Experimental Results on Thermal Boundary Resistance for Nb and

Nb/CuEnzo Palmieri1,2, A.A. Rossi1, R. Vaglio3

1 Legnaro National Laboratories of the INFN2 Università degli Studi di Padova3 Università degli Studi di Napoli

0 2 4 6 8 10 12 14

1E7

1E8

1E9

1st RF Test:

@4.2K

@1.8K

P=200mW

2nd RF Test:

@4.2K

@1.8K

P=200mW

Q

Eacc

[MV/m]

quench

Measured twice

0 2 4 6 8 10 12 14 16 1810

9

1.5x109

2x109

2.5x109

3x109

3.5x109

Q-switch

Q-slope

quench

T=1.8K

Before Anodization

After Anodization

After De-Anodization

Q0

Eacc

[MV/m]

quench

Q-switch

Q-slope

field emission

If we mirror finish the cavity

external surface, ….

this will behave as a Mirror for

thermal phonons!

A mirror-like external surface will

also decrease the nucleation sites

for Helium boiling nucleation,

promoting then

the Liquid He Super-heating

If liquid He Super-heating is

detrimental for Q(Eacc

),

Should we worry more about that

type of superheating rather than to

the Nb HSh

?

Since the qD

of the Cu is

higher than the one of Nb

and in Kapitza it plays as𝑇

𝜃𝐷

3,

does this contribute to the fact

that, at 1.8K, sputtered Nb

showsn lower performances?

Outer Lead coating of a sputtered Nb/Cu

BEFORE AFTER

0 2 4 6

106

107

108

Cu1_Comparison before&after Pb

1.8K_AfterPb

4.2K_AfterPb

4.2K_BeforePb

1.8K_BeforePb

Q

Eacc

[MV/m]

Lead however did not remain attached to CU

Can water micro-cristallites

on the external surface of Nb

promote film boiling and then

positively affect cavity

performances?

0 2 4 6 8 10 12 1410

7

108

109

Nb 127 with external EP

@ 4.2K

@ 1.8K

@ 4.2K After Grinding

@ 1.8K After Grinding

@ 4.2K After Anodization (Yellow)

@ 1.8K After Anodization (Yellow)

@ 4.2K After Ice Film

@ 1.8K After Ice Film

Q

Eacc (V/m)

For years we have considered a cavity

as an adiabatic system made by the

RF fields + Nb, because the He bath

has been considered as a stable and

infinite reservoir at fixed temperature.

Is it not the time now to consider

instead the adiabatic system

composed by RF fields + Nb + Liquid

Helium ?

Nb/Cu 6 GHz

cavities

The Cathodic Arc coated cavity deposited by Soltan Institute

and INFN- Roma2 was never measured ….

do you know why?

Bad adherence between Cu and Nb

is a common problem!

Tsurface

CuNbVacuum

ΔT1

Helium

sNb sCu

T4

T3

T2

T1

T0

ΔT2

ΔT3

ΔT4

Nb Cuthermal d d

TOT Nb CuNb Cu Cu He

R P P s sT

K h K h

1 1

If the adhesion of

Niobium to Copper is not

good, the cavity will go in

thermal runaway!!!!

What has high solubility

both in Niobium and in

Copper?

• Palladium

• Silver

• Tin

• Alluminum

“Silver Cathode”

0 1 2 3 4 5 6 7 8

106

107

108

109

Eacc

[MV/m]

Comparison between 1st, 2nd and 3rd Nb/Cu Sputtering

1st Sputtering:

@ 4.2K

@ 1.8K

2nd Sputtering:

after HPR @ 4.2K

after HPR @ 1.8K

3rd Sputtering:

@ 4.2K

@ 1.8K

Q

0 1 2 3 4 5 6 7 8

106

107

108

109

Eacc

[MV/m]

Comparison between 1st, 2nd and 3rd Nb/Cu Sputtering

1st Sputtering:

@ 4.2K

@ 1.8K

2nd Sputtering:

after HPR @ 4.2K

after HPR @ 1.8K

3rd Sputtering:

@ 4.2K

@ 1.8K

Q

If we want to improve SRF

performances

we must study more deeply

Cryogenics

and precisely Heat Transfer

mechanism from a Surface to Liquid

Helium

If the adhesion of

Niobium to Copper is not

good, the cavity will go in

thermal runaway!!!!

Acknowledgements

• We thank F. Stivanello and V. Pastushenko for the Chemical and electrochemical polishing of the cavities,

• Serguey Stark for the huge work on RF,

• M. Martinello and M. Checchin for the sputtering and rf test