Pei ZhangM. Therasse and W. Venturini Delsolaro

(BE-RF-SRF, CERN)

The influence of cooldown conditions at Tc on the Q0 of niobium sputtered

quarter-wave resonators

*This work received support from a Marie Curie Early Initial Training Network Fellowship of the European Community's 7th Programme under contract number PITN-GA-2010-264330-CATHI.

Outline

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• Brief introduction of HIE-ISOLDE & QWR

• The impact of thermal gradient & cooldown speed

• The impact of ambient magnetic field

• The frequency shift during transition

• The impact of helium processing on low-field Q0

HIE-ISOLDE Linac

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Niobium sputtered on copper

Boost the radioactive beam energy from 3MeV/u to 10MeV/u by using SC linac.

Quarter-wave resonator

Oct 7th, 2014Thin film workshop, LNL, Italy

More on A. Sublet’s talk

High-β QWR

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Frequency 101.28 MHz

Eacc 6 MV/m

βoptimum 10.9%

R/Q 553 Ω

Epeak/Eacc 5.0

Bpeak/Eacc 95.6 G/(MV/m)

G=RsQ 30.7 Ω

U/Eacc2 0.207 J/(MV/m)2

Pc at 6MV/m 10W

|E| field |H| field

Courtesy M. Fraser

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Cavity production flow

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Cavity reception

Frequency tuning

Surface treatment

Niobium coating

Cryostat preparation

RF cold test

Cavity storage

Niobium Stripping

4-week process

Oct 7th, 2014Thin film workshop, LNL, Italy

More on A. Sublet’s talk

From Feb to Jun 2014

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02.2014

06.2014

Initial cooldown: two full daysThermal cycle: one day

Initial cooldown

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initial cooldown from ~300K

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Initial cooldown & thermal cycle

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initial cooldown from ~300KThermal cycling to ~20K

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Low-field Q0 improvement

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The reduction of low-field Rs by thermal cycle

12% ~ 55%

Low-field: Eacc = 0.2 MV/m

Thermal gradient

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Thermal gradient [K]: average ΔT from T2=10.5 to 8.5K

Oct 7th, 2014Thin film workshop, LNL, Italy

topbottom TTT

Transition temperature

Error bar: standard deviation of ΔT from T2=10.5 to 8.5K

Cooldown speed

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Cooldown speed [mK/s]: average speed from 10.5K to 8.5K

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Cooling rates during thermal cycles are normally 10 ~ 20 times faster.

Rs vs. cooldown speed

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Low-field: Eacc = 0.2 MV/m

0

8.30

QRs

No obvious dependence

Rs vs. thermal gradient

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Low-field: Eacc = 0.2 MV/m

0

8.30

QRs

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Clear dependence on thermal gradient

Decompose Rs

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accERsRsRs 10

Residual resistance

Linear slope

QS1.1 initial cooldown

Rs0 & Rs1 vs. cooldown speed

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accERsRsRs 10

Residual resistance

Linear slope

Rs0 & Rs1 vs. thermal gradient

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accERsRsRs 10

Residual resistance

Linear slope

Ambient magnetic field

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Horizontal plane Vertical plane

Total reduction: 62.5 µT → 18 µT (~70% ↓)

Total enhancement: 62.5 µT → 115.5 µT (~85% ↑)

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Sensitivity to ambient magnetic field

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Ambient magnetic field

0µT 120µT

QP3.2 ~9% ↓ 8% ↑

QS2.1 ~7 % ↓ 6% ↑

• 1.3GHz bulk niobium cavities: 3.5n /Ω µT • Scale to 100MHz bulk niobium: ~1n /Ω µT• Measured 100MHz niobium sputtered QWR

- QP3.2: 0.024 nΩ/µT (1/42 x)

- QS2.1: 0.035 n /µΩ T (1/29 x)

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Frequency shift during transition

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Field emission at high E-field region

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Low-field Q0 degradation

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Low-field Q0 degradation

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44%

Low-field Q0 recovery

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Low-field Q0 recovery

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44%

36%

More helium processings

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Reproducible Q0 depression & recovery

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44%

36%20% 17%

39% 33%

Field emission at low E-field region

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During He processing

Summary

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Q0

Thermal gradient

Cooldown speed

Insensitive

Clear dependence

Ambient B-field

0.035 n /µΩ T

Backup slide: Magnetic field

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[Ref] P. Zhang et al., “The Impact of Defects on Q0 for HIE-ISOLDE High-Beta Quarter-Wave Resonators”, CERN-ACC-NOTE-2014-0034, 2014.