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A conduction-cooled SRF cavity: Apparatus and first results

2019 Cryogenic Engineering Conference, Hartford, Connecticut

• R. C. Dhuley1, M. I. Geelhoed1, Y. Zhao2, I. Terechkine1,
• M. Alvarez1, O. Prokofiev1, J. C. T. Thangaraj1

1Fermi National Accelerator Laboratory, Batavia, Illinois 2Euclid Techlabs LLC, Bolingbrook, Illinois

FERMILAB-SLIDES-19-044-AD-DI-LDRD-TD

This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics.

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7/18/2019 2

Take out liquid helium (and its complexities) Cool SRF cavities conductively with 4 K cryocoolers Goal: To demonstrate cryogen-free SRF cavity operation

Dhuley| Conduction cooled SRF cavity

Key thermal design criterion

• SRF cavities dissipate heat during operation (dynamic heat load)
• Cryocoolers have limited 4 K cooling capacity
• Need a high thermal conductance link to extract this dynamic load and

transport to the cryocooler.

Port He Fill Port Vacuum Insulation

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E-beam weld niobium rings around the equator to attach a thermal link

Cavity-cooler thermal link: Our design approach

Dhuley| Conduction cooled SRF cavity 2

1 2

diss s s

P R H ds =



Surface magnetic fields dissipate most heat near the equator

Hs

• O. Prokofiev
• DI

. ·

1n

. ~ '

~

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Cavity-cooler thermal link: Our design approach

Dhuley| Conduction cooled SRF cavity

Use high purity (5N) aluminum as the thermal link material

• R. C. Dhuley et al., Cryogenics 93, 86-93, 2018

Measure and design low thermal resistance pressed niobium-aluminum contacts Construct a thermal link for distributed cooling around the cavity equator

• R. C. Dhuley et al., IEEE TAS 29(5), 0500205, 2019

Henter block

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5

Conduction cooled cavity test setup

7/18/2019 Dhuley| Conduction cooled SRF cavity

SRF cavity

• Cooled by cryocooler stage-2
• Elliptical single cell, 650 MHz
• Niobium or Nb3Sn coated

Cryocooler

• Cryomech PT420

(2 W @ 4.2 K with 55 W @ 45 K)

Cavity and shield supports

• Ti64 rods

MLI wrapped thermal shield

• Cooled by cryocooler stage-1
• Copper 101 top plate
• Aluminum 1100 shell

Vacuum vessel

• SS304
• 5 feet tall
• M. Alvarez

Magnetic shield

• MuMetal
• Room temperature
• <10 mG total field at the

cavity location

• I. Terechkine

6

Conduction cooled cavity test setup

7/18/2019 Dhuley| Conduction cooled SRF cavity

RF driver with feedback for PLL

• Y. Zhao

FNAL

F

Control computer PXle Crate 10MHz Ref NIPXIND NI PXleFPGA

OCXO PLL Freq. Source

r--,

I RFL RF Monitor

r: - - "'"" - - J

14

_________________

FWD

__

R _ F_ Mon _ itor

.,

'---'

Detector Detector

• - --

1 /0demod

r---

Cavity Field monitor I

• - - .,

a

,------

630MHz Carrier

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Conduction cooled cavity test setup

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Cool down characteristics

Dhuley| Conduction cooled SRF cavity

The cryostat cooled to its base temperature within 24 hours

• Cryocooler stage I < 30 K, thermal shield top plate ≈ 32 K
• Cryocooler stage II ≈ 2.95 K
• Cavity cell ≈ 5 - 5.8 K (measured at multiple locations)

Thermal shield 4 K stage

T A possible reason for the significant cryocooler-cavity ΔT

• The estimated heat leak to cryocooler

4 K stage is ≈ 450 mW, mostly coming via the RF cables

• This heat flows through the cavity body

(4 mm thick niobium), then through the thermal link, and into the cryocooler

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7/18/2019 9 Dhuley| Conduction cooled SRF cavity

First results: Accelerating gradient > 1.5 MV/m

• First measurements used a single cell, 650 MHz, niobium cavity
• Cryocooler had available ~1.55 W @ 4.2 K after accounting for the static leaks

No load on cryocooler stage 1 55 W on cryocooler stage 1 Uncertainty = ±10% Limit of RF power supply

4.E+8

Cl

~"

.8 3.E+8

Q

~

• ~
• ~ ::s

cr-

0 2.E+8

• ~

~

u

1.E+8 0.0

·-

• ~- E>-c
• ~e

0.5 1.0 1.5 2.0 Accelerating gradient, Eacc [MV

Im]

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Fermilab

7/18/2019 10 Dhuley| Conduction cooled SRF cavity

(a measure of heat dissipation in the cavity)

≈5.7 K

Tcryocooler (measured)

≈5.1 K

Tcavity,RF = f(Q0)

Projections for a Nb3Sn coated cavity with the existing link Need to know the cavity RF surface and cryocooler temperatures

• Tcavity,RF is estimated from Q0, Tcryocooler is measured

6.0

r---,

\

~EJ-fr

• ·--•-
• - -

■-

~5.0

.,,,

..... ........

.,,

3.0

....

0.0

0.5 1.0

1.5

2.0

Accelerating gradient, Eacc [MV

Im]

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Fermilab

7/18/2019 11 Dhuley| Conduction cooled SRF cavity

Projections for a Nb3Sn coated cavity with the existing link

Surface resistance in Nb3Sn [nΩ] Eacc [MV/m] with the existing conduction-cooling link 20 (residual = 10) 11.5 60 (residual = 50) 6.5 110 (residual = 100) 5.0

Assume no changes to the link

• Tcavity,RF = 5.7 K
• Tcryocooler = 5.1 K

RBCS(T) RBCS(T) RBCS(T) + 10 nΩ RBCS(T) + 50 nΩ RBCS(T) + 100 nΩ Niobium Nb3Sn

RBCS(T) computed using https://www.classe.cornell.edu/~liepe/webpage/researchsrimp.html

1 ( )

acc BCS residual

E R T R  +

Nb3Sn << Niobium (see plot) Nb3Sn has demonstrated as low as 10 nΩ

Projected Eacc for Nb3Sn with different residuals

] 1000

..........

u

g

• 100

·

• f/l

.....

10

u

✓

c.s

3

r/J.

/

1 5.3

I I I I ·······························1 ··········

• ----- - -----r ---
• ------_. __ _

5.5 1

I I

5.7 Temperature [K] 5.9

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7/18/2019 12 Dhuley| Conduction cooled SRF cavity

Summary and outlook

First ever demonstration of accelerating gradients on a cryogen-free, cryocooler conduction-cooled SRF cavity

• Niobium cavity produced >1.5 MV/m with a 2 W @ 4.2 K

cryocooler

• There is considerable scope for improving the thermal

management in our setup

• Ongoing: mitigation of static heat leak
• An Nb3Sn coated cavity is projected to yield >10 MV/m

accelerating gradients on our existing setup

• Tests are planned for the near future
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Fermilab

7/18/2019 13 Dhuley| Conduction cooled SRF cavity

This presentation has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics.

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Fermilab

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Thank you.

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