1. Villigen, Dec 2nd 2010, RFTech 2nd Workshop
Unknown (?) disease of a rebuncher cavity
Marco Di Giacomo

2. Contents Workshop vs conference talks
Unknown (?) disease of a rebuncher cavity
Workshop vs conference talks
Better opportunity to discuss real life problems
Cavity description
Problem description
Discussion

3. ACKNOWLEDGEMENTS J.F. Leyge: rebuncher team leader
Unknown (?) disease of a rebuncher cavity
J.F. Leyge: rebuncher team leader
M. Michel : mechanical and technical design
P. Toussaint : manufacture
Ph. Robillard: vacuum system
J.C. Deroy : Labview command and control
Friendly consultants:
M. Malabaila, M. Vretenar, R. Bourquin, Y. Cuvet (CERN),
W. Barth, H. Vormann, T. Dettinger (GSI),
A. Schempp (IAP Frankfurt)

4. Cavity description SP2 MEBT Line Parameters Maximum beam voltage
Maximum beam voltage
R1 : 120 kV in CW for normal operation 190 kV pulsed for bunch length measurement
R2 : 60 kV ( 1/6)
R3 : 120 kV (1/6)
Minimum beam voltage
All cavities: 20 kV (for proton beam))
Vacuum pressure 1·10-7mbar
Alignment precision : <±0.5 mm on x and y

5. Cavity description
The cavity
First Cu plated cavity at Ganil

6. Cavity description Technology Technology: Cu plated tank,
Solid Cu for all other parts
Water cooled stems and tank
Conduction cooled rings and trimmers
Cu plait contacts for RF and thermal conduction
Helicoflex joints for RF and vacuum
Inductive power couplers and probes

7. Problem description First assembling
Beam axis pressure: 8.1x10-8 mb
Pump flange pressure : 5.1x10-8 mb
Calculated Désorption : 2.8x10-6 Pa.m3.s-1
First RF test: different power levels tested for tens of minutes
Cavity well matched up to few Watt
Strong mismatch between 10 and 800W (cavity voltage lower but still there)
Cavity well matched between 800 W and 5 kW
but vacuum rapidly increasing.
190 kV beam = 80 kV =
160 kV inter-electrode

8. Problem description What to do? QUESTIONS
Why such an extended forbidden area?
Why does the pressure raise so much?
Multipactor or something else?
Where?
Why?
POSSIBLE ANSWERS from:
Residual gaz spectra and
Cavity inspection

9. Exemple of clean chamber
Problem description
Residual gas spectra
Hydrogen,
18 H20
28 N and C02
32 Oxygen (leaks)
44 C02
higher mass due to hydro-carburs
Log scale to see pollution otherwise not visible
Cavity probably polluted
Rebuncher Cavity
Channeltron
Vendredi 23 juillet 2010
72 hours
P = 1.75x10-8 mbar
Spectre LBE1 LPSC Channeltron
Exemple of clean chamber
Pression = 1.0x10-8 mbar
E-9
E-10
E-11
E-9
E-10
E-11

10. Problem description Inspection Traces of ……… ?
Only the upper side of the tank is undomaged,
The pump is on the bottom side
Traces of ……… ?

11. Problem description What to do?
New partial assembly: stem and tank only, f=95 MHz
Tank and stems cleaned with alcohol, spots tend to disappear, but some are small craters.
Glass windows: on the beam axis and fro the top
Before cleaining
after cleaning

12. Prototype Groupeur LME
Problem description
95 MHz (<150W)
First test
First assembling
Beam axis pressure: 8.1x10-8 mb
Pump flange pressure : 5.1x10-8 mb
Calculated Désorption : 2.8x10-6 Pa.m3.s-1
Second test :
Better Cleaned parts
Tank and stems only (95 MHz)
(no ports, trimmers, plaits)
But limited to 150 W
Beam axis pressure : 1.1x10-8 mb
Pump flange pressure : 9.9x10-9 mb
Calculated Désorption : 4.6x10-7 Pa.m3.s-1
Factor 5 smaller desorption
Still a factor 7 higher than our Cu reference
CERN experience: with baking 200°C,2h
2,66x10-9 Pa.m3s-1m-2 « bains acide de cuivre sulfate sans brillanteur »
Prototype Groupeur LME
Channeltron
Mercredi 06 Octobre 2010
Sous vide depuis 30 jours
Paxe = mbar
Pturbo = mbar
E-7
E-8
E-9

13. Problem description 95 MHz (<150W)
Tests limited to 150 W, (Xrays <30keV)
Disease still present but seems to condition with time
Reconditioning required after a long rest
Nothing visible through the glass windows

14. Problem description
95 MHz (<150 W)
Disease processing

15. Problem description 88 MHz (<150 W)
The disease seems to be there but processable.
New inspection, no new visible surface spot areas observed
Cleaning of other parts: beam ports, Cu plait and trimmers,
Second assembly of the complete cavity: 88 MHz
Low power tests as follows
High power tests in December

16. Problem description 88 MHz (<150 W) 88 vs 95 MHz configurations
No visible changes with RF: disease present but processed
No visible changes in vacuum
Nothing visible through the glass windows

17. Cu plating defects Problem description But:
Were the spots already there, just less visible?

18. Conclusion Discussion Some questions to initiate the debate
Multipactor (mp) or something else?
We have never observed mp loading a cavity in this way: usually no voltage when you are in the barrier. Continuous sparks, plasma?
Why such an extended forbidden area? (frow few W to almost 1 kW) ?
Where?
Nothing visible (<150W) unless strange spots on the Cu plated surface. But they were probably already there. Only one half of the cavity is concerned. Cu Plating problems?
Why?
Cavity polluted? Cleaning seems to help: the disease is still there but can be processed. What kind of pollutant?
Why is the vacuum raising so much, so quickly. Have you any information on how the desorption changes with RF in your cavities?
Other suggestions?

19. Conclusion
Thank you for your help