1. The broken HOM Couplers on 3.9 GHz Cavity #2
Welding
Cavity processing
RF measurement
Findings / optical inspection
tests, bending / welding
Investigations on hardness, grain size
Simulations
W.-D. Moeller
TTC Meeting at KEK, Sept

2. TTC Meeting at KEK, Sept. 25-28 2006
Assembling procedure
Welding tool
Welding HOM can to beam pipe
Assemble antenna on welding tool
Tag weld both antenna legs
Weld first leg (2)
Weld leg with x-weld (3)
Leave under vacuum 15 min
Purge N2 at 25mbar, 15 min
Open to atmosphere 1 2 3
Antenna welding
W.-D. Moeller
TTC Meeting at KEK, Sept

3. TTC Meeting at KEK, Sept. 25-28 2006
Cavity processing
Cavity and the HOM coupler underwent all the normal processing that would be expected:
several times US degrease & BCP etching steps
800 C heat treatment, 2 h
BCP etching & US degrease
final BCP
high pressure rinse
cool down to 2 K
W.-D. Moeller
TTC Meeting at KEK, Sept

4. 1st & 2nd cavity measurement at 2 K
Q/T only low power test, low Q → new BCP & HPR
Q/T, Q/E up to 2.2 MV/m
no limitation
multipacting was observed
mode measurement show higher losses in end cells
lower residual resistance after mode measurement
measurement stopped because of the low Q
→ new BCP & HPR
2nd
W.-D. Moeller
TTC Meeting at KEK, Sept
Timergali Khabiboulline

5. 3rd cavity measurement at 2 K
Q/T, Rs improved after first Q/E in Pi and 0 mode
Q/E, Q improvement with high power ( processing with pulses)
multipacting observed
8-11 MV/m Eacc reached in pulsed regime. No quench, power limitation due to low Q.
3rd
W.-D. Moeller
TTC Meeting at KEK, Sept
Timergali Khabiboulline

6. 4th Cavity measurement at 2 K
temp sensors attached
one HOM coupler detuned (bottom) →no heating
pulse up to 14 MV/m possible, higher than 3rd test
temp rise on membrane on top HOM ,7 ,6 ,5
Timergali Khabiboulline
W.-D. Moeller
TTC Meeting at KEK, Sept

7. Optical inspection after warm up
After RF the test, the leg on the x-weld is broken on both HOM antennas
Parts are not investigated between welding the parts and after RF test
after welding (different part shown)
after cold RF test
W.-D. Moeller
TTC Meeting at KEK, Sept

8. TTC Meeting at KEK, Sept. 25-28 2006
Tests on samples
Bending tests at room temperature:
HOM coupler spare part:
Antenna welded (same welding procedure than the original) to a can was bended and hit with a hammer: → ductile
Test weld:
‘dirty’ (off the shelf) Nb sheets, chamber vented to air right after the weld, then bended at 90°:
→ ductile
It is hard to imagine that the antenna can break at RT when the material is ductile.
But, small size gaps could open.
W.-D. Moeller
TTC Meeting at KEK, Sept

9. Investigations on the broken parts SEM (1)
fractured antenna leg on the can
looks like brittle fracture
also cracks in the weld area
wire cut of NOT fractured antenna leg on the can
cracks in the weld area all around the weld, they look brittle
before or after etching?
W.-D. Moeller
TTC Meeting at KEK, Sept

10. Investigations on the broken parts SEM (2)
fractured antenna leg
scrape mark (maybe added late)
small pits, 40microns
W.-D. Moeller
TTC Meeting at KEK, Sept

11. Investigations on the broken parts cross section
cross section through a fractured antenna leg on the can
cracks also in the welding area
large grains up to 2-3 mm
etching marks ????
W.-D. Moeller
TTC Meeting at KEK, Sept

12. TTC Meeting at KEK, Sept. 25-28 2006
Investigations on the broken parts grinded cuts with hardness measurements
W.-D. Moeller
TTC Meeting at KEK, Sept

13. Simulations on multipacting
Nikolay:
W.-D. Moeller
TTC Meeting at KEK, Sept

14. Simulations on thermal heating
Case 4:
Top tube heat flux
7K can bc
Case 3:
Top tube heat flux
Case 1:
End gap
heat flux
FEA Summary:
• transient analysis, heat flux
applied over 0.05 sec
• 4 cases
20W, 50W, & 100W
• thermal stresses
calculated for each case
Case 2:
Dog leg
heat flux
can was kept at 1.8 K
Salman Tariq, Rob Polera Accelerator Division/MSD /29/2006
W.-D. Moeller
TTC Meeting at KEK, Sept

15. Simulations on thermal heating, cont’d
heat flux over the whole length
case 3 with heat applied
heat flux of 50 W
critical spot was same for all simulated cases = pretty much ‘exact’ location where failures occurred!!
max stress at critical spot: 112 MPa (100 W→300 MPa)
cycling at brittle temp.’s could cause failure at much lower stresses
W.-D. Moeller
TTC Meeting at KEK, Sept

16. TTC Meeting at KEK, Sept. 25-28 2006
Heat flux to the Helium
Can we keep the surface at 2 K?
what is the heat flux?
40 W/cm2 far above the film boiling
for higher can surface temperature it is worth
2 K boundary conditions
Heat input
peak temperature [K]
outside surface flux [W/cm2]
20 W
133 40
50 W
507 99
W.-D. Moeller
TTC Meeting at KEK, Sept

17. Stress simulation during and after welding
After welding the temperature is measured between the posts
The ANSYS model has a heat sink at the beam pipe end. The resistance of the sink is arranged, so that the measured temperature is realized in model
picture shows the temperature after last weld
heat sink
W.-D. Moeller
TTC Meeting at KEK, Sept

18. Stress simulation during and after welding, cont’d
At elastic deformation only, the stress around the weld is very big.
Plastic case will be implemented soon
W.-D. Moeller
TTC Meeting at KEK, Sept

19. Conclusion (or open questions)
All cracks look brittle →this is only at cryogenic temperature possible
The only force at cryo temp could come from thermal expansion →can the broken area stay cold & brittle? (He-boiling)
Is multipacting heating the inner conductor?
Cracks in the weld area around the leg →welding procedure ok?
W.-D. Moeller
TTC Meeting at KEK, Sept

20. This is the work of many people:
C. Antoine, C. Cooper, E. Harms, A. Rowe, T. Arkan N.Dhanaraj, T. Khabiboulline, N. Solyak, L. Bellantoni, H. Edwards, D. Mitchell, C. Boffo, M. Foley, D. Olis, G. Wu, H. Carter, C. Ginsburg, P. Pfund, Sergio, I. Gonin, W. Singer
Thank you
W.-D. Moeller
TTC Meeting at KEK, Sept

21. Hardness of Niobium at low temperatures
W.-D. Moeller
TTC Meeting at KEK, Sept

22. TTC Meeting at KEK, Sept. 25-28 2006
Stress Simulation Results Table (3.9GHz Formteil)
100K Nb (heat treated) physical properties
Yield strength: 400 MPa (58 ksi)
Ultimate strength: 470 MPa (68.2 ksi)
300K Nb (heat treated) physical properties
Yield strength: 60 MPa (8.7 ksi)
Ultimate strength: 125 MPa (18 ksi)
Ref:
Mech Prop. of High RRR Nb
at Cryogenic Temp.’s
M.G. Rao & P. Kneisel, JLab
Main conclusions to be drawn:
□ Critical spot was same for all cases = pretty much ‘exact’ location where failures occurred!!
□ Fatigue at brittle temp.’s could cause failure at much lower stresses (from thermal cycling)
□ Necking at the weld will further weaken the Formteil (not included in the FEA model)
□ Radiation was found to have negligible effects on these results (small radiative areas)
□ Refining the mesh at the critical location yields 15-20% higher stresses
Salman Tariq, Rob Polera Accelerator Division/MSD /29/2006
W.-D. Moeller
TTC Meeting at KEK, Sept