1. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.20051 RF Fingers for Secondary Collimators What is that? Constraints for design Choice of materials: –Contact resistance –Mechanical load –Thermal load Experimental results on contact resistance and wear robustness Conclusion

2. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.20052 Collimator tank and RF contacts (SPS prototype)

3. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.20053 Proposed layout

4. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.20054 Boundary conditions (nominal beam, normal run) Power from particle bombardment on the RF contacts: –Peak power deposition: 1.1 W/cm 3 –Average power deposition: 0.6 W/cm 3 Power deposition from RF trapped modes: –2.4 W per each set of 15 fingers assuming they are made of Cu –Same power for CuBe fingers plated with at least 6 µm of Ag (  3  ) Contact resistance for each side of a collimator jaw: –< 1 mOhm Vacuum bakeout at 250 ºC

5. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.20055 Choice of materials: start from contact resistance Based on the experience of the cold interconnects and LEP –For the best contact resistance a pair of noble metals (soft-soft or hard-soft) has to be used. No oxide that reduces contact resistance. RF fingers of CuBe, Ag plated 25 µm –Au cannot be used because of bakeout at 250 ºC: risk of diffusion –Thick plating withstands wear (verified experimentally) Centring rings of s.steel Rh plated 2 µm –Round contact corner, electropolished –Rh is very hard –No interdiffusion Rh-Ag up to 600 ºC Plating of CuBe finger Plating of base material (inox) Resistance [m  ] 1AgNone500 2AgAu3 3AgRh10 4AgRh (on copper base plate)2 5AuRh10 6AuRh (on copper base plate)3 Contact resistance

6. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.20056 Choice of materials: which CuBe alloy? General properties C17200 - Standard alloy (CERN stores), low conductivity, high yield strength (~“elastic limit”) C17410 - High conductivity alloy (ex. “cold interconnects”), lower yield strength All have E=130÷140 GPa independent of composition or of thermal treatment (“age hardening”)

7. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.20057 Calculation of bending stress CuBe thickness 0.5 mm Force = 13 grams on centre finger Max stress  max = 130 MPa CuBe thickness 0.3 mm Force = 2.9 grams on centre finger Max stress  max = 80 MPa Beam axis Jaw ~ 20 mm CuBe standard C17200 ½ HT   0.2 ≈ 1200 MPa CuBe high conductivity C17410 ½ HT   0.2 ≈ 600 MPa Even with the high-conductivity alloy the maximum stress is ~ 15 % of the yield strength

8. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.20058 Heating effect including particle bombardment and RF Beam axis The Wiedemann-Franz law relates the bulk thermal conductivity to the electrical conductivity for metals. K thermal /  electrical T = 2.45x10 -8 W  / K 2 (Lorenz number) This law can be extrapolated to estimate the contact thermal conductance from the measured contact electrical resistance (extrapolation at 300 K) R contact  10 m   C contact  8x10 -4 W/K Jaw T=T 0 T=T m

9. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.20059 Heating effect including particle bombardment and RF Assuming power density by particle bombardment in Ag  Cu Both the thermal conductivity and contact thermal conductance would increase with temperature  safety margin Values in parenthesis: when contact at the flange side is lost (accident)  T max for 0.3 mm thickness of the fingers  T max for 0.5 mm thickness of the fingers CuBe C17200 ½ HT (standard alloy) + Ag plating 2*25 µm  T max = 125 ºC (  T max = 200 ºC)  T max = 125 ºC (  T max = 200 ºC) CuBe C17410 ½ HT (high-conductivity alloy) + Ag plating 2*25 µm  T max = 100 ºC (  T max = 150 ºC)  T max = 100 ºC (  T max = 150 ºC) Power deposition:- From particles 0.23 W / finger at 0.55 mm thickness - From trapped modes 0.16 W / finger if Ag plated - Blackbody heat radiated: -0.02 W per finger at 100 ºC Includes contact resistance of 10 mOhm per finger at flange side and 1.5 mOhm per finger at C-C side

10. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.200510 Stress relaxation resistance for CuBe (applied stress = 75% of yield strength) 10 5 h – LHC life Bakeout time in LHC life ? C17200 10 5 h – LHC life Bakeout time in LHC life C17410 Standard alloyHigh-conductivity alloy

11. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.200511 Experimental verification: open-close cycles in vacuum

12. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.200512 Contact resistance and wear resistance for 15000 open-close cycles (2x LHC life) CuBe C17200 ½ HT, 0.5 mm thick, 8 µm Ag plating R contact < 0.5 mOhm for the entire set of fingers (value limited by the bulk resistance of the fingers)

13. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.200513 Wear after 1500 cycles (4 years LHC life) CuBe 0.5 mm thick Ag thickness in µm on

14. Sergio Calatroni TS/MMETS Workshop, Archamps 25.5.200514 Conclusion & future This is a small item of LHC hardware. Nevertheless, an extensive study was needed in order to be able to satisfy specifications and constraints All requirements are satisfied and the long term reliability guaranteed with the chosen materials However, a reduction of thickness will be tested in order to reduce the force on the motors: –C17410 alloy 0.4 mm thickness, because of material availability. –No problems are anticipated for thickness reduction, but mechanical behaviour has to be checked The help of many people from TS/MME, the Collimators Working Group and the Collimators Design Group is gratefully acknowledged