 5 10 15 20 25

2 H218 H2O28 N232 O244 CO2

Total pressure

Ion

cu

rre

nt

(A)

To

tal (P

a)

Time (hour)

J-PARC (800oC x 3h)

KEK Large (750oC x 3h)

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

0 20 40 60 80 100

after 800x3hafter cooling

Ion

cu

rre

nt

(A)

Mass

H2

H2O N

2CO

2

hydrocarbon

Butyl (57)

• Residual gas doesn’t show the big difference between Oil furnace and Oil free furnace.

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

0 20 40 60 80 100

after 750 x 3hafter cooling

Ion

cu

rre

nt

(A)

Mass

H2

H2O

N2

CO2 hydrocarbon

Butyl (57)

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

0 5 10 15 20 25

2 H218 H2O28 N232 O244 CO2

Total pressure

Ion

cu

rren

t (A

)

To

tal P

ressu

re (Pa

)

Time (hour)

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

0 20 40 60 80 100

after 800x3hafter cooling

Ion

cu

rren

t (A

)

Mass

COI (800oC x 3h)

Vertical test result

18

• The cavity performance with oil free furnance can reach same performance as standard without final EP.

109

1010

1011

0 10 20 30 40 50

2K VT w/ and w/o EP

R8_1st 2KR8b_1st 2K_1stR8c_1st 2KR9b_1st 2KR6 4th(J-PARC)R9a_1st (KEKS)

Qo

Eacc(MV/m)

w/ Final EP

XPS (X-ray photoemission spectroscopy) mea

19

280285290295

0

1000

2000

3000

4000

5000

02270040_1a.PRO

Binding Energy (eV)

c/s

200205210215

0

2

4

6

8

10

12

14

x 104 02270040_1a.PRO

Binding Energy (eV)

c/s

C1s

Nb3d

before annealing

280285290295

0

1000

2000

3000

4000

5000

6000

7000

8000

02270042_1a.PRO

Binding Energy (eV)

c/s

200205210215

0

1

2

3

4

5

6

7

8

9

10

x 104 02270042_1a.PRO

Binding Energy (eV)

c/s

KEK large 750oC

280285290295

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

02270046_1a.PRO

Binding Energy (eV)

c/s

200205210215

0

5

10

15x 104 02270046_1a.PRO

Binding Energy (eV)

c/s

KEK small 900oC

280285290295

0

1000

2000

3000

4000

5000

02270052_1a.PRO

Binding Energy (eV)

c/s

200205210215

0

2

4

6

8

10

12

14

x 104 02270052_1a.PRO

Binding Energy (eV)

c/s

J-PARC 800oC

• Top layer coved with carbon and oxygen.• After stripping the top surface layer (9.1 nm), clean surface less than XPS sensitivity appears.

• KEK large furnaces are contaminated with carbon more than 45 nm.

Analyzer: Versa Probe XPSX-ray source: Al K-alpha(1486.6eV), f200 um, 45 W Spattering condition: 2kV , 2mmx2mm(Spattering rate: 9.1nm/min measured by SiO2 reference sample )

0nm

9.1nm

18.2nm

27.3nm

36.4nm

45.5nm

CーNb bond

C-N bond

Nb2O5

Nb

測定：ULVAC(株)

Nitrogen dope

20

Nitrogen injection system

21

• Turbo pump 3unints: SIMADZU TMP3202M (3000L/sec x3)

• Scroll pump 3unints:ANEST IWATA ISP500 (500L/min x3)

• Cryopump 1unint:

ANELVA CAP220 (10000L/sec)

• Same system is used in all furnace.• N2 Flow is controlled by variable valve.

Portable TMP unit

Argon

bottle

Scroll Pump

Variable

Leak valveFilter

Nitrogen bottle

Scroll Pump

Pirani gauge

Turbo PumpCapacitance gauge

CCG

Turbo Pump

Pirani gauge

BA gauge

J-PARC Furnace

Nitrogen inlet line

cryopump

3 Turbo pump

3 Scroll pump

Main pumps

Portable pump unit

QMS

Leak detector

• Cavity is cleaned in clean room by using high pressure rinsing.• All components are transfer in clean pack.• All cavity ports are covered with niobium foil to protect inner surface from Ti contamination

because flanges are made of NbTi or Ti.

Transfer cavity

22

Nitrogen injection

23

10-6

10-5

10-4

10-3

10-2

10-1

100

101

0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25 30 35

J-PARC N-Dope

Vac.

Temp.

Vacu

um

(P

a) Tem

p (o

C)

Time(hour)

10-6

10-5

10-4

10-3

10-2

10-1

100

101

0

200

400

600

800

1000

1200

1400

9 9.5 10 10.5 11

J-PARC N-Dope

Vac.

Temp.

Vacu

um

(P

a) T

emp

(oC

)

Time(hour)

20 min.

Zoom

• 800℃ , N2 pressure ~3Pa、injection time several minutes. • After heat process, apply EP (5um ~10um ) to remove N2

rich layer.

VT results N-dope

24

109

1010

1011

0 10 20 30 40 50

R6 single cell

N-Dope @ J-PARCReference (ILC recipe)

Qo

Eacc(MV/m)

No F.E.

Limited by Quench

Without N-dope↓ Without N-dope

↓

KEK Large KEK Small

3.3Pa N-dope, 2min

J PARC(N-Dope 1st trial )

2.7Pa N-dope, 20min

↑Without N-dope

2.7Pa N-dope, 20min

10-6

10-5

10-4

10-3

10-2

10-1

100

101

0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25 30 35

J-PARC N-Dope

Vac.

Temp.

Va

cuu

m (

Pa

) Tem

p (o

C)

Time(hour)

Although many surface treatments in KEK furnaces did not work, we easily succeeded by using oil free furnace.

N density measured by GD-OES

25

• Density profile of nitrogen was not changed ⇒Impurities (carbon, oxygen,… ) degrader the cavity performance

10-5

10-4

10-3

10-2

10-1

0 10 20 30 40 50 60

KEK Large Nitorogen

#1_NoHeating_N#12_KEKL_750anneal_N#2_KEKL_Ndope_N

den

sity

(a

.u.)

depth (um)

10-4

10-3

10-2

10-1

0 10 20 30 40 50 60

JPARC Nitrogen

#20_JPARC_800anneal_N#3_JPARC_Ndope_N

den

sity

(a

.u.)

depth (um)

Rem

ove b

y EP

Rem

ove b

y EP

Nitrogen infusion

26

COI furnace

27

• We have been studying nitrogen infusion using J-PARC vacuum furnace and COI vacuum furnace.• For nitrogen infusion, the quality of the degree of vacuum is important because the heat treatment in a

vacuum furnace becomes the final surface of RF.• The COI furnace is set up so that it can analyze the introduced gas.

RGA

TMP

RGA

TMPfurnace

Furnace RGA

Variable valve

Angle valve

Variable valve

N2 flow (0.53 SLM)

N2 line RGA

N2injection panel

N2 Purity monitor

28

• Purity was monitored using RGA while injecting nitrogen gas so as to keep the vacuum furnace at 3.3 Pa.• The N2 cylinder is guaranteed 6N purity.• Other gas is the background level of RGA.

10-13

10-12

10-11

10-10

10-9

10-8

0 0.2 0.4 0.6 0.8 1

21828324044

Ion

cu

rren

t (A

)

Time (hour)

N2 line RGA

0

200

400

600

800

1000

1200

10-7

10-5

10-3

10-1

101

103

105

0 100 200 300 400 500 600 700 800

20190421_tempandvac_v2Temp. (Left back)

Pressure (Pa)

Tem

per

atu

re (

oC

)

Pressu

re (P

a)

Time (hour)

10-9

10-8

10-7

10-6

10-5

10-4

10-3

0 100 200 300 400 500 600 700 800

20190421 furnace RGA dataMass 2Mass 18Mass 28Mass 32Mass 40Mass 44

Ion

curr

ent

(A)

Time (hour)

Purity monitor

29

• Even after open to the atmosphere, it is sufficiently low except for hydrogen (~5N).• After heating (1100 ° C x 3h), the purity is initially poor.

• N2 flow seems to carry impurity gas covered the heater or inner surface of furnace

After 1100℃ annealing

10-9

10-8

10-7

10-6

10-5

10-4

10-3

130 135 140 145 150 155 160 165 170

Mass 2

Mass 18

Mass 28

Mass 32

Mass 40

Mass 44

Ion

curr

ent

(A)

Time (hour)

10-9

10-8

10-7

10-6

10-5

10-4

10-3

0 0.2 0.4 0.6 0.8 1

Mass 2Mass 18Mass 28Mass 32Mass 40Mass 44

Ion

curr

ent

(A)

Time (hour)

After open to airN2 flow N2 flowN2 flow

N2 infusion @COI

30

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

100 120 140 160 180 200 220

20190128 AES018

Mass 2Mass 18Mass 28Mass 32Mass 40Mass 44

Inte

nsi

ty (

a.u

.)

Time (hour)

0

200

400

600

800

1000

1200

1400

10-6

10-5

10-4

10-3

10-2

10-1

100

101

100 120 140 160 180 200 220

20190125 AES018

Temp.

Vac.

Tem

per

atu

re (

Pa

)

Pressu

re (Pa

)

Time (hour)

Monitored with variable valveN2：3.3Pa x48h

Result of N-infusion @COI

31

Unfortunately, FE masked high gradient performance, but the Q value was improved at low gradient. A big difference couldn’t seen in BCS resistance. Residual resistance become lower

108

109

1010

1011

1012

10-1

100

101

102

103

0 5 10 15 20 25 30 35 40

Standard_QoNinf_Qo

Standard_XrayNinf_Xray

Qo

Xra

y (u

Sv/h

)Eacc (MV/m)

0

5

10

15

20

0 5 10 15 20 25 30 35 40

Standard_RresNinf_Rres

Standard_RbcsNinf_Rbcs

0

5

10

15

20

RB

CS &

Rre

s (n

Oh

m)

Eacc (MV/m)

Surface analysis of N infusion

32

10-1

100

101

102

103

0 5 10 15 20 25

typical

C_N307_20190110_800Canneal_R6C_N301_20181120_Ninf120C_R8C_N302_20181218_Ninf160C_R9bC_N207_20181015_Ninf200C_sampleonly

Inte

nsi

ty (

norm

ali

zed

by

Nb

)

Depth (nm)

10-4

10-3

10-2

10-1

100

101

102

0 5 10 15 20 25

typical

Nb2+O5_N307_20190110_800Canneal_R6Nb2+O5_N301_20181120_Ninf120C_R8Nb2+O5_N302_20181218_Ninf160C_R9bNb2O5_N207_20181015_Ninf200C_sampleonly

Inte

nsi

ty (

no

rma

lize

d b

y N

b)

Depth (nm)

10-1

100

101

102

0 5 10 15 20 25

typical

Nb+O_N307_20190110_800Canneal_R6Nb+O_N301_20181120_Ninf120C_R8Nb+O_N302_20181218_Ninf160C_R9bNbO_N207_20181015_Ninf200C_sampleonly

Inte

nsi

ty (

no

rma

lize

d b

y N

b)

Depth (nm)

10-3

10-2

10-1

100

0 5 10 15 20 25

typical

Nb+N_N307_20190110_800Canneal_R6Nb+N_N301_20181120_Ninf120C_R8Nb+N_N302_20181218_Ninf160C_R9bNbN_N207_20181015_Ninf200C_sampleonly

Inte

nsi

ty (

no

rma

lized

by

Nb

)

Depth (nm)

• Holding temperature during the nitrogen injection was changed.

• The nitrogen loading appears to increase with temperature.

0

200

400

600

800

1000

10-8

10-6

10-4

10-2

100

102

380 400 420 440 460 480 500

20181015 200 oC N-infusion

Temp.

Vac.

Tem

per

atu

re (

oC

)

Vacu

um

(Pa

)

Time (hour)

Example: 200℃ N-infusion

Changed this temperature

Summary

33

• FNAL has developed nitrogen doping that greatly improves the Q value and nitrogen infusion that improves the Q value and RF critical magnetic field.• Nitrogen dope contains nitrogen to a depth of ~ 30um,• Nitrogen infusion has the difference that nitrogen enters up to ~ 10 nm.

• Nitrogen doping has also been succeeded at KEK.

• Vacuum furnace cleanliness is more important for N-infusion. • Sample measurement requires top surface analysis, which is difficult to analyze.