Unless of a few eclectic scientists that have studied the problem of Kapitza resistance, a superconducting cavity is almost always seen as an adiabatic system of the electromagnetic fields interacting with a Niobium sheet cooled at 4,2K or at temperatures between 1,8K and 2 K. In this talk it will be shown that the real system to consider is that of Electromagnetic fields, Niobium and liquid Helium. And liquid Helium is a component much more complex than an infinite heat exchanger at fixed temperature. Even at superfluid temperatures indeed, at the interface between Helium and the superfluid, it will be always a nanometric thick layer of normal helium and the thickness of this layer depends on the RF power. Niobium sputtered cavities will be also examined under the point of view of thermal boundary Resistance, arriving to the conclusion that more advanced comprehension of SRF cannot prescind from a deep understanding of Cryogenics

1. Experimental Results on Thermal
Boundary Resistance for Nb and
Nb/Cu
Enzo Palmieri1,2, A.A. Rossi1, R. Vaglio3
1 Legnaro National Laboratories of the INFN
2 Università degli Studi di Padova
3 Università degli Studi di Napoli

3. 0 2 4 6 8 10 12 14
1E9
1E8
1E7
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

7. quench
0 2 4 6 8 10 12 14 16 18
3.5x109
3x109
2.5x109
2x109
1.5x109
109
Q-switch
Q-slope
quench
T=1.8K
Before Anodization
After Anodization
After De-Anodization
Q0
Eacc [MV/m]
Q-switch
Q-slope
field emission

9. If we mirror finish the cavity
external surface, ….
this will behave as a Mirror for
thermal phonons!

11. A mirror-like external surface will
also decrease the nucleation sites
for Helium boiling nucleation,
promoting then
the Liquid He Super-heating

12. If liquid He Super-heating is
detrimental for Q(E
),
acc
Should we worry more about that
type of superheating rather than to
the Nb H
?
Sh

13. Since the qD
of the Cu is
higher than the one of Nb
and in Kapitza it plays as 푇
휃퐷
3
,
does this contribute to the fact
that, at 1.8K, sputtered Nb
showsn lower performances?

14. Outer Lead coating of a sputtered Nb/Cu
BEFORE AFTER

15. Lead however did not remain attached to CU
0 2 4 6
108
107
106
Cu1_Comparison before&after Pb
1.8K_AfterPb
4.2K_AfterPb
4.2K_BeforePb
1.8K_BeforePb
Q
E
acc
[MV/m]

16. Can water micro-cristallites
on the external surface of Nb
promote film boiling and then
positively affect cavity
performances?

17. 0 2 4 6 8 10 12 14
109
108
107
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)

18. 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 ?

20. Nb/Cu 6 GHz
cavities

24. 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!

25. Tsurface
Vacuum Nb Cu
ΔT1
Helium
sNb sCu
T4
T3
T2
T1
T0
ΔT2
ΔT3
ΔT4
 Nb Cu

R P P s s
thermal d d
1 1
       
TOT Nb Cu
     Nb Cu Cu He

T
K h K h

26. If the adhesion of
Niobium to Copper is not
good, the cavity will go in
thermal runaway!!!!

31. What has high solubility
both in Niobium and in
Copper?
• Palladium
• Silver
• Tin
• Alluminum

32. “Silver Cathode”

33. Comparison between 1st, 2nd and 3rd Nb/Cu Sputtering
0 1 2 3 4 5 6 7 8
109
108
107
106
E
acc
[MV/m]
1st Sputtering:
@ 4.2K
@ 1.8K
2nd Sputtering:
after HPR @ 4.2K
after HPR @ 1.8K
3rd Sputtering:
@ 4.2K
@ 1.8K
Q

34. Comparison between 1st, 2nd and 3rd Nb/Cu Sputtering
0 1 2 3 4 5 6 7 8
109
108
107
106
E
acc
[MV/m]
1st Sputtering:
@ 4.2K
@ 1.8K
2nd Sputtering:
after HPR @ 4.2K
after HPR @ 1.8K
3rd Sputtering:
@ 4.2K
@ 1.8K
Q

35. If we want to improve SRF
performances
we must study more deeply
Cryogenics
and precisely Heat Transfer
mechanism from a Surface to Liquid
Helium

36. If the adhesion of
Niobium to Copper is not
good, the cavity will go in
thermal runaway!!!!

37. 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