Overview of Flux Trapping at Cornell · 1.3 GHz high-T N-doping 11/9/18 James Maniscalco 10...

Overview of Flux Trapping at Cornell

J. T. ManiscalcoCLASSE, Cornell University

11/9/18 James Maniscalco 1

Introduction / Teaser Slide


Mean Free Path [nm]100 101 102 103

Rre

s,B/B

trapp

ed

[n Ω

/mG

]

0

1

2

3

4

5Experimental Data

Gurevich Theory, ℓp = 75ℓPower Fit: y = 16.2/

√

ℓ

• In general linear sensitivity with an “offset” (y-intercept) parameter and a “slope” parameter. Linear slope predicted by “collective weak pinning” (see D. Liarte’s talk later today!)

• Parameters dependent on cavity treatment and material parameters, especially the electron mean free path, doping depth, and frequency.

• All Cornell cavities made from the same Nb stock

Outline• Overview of flux sensitivity measurement procedure @ Cornell• Results for doped niobium

• 1.3 GHz high-T (800 ºC) N-doping (including “2/6” recipe)• 1.3 GHz 160 ºC impurity doping• 2.6 GHz “2/6” N-doping

• Results for Nb3Sn• 1.3 GHz cavities


Flux Sensitivity Test Procedure


Helmholz coil

I

I

T3

T2

T1B1

B2

B3

• Fast cool to expel mag. flux; slow cool to trap it

• Temperature sensors (Cernox) to measure ∆T at Tc

• Flux gate magnetometers to measure ambient field, applied field, expulsion, trapping

• Note: dT/dx across cellmatters, not dT/dt

• Though cooling quickly is aneasy way to get a large dT/dx.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Bpk

(T)

0

0.5

1

1.5

2

2.5

3

3.5

4

RB

CS

()

10-8

1.6

1.7

1.8

1.9

2

2.1

2.2

Bath

tem

pera

ture

(K

)

RBCS

R0

Flux Sensitivity Test Procedure• One test with fast cool• One or more tests with slow

cools, varying trapped flux• Collect RF test data:

Rs(BRF , T) = G / Q0

• Exponential fitting (SRIMP algorithm) to separate Rs:


𝑅" = 𝑅$%& 𝐵(), 𝑇 + 𝑅-(𝐵())

Flux Sensitivity Test Procedure• Compare R0 from different

cooldowns to determine sensitivity:


𝑅- 𝐵trapped =𝑅- slowcool − 𝑅-(fastcool)

𝑆 = 𝑅- 𝐵trapped /𝐵trapped= 𝑎 @ 𝐵() + 𝑏

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

BRF

(T)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

R0/B

tra

pp

ed (

/mG

)

10-8

R0 = a*B

RF*B

trapped + b*B

trapped

a = 0.021 0.003 n /mTRF

/mGtrapped

b = 4.8 0.6 n /mGtrapped

a = 0.018 0.003 n /mTRF

/mGtrapped


16 2 mG trapped

36 3 mG trapped

Multiple cooldowns, same sensitivity!

Peak surface magnetic field (mT)

0 20 40 60 80 100 120 140

Qualit

y fa

ctor

Q0

109

1010

1011

C3(P1)

VEP niobium

1.3 GHz high-T N-doping• Treatment protocol:

• 800 ºC UHV degas bake (5-8 hr)• 800 ºC 40 mTorr (6 Pa) N2 doping bake (2-60 minutes)• 800 ºC UHV anneal bake (0-60 minutes)• Final VEP (2-20 µm)


Depth [ µm]0 20 40 60 80

Nitr

og

en

Co

nce

ntra

tion

[Ato

ms/

cm

3 ]

1018

1019

1020

1021

800°C: 20 min, 30 min anneal

990°C: 5 min

1.3 GHz high-T N-doping


Strong “offset”,no slope in BRF

“offset” highly sensitive to ℓ𝒆

See D. Gonnella et al. (J. Appl. Phys. 2016)and J. T. Maniscalco et al. (J. Appl. Phys. 2017)

E acc [MV/m]0 5 10 15 20

Rre

s [Ω]

10-9

10-8

10-7 Btrapped = 0 mG

Btrapped = 3 mG

Btrapped = 7 mG


Rre

s,B/B

trapp

ed

[n Ω

/mG

]

0

1

2

3

4

5Experimental Data


√

ℓ


Rre

s,B/B

trapp

ed

[n Ω

/mG

]

0

1

2

3

4

5Experimental Data


√

ℓ



Sensitivity also likely dependent on properties of bulk Nb!(different cavity material at different labs)

M. Martinello et al.(J. Appl. Phys. 2016)

Flux expulsion also linked to material stock in doped cavities!see D. Gonnella et al. (NIM-A 2018), S. Posen et al. (J. Appl. Phys. 2016)



Takeaway message:

Doping strongly impacts trapped flux losses:1) lowers the mean free path, which increases losses2) increases pinning effects which decrease losses

Nitrogen’s link to pinning is still not well understood.Other material parameters might be relevant too!

(see Danilo’s work on collective weak pinning – how material parameters could affect pinning strength and vortex loss magnitude)

1.3 GHz 160 ºC N-doping


• Treatment protocol:• 800 ºC UHV degas bake (5 hr)• UHV ramp down to 160 ºC• 160 ºC UHV rest (3 hr)• 160 ºC, 40 mTorr (6 Pa) N2 doping

bake (1 day for test shown here)• 160 ºC optional UHV anneal bake

(not used for tests shown here)

• Little/no post chemistry! Maybe HF rinse or oxypolish for light surface removal ≪ 1 µm.

Thinner doped layer, different impurity content,

different impurities

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

BRF

(T)

0

0.2

0.4

0.6

0.8

1

1.2

R0/B

trapped (

/mG

)

10-9

R0 = a*B

RF*B

trapped + b*B

trapped

a = 0.0034 0.0006 n /mTRF

/mGtrapped


42 2 mG trapped


Rre

s,B/B

trapp

ed

[n Ω

/mG

]

0

1

2

3

4

5Experimental Data


√

ℓ



Lower “offset” sensitivity, higher “slope” sensitivitycompared to 800 ºC-doped cavities

(preliminary)



Takeaway message:

Shallow 160 ºC doping gives lower sensitivity than 800 ºC dopingNote: dopant may be different…

Results indicate linear field dependencemore prominent for low-temperature dopings

(see again Danilo’s work/talk)

2.6 GHz “2/6” N-doping• Treatment protocol:

• 800 ºC UHV degas bake (3 hr)• 800 ºC 40 mTorr (6 Pa) N2 doping bake (2 minutes)• 800 ºC UHV anneal bake (6 minutes)• Final VEP (6 µm)



Rre

s,B/B

trapp

ed

[n Ω

/mG

]

0

1

2

3

4

5Experimental Data


√

ℓ

2.6 GHz “2/6” N-doping


“offset” sensitivity 2x higher than 1.3 GHz cavities with similar ℓDR0/Btrapped∝ ω?

see J. T. Maniscalco et al. (LINAC 2018)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

BRF

(T)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

R0/B

trapped (

/mG

)

10-8

R0 = a*B

RF*B

trapped + b*B

trapped

a = 0.021 0.003 n /mTRF

/mGtrapped


a = 0.018 0.003 n /mTRF

/mGtrapped


16 2 mG trapped

36 3 mG trapped

2.6 GHz “2/6” N-doping


Takeaway message:

High frequency → stronger sensitivity

Initial results indicate linear behavior with frequency, i.e.R0/Btrapped∝ ω

1.3 GHz Nb3Sn• Treatment protocol:

• Nb3Sn grown on Nb cavity substrate by vapor diffusion


20× more efficient than Nb at 4.2 K!

1.3 GHz Nb3Sn


Typical results: linear sensitivity

More sensitive slope than doped Nb, similar

“offset” sensitivity

see D. L. Hall et al. (IPAC 2017)𝑑𝑅

𝑑𝐵FGHI= 21 ± 1 pΩ mG⁄ mT⁄ ⋅ 𝐵() + 0.47 ± 0.02 nΩ ∕ mG

3 pΩ mG⁄ mT⁄for 160 ºC doping

≤ 0.5 nΩ/mGfor clean Nb

compare:

M1 M2

1.3 GHz Nb3Sn


Also sensitive to thermoelectric currents

(Seebeck effect)

Cavity must be cooled uniformly!Need to limit flux sensitivity.

hot

cold

V1 →

V2 →

II

1.3 GHz Nb3Sn


Takeaway message:

Clear linear behavior, strong slopeLow “offset” sensitivity – similar to undoped cavities

Review


• Flux sensitivity typically linear in RF field• Coefficients of sensitivity linked to material parameters

• Electron mean free path• Doping depth• Impurity species• Frequency• Niobium stock• Nb3Sn vs. doped Nb vs. clean Nb

(vs. Nb/Cu films not addressed here)• Nb3Sn: R0 from trapped flux important because RBCS is so small!

• Higher “slope” sensitivity than doped Nb• Bimetallic interface necessitates slow, uniform cooling

and good magnetic shielding/hygiene


Thanks for your attention!

See also:

D. B. Liarte’s talk later today (11h59):“Vortex dynamics and hysteretic flux losses due to pinning”

Overview of Flux Trapping at Cornell · 1.3 GHz high-T N-doping 11/9/18 James Maniscalco 10...

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