20180313 saravan - 650 mhz q studies & material...

Saravan K. Chandrasekaran, Martina Martinello, Anna GrassellinoPIP-II meeting13 March 2018

Update on Q0 studies & Nb for PIP-II 650 MHz cavities

• Primarily focused on TESLA type 1.3 GHz cavities, with sufficient statistics to industrialize and implement in LCLS-II– N-doping– Surface resistance contributions– Sensitivity to magnetic flux trapping with & without N-doping

• Cool down rates & their influence on flux trapping

• Preliminary studies on 650 MHz 1-cell & 5-cell cavities – N-doping (1.3 GHz optimized recipe)– Sensitivity to magnetic flux trapping with & without N-doping

Studies so far…

3/13/2018 S.K. Chandrasekaran | Update on Q & Nb studies for PIP-II 650 MHz cavities2

The discovery of N-doping

S.K. Chandrasekaran & A. Grassellino

0 5 10 15 20 25 30 35 40109

1010

1011

Q0

Eacc (MV/m)

T= 2K

Q‐factor improvement after N‐doping – up to 4 times higher Q than standard Nb cavities

Typical Q vs Eacc curve obtained with 120C bake (standard ILC treatment)

Average Q at 16MV/m:

• Standard ILC  treatment:

~1.7 · 10

• N‐doping:

~3.5 · 10

Anti‐Q‐slope

A. Grassellino et al., Supercond. Sci. Technol. 26, 102001 (2013) – Rapid Communications

Slide courtesy M. Martinello 3 9/20/17

N-doping process

• 800 C standard degas• Inject N2 at 25 mTorr

– 99.9999% purity• Pump to high vacuum &

short soak• Or cooldown

S.K. Chandrasekaran & A. Grassellino

M. Merio, MOPB111 SRF ‘15

4 9/20/17

N-Doping: small variation from standard protocol, large difference in performance

9/20/17 S.K. Chandrasekaran & A. Grassellino

Example from a doping process developedfor LCLS-2:

• Bulk EP• 800 C anneal for 3 hours in vacuum• 2 minutes @ 800C nitrogen diffusion• 800 C for 6 minutes in vacuum• Vacuum cooling• 5 microns EP

Cavity after Equator Welding

EP 140 um

Ethanol Rinse

External 20 um BCP

Short HPR

800C HT Bake

RF Tuning

EP 40 um

Ethanol Rinse

Long HPR

Final Assembly

Long HPR

Helium Tank Welding

Procedure

VT Assembly

HPR

HOM Tuning

Ship to DESY

Leak Check

120C bake

XFE

L

X

5

9/20/17 S.K. Chandrasekaran & A. Grassellino

N2

Nb

Slides adapted from M. Martinello

First breakthrough for Q: N doping of Nb cavities800C UHV, 3 

hours800C UHV, 6 minutes

800C UHV, 6 minutes UHV coolingUHV cooling 5 um EP5 um EP

N

Final RF Surface

800C UHV, 6 minutes UHV cooling 5 um EP

800C N2p = 25 mTorr 2 

minutes

800C N2p = 25 mTorr 2 

minutes

800C N2p = 25 mTorr 2 

minutes

N Interstitial

Nbx N

y

Y. Trenikhina et al, Proc. of SRF 2015

Y. Trenikhina et al, Proc. of SRF 2015

6

9/20/17S.K. Chandrasekaran & A. Grassellino7

HB650 RF Test Results: N-doping vs 120C baking

Martina Martinello | P2MAC - Apr 2017

PIP‐II specs

HB650 RF Test Results: N-doping vs 120C baking

Martina Martinello | P2MAC - Apr 2017

PIP‐II specs

ANTI‐Q SLOPE NOT PRESENT, YET.DOPING PARAMETERS NEED TO BE OPTIMIZED

HB650 5-cells Test Result

Martina Martinello | P2MAC - Apr 2017

PIP‐II specs

First Results of Rs(Eacc) at 650 MHz

Martina Martinello | P2MAC - Apr 2017

·

• 2/6 N‐doped gives very low R0 but higher sensitivity

• RBCS seems to be very similar for the 2/6 N‐doped and the 120C baked (not optimal doping recipe!), but still lower at high fields

Trapped flux surface resistance

Martina Martinello | P2MAC - Apr 2017

These losses can be reduced by minimizing these contributions:

• Magnetic shielding/hygiene improvement

• Fast Cooling• Material Optimization

S• Optimizing mean free path

Trapped flux surface resistance

Martina Martinello | P2MAC - Apr 2017

These losses can be reduced by minimizing these contributions:

• Magnetic shielding/hygiene improvement

• Fast Cooling• Material Optimization

S• Optimizing mean free path

External magnetic field

Minimization of remnant field in the cryomodule

Martina Martinello | P2MAC - Apr 2017

Demagnetization

Empty vacuum vessel Assembled Cryomodule

Coils for magnetic field demagnetization

Demagnetization

Minimization of remnant field in the cryomodule

Martina Martinello | P2MAC - Apr 2017

Demagnetization

Empty vacuum vessel Assembled Cryomodule

Coils for magnetic field demagnetization

Demagnetization

SIMILAR TECHNIQUES CAN BE OPTIMEZED FOR 650 MHz CMs

Trapped flux surface resistance

Martina Martinello | P2MAC - Apr 2017

These losses can be reduced by minimizing these contributions:

• Magnetic shielding/hygiene improvement

• Fast Cooling• Material Optimization

S• Optimizing mean free path

Flux Expulsion Efficiency

Fast cooldown helps flux expulsion

• Fast cool‐down lead to large thermal gradients→ efficient flux expulsion• Slow cool‐down lead to small thermal gradients→ poor flux expulsion

A. Romanenko et al., Appl. Phys. Lett. 105, 234103 (2014)A. Romanenko et al., J. Appl. Phys. 115, 184903 (2014)D. Gonnella et al, J. Appl. Phys. 117, 023908 (2015)M. Martinello et al., J. Appl. Phys. 118, 044505 (2015)S. Posen et al., J. Appl. Phys. 119, 213903 (2016)S. Huang, Phys. Rev. Accel. Beams 19, 082001 (2016)

T1

T2

All flux trapped

Efficient flux expulsion

Bsc/Bnc=1.74 after complete Meissner effect

Bsc/Bnc=1 after full flux trapping

B sc/B n

c

Q0

Martina Martinello | P2MAC - Apr 2017

Current understanding of flux expulsion issue

3/13/2018 S.K. Chandrasekaran | Nb Specification update for PIP-II18

• Problem: Substantial magnetic flux may get trapped in cavities’ bulk Nb decreasing Q0– Observed in LCLS-II, despite cavities being manufactured from

Nb of XFEL/007 (or LCLS-II) spec. & fast cool down

Figures courtesy: S. Posen

Current understanding of flux expulsion issue

3/13/2018 S.K. Chandrasekaran | Nb Specification update for PIP-II19

• Problem: Substantial magnetic flux may get trapped in cavities’ bulk Nb decreasing Q0– Observed in LCLS-II, despite cavities being manufactured from

Nb of XFEL/007 (or LCLS-II) spec. & fast cool down

Figures courtesy: S. Posen

LCLS‐II 1.3 GHz

Current understanding of flux expulsion issue

3/13/2018 S.K. Chandrasekaran | Nb Specification update for PIP-II20

• Solution: Heat treat cavity at T>900 C, some even 950 C– Grain growth & lowering of tensile properties seem to correlate

to better flux expulsion in these problematic Nb

Figures courtesy: S. Posen

Current understanding of flux expulsion issue

3/13/2018 S.K. Chandrasekaran | Nb Specification update for PIP-II21

• Solution: Heat treat cavity at T>900 C, some even 950 C– Grain growth & lowering of tensile properties seem to correlate

to better flux expulsion in these problematic Nb

Figures courtesy: S. Posen

LCLS‐II 1.3 GHz

• Dislocations and unrecovered/unrecrystallized regions may be the key…

• Role of purity (RRR) unclear at this time

Current understanding of flux expulsion issue

3/13/2018 S.K. Chandrasekaran | Update on Q & Nb studies for PIP-II 650 MHz cavities22

Large grain cavities – lower grain boundary contribution

Sliced & formed

Sliced, rolled, & formed

Figures courtesy: S. Posen

Available:• Fermilab ED0371037 Rev C

– Aimed at high gradient cavities• DESY 2009

– Aimed at high gradient 1.3 GHz cavities for XFEL• XFEL/007, 2010 (also LCLS-II)

– Aimed at high gradient 1.3 GHz cavities– Must have license with DESY to view/use

What is needed for PIP-II• Above + high Q (i.e. good flux expulsion)

– Fermilab version may already address this…

Nb specifications

3/13/2018 S.K. Chandrasekaran | Nb Specification update for PIP-II23

• ASTM B393 Type 5 RRR Nb

Fermilab ED0371037 Rev C

3/13/2018 S.K. Chandrasekaran | Nb Specification update for PIP-II24

Trapped flux surface resistance

Martina Martinello | P2MAC - Apr 2017

These losses can be reduced by minimizing these contributions:

• Magnetic shielding/hygiene improvement

• Fast Cooling• Material Optimization

S• Optimizing mean free pathTrapped 

Flux Sensitivity

10 100 10000.0

0.5

1.0

1.5

2.0

(nm)

Doped

EP/BCP

Sensitivity data @ 5 MV/m Sensitivity data @ 16 MV/m

S (n

/m

G)

120 Cbaked

Light doping to minimize trapped flux sensitivity

Martina Martinello | P2MAC - Apr 2017

Trapped flux sensitivity:

• Bell‐shaped trend of asa function of mean freepath

• N‐doping cavities presenthigher sensitivity thanstandard treated cavities

• Light doping is neededto minimize trapped fluxsensitivityM. Martinello et al., App. Phys. Lett. 109, 062601 (2016)

Minimizing at 16 MV/m

Martina Martinello | P2MAC - Apr 2017

• Very low values of RBCS

• Acceptable values of sensitivity

• Low residual resistance

The best surface treatmentminimizes both andcontributions:

For LCLS‐II was chosen lightdoping:

Minimizing at 16 MV/m

Martina Martinello | P2MAC - Apr 2017

• Very low values of RBCS

• Acceptable values of sensitivity

• Low residual resistance

The best surface treatmentminimizes both andcontributions:

For LCLS‐II was chosen lightdoping:

SIMILAR STUDIES TO BE DONE FOR 650 MHz

9/20/17S.K. Chandrasekaran & A. Grassellino

The advantage of N-doping in condition of full flux-trapping

12

15

N-doping wins over standardtreatments as long as the fieldtrapped is <10 mG

M. Martinello et al., App. Phys. Lett. 109, 062601 (2016)

Martina Martinello | APT Seminar29

9/20/17S.K. Chandrasekaran & A. Grassellino

The advantage of N-doping in condition of full flux-trapping

12

15

N-doping wins over standardtreatments as long as the fieldtrapped is <10 mG

M. Martinello et al., App. Phys. Lett. 109, 062601 (2016)

Martina Martinello | APT Seminar30

SIMILAR ANALYSIS REMAINS TO BE PERFORMED FOR 650 MHz CAVITIES WITH 

SIGNIFICANT STATISTICS

• Extensive studies performed for 1.3 GHz cavities– Same level of statistics may not be obtained for 650 MHz

• Several studies remain for 650 MHz cavities– Studies structure determined from 1.3 GHz experience– Optimized N-doping for 16—18 MV/m

• Lower RBCS & obtain anti-Q slope– Flux trapping sensitivity of N-doped vs 120 C

• More statistics needed

Summary

3/13/2018 S.K. Chandrasekaran | Update on Q & Nb studies for PIP-II 650 MHz cavities31