SPL power coupler

1. SPL power coupler Possible proposed designs Part I : general considerations Eric Montesinos / CERN-BE-RF-SR

2. Summary • Preliminary remarks • Set of questions and preliminary answers: • A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? • Conclusion Eric Montesinos / CERN-BE-RF-SR

3. Preliminary remarks: Tight planning Coupler design review March 2010 • We wanted eight couplers fully operational, ready to be mounted on the cryomodule by end 2011 ! → very tight planning • Crucial component of a power coupler is the main ceramic window • Due to that very tight planning, we do NOT have time available for a new ceramic window design, so we must use : • An existing design • A new design using a very well known brazing process Interfaces definition for end 2009 8 couplers fully RF conditioned for beginning 2012 Eric Montesinos / CERN-BE-RF-SR

4. Preliminary remarks: Chamonix • Chamonix workshop: New R&D program • LP-SPL parameters not required anymore • From the coupler point of view we would prefer to stick to the initial planning (which is optimistic) • Still have NO time for a new ceramic window design Eric Montesinos / CERN-BE-RF-SR

5. Preliminary remarks:Proposed designs November last year (2009) • This R&D program could (will?) be followed with a whole project requiring more than 250 couplers • So, with the design of these SPL couplers, we would like to always keep in mind some key parameters: • As reliable as possible • Easy for operation and maintenance • Avoid mass production difficulties • As simple as possible • Reducing the costs • This raises several questions… CEA Saclay HIPPI Coaxial disk 820 kCHF LHC cylindrical 560 kCHF SPS coaxial disk 640 kCHF SPS waveguide disk 560 kCHF Eric Montesinos / CERN-BE-RF-SR

6. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Double window couplers • Double window coupler is for: • Clean beam vacuum • Safe operation • Typical example of double window couplers is the TTF – XFEL family couplers (1.3GHz, 1MW pulsed peak , 5kW average) • “Cold Window” for: • Beam vacuum / line vacuum insulation • Closing the beam vacuum • No coolant (water, gas) in contact with the “cold window” • “Warm Window” for: • line vacuum / air side • Closing the line vacuum Eric Montesinos / CERN-BE-RF-SR

7. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Double window couplers XFEL coupler • Unfortunately, with more average power it is difficult to use such a design • Maximum power reached with the Cornell ERL power coupler was 61kW cw (at 1.3GHz) (2007: HIGH POWER TESTS OF INPUT COUPLERS FOR CORNELL ERL INJECTOR*, V. Veshcherevich†, S. Belomestnykh, P. Quigley, J. Reilly, and J. Sears (Cornell University, Ithaca, NY, USA)) • Ceramics have losses • The cold window is difficult to cool down because fully vacuum insulated (both sides) • More thermal intercept points • Increase the sizes • Use a coolant but this increases the line vacuum leak risk Cold window: difficult to cool down Cornell ERL coupler with bigger cold ceramic and air cooling of a radiator for the cold ceramic Eric Montesinos / CERN-BE-RF-SR

8. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Double window couplers • Another possibility is a double window coupler with a coolant in contact with the two ceramics to allow high average power • The APT coupler is a possible example (APT: Accelerator Production of Tritium, Los Alamos National Laboratory) • Very interesting possibility, but requires a full development, → impossible to be ready with eight couplers conditioned beginning 2012 APT coupler: two disk ceramic windows coupler Eric Montesinos / CERN-BE-RF-SR

9. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Single window couplers • We have at CERN a long experience with single window couplers • SPS, 16 couplers in operation: • From 1975 to 2000 (~25 years) with a few couplers broken, mainly during tests procedures, when they were not properly conditioned • Since 2000, new coupler design (for more power, 550 kW cw) without any problem (already 10 years) • LEP I couplers at ESRF: • 20 years of operation with NO fault • Still in operation even if some couplers have been faulty after 20 years with small pinehole in the equator of the ceramic • We are building a new coupler based on LHC ceramic (collaboration with ESRF and SOLEIL) SPS single cylindrical window coupler 375 kW cw in operation from 1975 to 2000 New SPS single coaxial window coupler 550 kW cw in operation since 2000 (10 years) Eric Montesinos / CERN-BE-RF-SR

10. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Single window couplers • More than 250 couplers in operation with LEP II, even if we had only five years of operation: No vacuum leaks • 16 couplers in operation with LHC, operation just started, but all couplers were already built since 2005, and tested up to 550 kW cw full reflection: No vacuum leaks • Oak Ridge SNS linac also has 81 single window couplers in operation since 2005, no major problem reported • We believe that single window couplers are increasingly becoming more robust and reliable LEP II single cylindrical window coupler in operation from 1995 to 2000 New LHC single cylindrical window coupler Eric Montesinos / CERN-BE-RF-SR

11. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Single window couplers • Nevertheless, in situ repair will be absolutely NOT an option: • The LHC test cavity presented a very small leak (2 x 10-9 mbar l/s) at a coupler second ceramic • We tried a vacuum liquid sealant repair (as often successfully done with SPS warm cavities) • The leak was repaired • It was absolutely catastrophic: • We could not reach more than a few MV/m • We had to completely dismantle everything • All the components were polluted • It took a year to repair it • We re-learnt in-situ repair with high gradient cavities is forbidden • Must have spare modules for exchange Vacuum sealant Leaky biasing ceramic LHC test cavity #21 Eric Montesinos / CERN-BE-RF-SR

12. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Single window couplers: conclusion • Based on past experiences, we believe a single ceramic coupler is now reliable enough for ~ 20 years of operation • A double window coupler would be difficult to design for 100 kW average and would need to be carefully studied: • It is much more complex than a single window coupler • Unfortunately, we have NOT enough time available • This could be a very interesting parallel study program • But if we believe on a single window coupler, do we really need it ? • In case of breakage, we should be aware that: • NO in-situ repair is possible • We must be able to exchange a whole module • This is a huge job • This has to be taken into account for sectorization issues Eric Montesinos / CERN-BE-RF-SR

13. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Adjustable coupler or not ? • Advantages: • Better for Qext matching • Better for power distribution • Drawbacks: • More complex to design • Need a moving system not stressing the ceramic • Need bellows somewhere: • More EB welding • Increases the vacuum leak risk • Increases mutipacting difficulties • Need an alignment system to keep the antenna at the right position • Increases the number of mechanical operations • Subsequently increases the total price Disk window - fixed coupler Disk window - adjustable coupler Eric Montesinos / CERN-BE-RF-SR

14. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Adjustable coupler or not ? Ø 100 • Do we really need it ? • For the HP-SPL we will (probably) have one power source per cavity • The coupler will probably be machined accurately enough with an antenna position within +/- 0.3 mm, this should guarantee a Qext spread within 1.18 to 1.23 x 106 (~ +/- 2% Qext, hopefully acceptable) • The cavity will be previously measured, so we can choose which coupler to combine with which cavity to reduce the total error Ø 43.5 35 60 60 mm +/- 0.3 mm Qext = 1.18 to 1.23 x 106 55.5 mm +/- 0.3 mm Qext = 1.18 to 1.23 x 106 Juliette Plouin CEA-Saclay 25th February 2010 Eric Montesinos / CERN-BE-RF-SR

15. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Why a double walled tube ? • Due to the high average power, thermal screens are not enough to insulate the coupler from cold to warm • A double walled tube is needed • It has several features: • It is a part of the coupler, because it is the outer line of the antenna, i.e. outer part of a 50 Ω coaxial line • From an RF point a view, it is a simple outer conductor tube with a guaranteed minimum of 3 µm of copper (+ margin) • It is a cryogenic pipe which ensures the thermal interface between 2K to ambient air temperature δ : skin depth [m] f : frequency [Hz] μ0 : permeability of free space = 4π x 10-7 [H/m] µr : relative permeability of the conductor for copper = 1 ρ : resistivity of the conductor [Ω.m] for copper = 1.68×10-8 [Ω·m] For copper at 704.4 MHz : δ = 2.45 [µm] Outer gas above dew point ≈300K Inner gas ≈5K Helium vessel ≈2K Eric Montesinos / CERN-BE-RF-SR

16. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Why a double walled tube ? • All the important parameters will be taken into account in Ofelia’s, Vittorio’s and Sergio’s calculations: • Length • Input gas temperature • Gas flow • Stainless steel thickness • Copper plating issues and high pressure water tests • … • Its contraction must be perfectly pre-calculated, because this will give the coupling value (Qext) • It may be used as support for the cavity, independently of the chosen coupler designs • We will have on both sides the same “all in one RF and vacuum” seal as with LEP and LHC double walled tube Eric Montesinos / CERN-BE-RF-SR

17. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Specific Double walled tube “RF and vacuum” seal • Specific shape designed for RF perfect continuity and vacuum sealing • It has been designed for LEP couplers, then re-used with LHC couplers • For bake out compatibility: • MATERIAL : Cu OFS 0.1% Ag • SURFACE STATE : Rolled bright, free of scratches and burrs • STANDARD : ASTM F68-82 • ALLOY SYMBOL : Cu -OF + Ag ASTM C 10700 • STATE : HA (half-hard) • CHEMICAL COMPOSITION : Cu (+Ag) = 99.95% min. Ag = 0.1 + 0.02% • We have asked a specialized company (GarlockCefilac) to build a series for the prototypes Eric Montesinos / CERN-BE-RF-SR

18. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? What about cryomodule integration requirements ? • As already presented in November, the RF conditioning of the couplers will be done prior to joining to the cavity • A pair of couplers will be brought into the clean room with two cavities • The couplers will be mounted onto the cavity horizontally with its double walled tube (at the time as HOM, antenna, …) to not pollute the cavity: XFEL “cold window” process Non perturbed clean room laminar air flow Coupler Trolley Cavity Trolley Eric Montesinos / CERN-BE-RF-SR

19. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? What about cryomodule integration requirements ? • The eight cavities will be connected together • After that, the beam vacuum should remain closed until installation into the machine • Very important decision for the design of the coupler → imposes a short distance from the ceramic to the beam axis … Non perturbed clean room laminar air flow Eric Montesinos / CERN-BE-RF-SR

20. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? What about integration into the cryomodule requirements ? • … because this defines the croymodule internal size • As we would like to test several coupler designs, the “worst” sized one will define the size of the cryomodule • With the next Talk, Vittorio will present us all these issues Eric Montesinos / CERN-BE-RF-SR

21. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? What about integration into the cryomodule requirements ? • We tried to make the different coupler designs compatible: • Same double walled tube • Same “integrated” length • Only 61 mm difference • but different ways to connect the waveguides • This has naturally eliminated the waveguide window coupler solution: • Impossible to insert it inside the cryomodule if already mounted onto the cavity Eric Montesinos / CERN-BE-RF-SR

22. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Why Vertically above/below the cavity ? • Within the last SPL workshop, we received the following comments: • Particles will fall down forever from the coupler and will slowly pollute the cavity degrading its performances • Nevertheless, effect of pollution to the cavity is very difficult to predict • Before vertically, why not horizontally ? Because, an horizontal position will: • Not be better than below the cavity from the cavity pollution point of view • Not help for integration • Makes the coupler vulnerable to shocks (it would be in, or near the transport area, not enough space at the opposite wall side with the foreseen tunnel) • Give more stress to the coupler (effect of the weight of the “pretty long” antenna over 25 years ?, mechanical vibrations, …) • No clear advantages for an horizontal positioning of such a coupler Eric Montesinos / CERN-BE-RF-SR

23. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Why Vertically above/below the cavity ? LHC cryomodule with couplers above the cavities • Our experience with LEP and LHC couplers: • We always had couplers above the cavity • With various angle, 30°and 45° with LEP couplers, vertically with LHC couplers • We always had a “dust bin” in front of the couplers, at the bottom side of the cavity • While repairing a LEP cavity which had been in operation we were very surprised when finding “big” dust particles in the “dust bin” • SNS cavities have their couplers below the cavity, no problems were reported due to this position • The two positions seem to be acceptable SNS cryomodule with couplers below the cavities Eric Montesinos / CERN-BE-RF-SR

24. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Why Vertically above/below the cavity ? Eric Montesinos / CERN-BE-RF-SR

25. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Multipacting • Because we want to close the beam vacuum only once, the distance between the beam axis and the ceramic is given by : • The helium vessel, which gives the coupler port flange position, 130 mm from beam axis • The double walled tube length (See Ofelia’s talk) • The cryomodule size (see Vittorio’s talk) • Within all the proposed designs, the position of the ceramic will be given by these parameters • We will have to handle multipacting regardless of how it will appear • Amos’ and Rama’s talks this afternoon 3 2 1 Beam axis Eric Montesinos / CERN-BE-RF-SR

26. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Multipacting and DC biasing • For long term operation it will be extremely useful to have DC biasing, because it suppresses multipacting • After a careful conditioning of the coupler without DC biasing, applying it will protect us from operational mistakes (remember SPS breakage during “wrong” tests) • The DC biasing has proven to be very efficient even if other methods could be used such as second frequency, magnetic surface roughness,… • It can be easily implemented with an air cooled antenna and a “doorknob free” waveguide (see talk part II) Eric Montesinos / CERN-BE-RF-SR

27. A single or a double window coupler ? • Adjustable or fixed ? • Why a double walled tube ? • What about cryomodule integration requirements ? • Vertically above or below the cavity ? • DC biasing ? • Air or water cooled ? Air or water cooled ? • Mandatory from vacuum considerations: • NO water directly in contact with brazing making vacuum/water insulation (except… exceptions…) • Water cooling of the antenna: • Machine access in case of fault ? • Risk of freezing water ? • Time to warm-up a cavity ? • More complex to apply the DC biasing, need “insulating” pipes • No major difficulties to run air cooled coupler with LHC and SPS SC cavities couplers Brazing making insulation between water circuit and beam vacuum Eric Montesinos / CERN-BE-RF-SR

28. Conclusion: the SPL coupler should be… Eric Montesinos / CERN-BE-RF-SR

29. End of part I, to be continued… 11h30 • Many thanks for your attention Eric Montesinos / CERN-BE-RF-SR