1. 650 MHz Helium Vessel Chuck Grimm, Serena Barbanotti, Harry Carter, Mike Foley, Camille Ginsburg, Tom Peterson

2. Outline 650 MHz helium vessel design criteria and comments –Start to develop a “functional specification” and a “technical specification” –Here only a general summary of design considerations Background information –Technical comments from Tom Peterson Copies of slides developed during consideration of various thermal, pressure, and piping issues for 325 MHz and 1.3 GHz cryomodules as well as 650 MHz –Background and status details from Chuck Grimm, Serena Barbanotti, Mike Foley Slides from a recent talk assembled by Chuck, Serena, and Mike

3. 650 MHz helium vessel design criteria and comments Tom Peterson Fermilab

4. Introduction We have only just begun to think about 650 MHz helium vessel design (and 650 MHz cryomodule design) –Perhaps I should just speak for myself -- I have been involved in very few discussions about these issues Fundamental questions remain to be addressed regarding features and constraints imposed by differences from 1.3 GHz pulsed operation such as the following –High heat loads of ~25 W per cavity in CW operation –Need to minimize microphonics for CW operation –Need to minimize helium pressure sensitivity of cavity tune –Effect of liquid and vapor volumes in buffering pressure changes –Desire to maintain Maximum Allowable Working Pressure (MAWP) of 2 bar warm, 4 bar cold –Venting with larger surface areas than 1.3 GHz cavities

5. Considerations in design of a 650 MHz helium vessel -- 1 Cavity performance –accelerating gradient (MV/m) –cavity Q0 (2K, perhaps also 1.8 K if doing temperature optimization) –average dynamic heat per cavity to 2 K (W) –worst case dynamic heat per cavity to 2 K (W) Cavity physical dimensions –cavity length (mm) –cavity length definition –cavity diameter (mm) –cavity surface area (sq. cm.) –helium vessel helium volume (liters) –periodic distance (mm) –definition of period (e.g., cavity-cavity-magnet repeating assembly) –beam tube aperture ID (mm) –beam tube flange ID (mm)

6. Considerations in design of a 650 MHz helium vessel -- 2 Cavity alignment requirements –cavity lateral alignment RMS (mm) –cavity vertical alignment RMS (mm) –Dressed cavity support mechanism, structure to meet alignment requirements Vibration –allowable frequencies for mechanical vibration (aim for highest possible mechanical resonances is important.) Helium vessel material choice (stainless vs. titanium) Helium vessel pressure limits and sensitivity –liquid helium volume liquid volume provides stability against pressure change with heat change –warm MAWP (bar) (at least 2 bar differential!) –cold MAWP (bar) (would like 4 bar differential!) –delta frequency / delta pressure (Hz/mbar) –pressure stability tolerance (+/- mbar from +/- allowable Hz)

7. Considerations in design of a 650 MHz helium vessel -- 3 Piping –two-phase pipe connection or flow through (Jlab, SNS type) liquid volume for stability and advantage of large surface area might indicate Jlab/SNS flow through type of vessel –vapor volume required for pressure stability big 2-phase pipe and/or vapor return pipe provide buffer against pressure fluctuation due to vapor flow change at compressor –2-phase pipe and chimney pipe diameters for maximum heat and flow conditions peak heat flow out chimney with about 1 W/sq.cm. loss of cavity vacuum flow rate so as not to exceed MAWP Main RF power couplers –angular orientation to cavity –coupler size, geometry, possible interference with vessel end –vacuum flange diameter (mm) –thermal intercept at 5 K? (yes, no) –estimated heat (W) at 2 K, 5 K, 70 K (or other selected temperatures)

8. Considerations in design of a 650 MHz helium vessel -- 4 Cavity tuners –tuner type Saclay lever, Jlab scissor, INFN blade, KEK, other –allowance in support structure, etc., for tuning motion –tuning force and travel, load on the tuner –cable requirements –motor location, size, overall tuner envelope size –thermal intercept temperature requirements –estimated heat at each intercept –fast tuners? Instrumentation on or in helium vessel –RF –temperature –pressure –position monitors (e.g., wire position) –vacuum (beam vacuum adjacent to vessel)

9. Considerations in design of a 650 MHz helium vessel -- 5 Beam Position Monitors (BPM) –locations –geometry –support and alignment –temperature –electrical connections Helium vessel assembly and welding procedure Magnetic shielding arrangement Liquid management features (heater at each helium vessel? probably yes!) More topics?

10. Helium vessels CW heat loads and pressure stability might imply different design from TESLA SNS/CEBAF style dressed cavity with large helium vessel. SNS dressed cavity image from “  esignof the    avity  upport  tructure” by  , et. al. Liquid volume provides pressure stability advantage. (Consider design and volumes when we quote pressure stability experience.) 1.3 GHz dressed cavity with blade tuner, showing close- fitting helium vessel with separate 2-phase helium supply pipe. Developed for TESLA to reduce helium inventory with very long strings and improve tuner efficiency.

11. Conclusions of a cavity detuning study at Fermilab Control of cavity detuning is manageable Allowable microphonics amplitude for 650 MHz cavity = 15 Hz * –Half of that available for pressure fluctuations –df/dP = 75 Hz/mbar –Allowable helium pressure fluctuations = +/- 0.1 torr (0.13 mbar) –Design goal is +/- 0.1 mbar –Crucial to provide adequate power for all cavities –Cannot rely on active compensation alone * from “Status of the Design of 650 MHZ Elliptical Cavities for Project X,” by S. Barbanotti#, I. Gonin, C. Grimm, T. Khabibouline, M. Foley, L. Ristori, N.Solyak, V. Yakovlev, FNAL, Batavia, IL 60510, U.S.A.

12. Pressure stability conclusion Project X CW linac goal of 0.1 mbar pressure stability 0.1 mbar = just 7 mm of liquid helium head Heat pulse buffered well by liquid mass –Latent heat of liquid absorbs energy –Vaporization takes most of the rest of the energy Flow change –Buffered in short term by vapor volume –Longer term equilibrium for a few % change easily exceeds 0.1 mbar Look at experience at existing SRF systems –Pressure stability, long-term and short-term excursions –How system volumes and features may correlate with their experience

13. 2-pipe 2 Kelvin vapor system TESLA-type cryomodule

14. Cryomodule Pipe Sizing Criteria Heat transport from cavity to 2-phase pipe –1 Watt/sq.cm. is a conservative rule for a vertical pipe (less heat flux with horizontal offset, i.e., if not vertical) I calculated 1.4 W/sq.cm. in 1993 for TESLA BESSY (Berlin) measured 1.5 W/sq.cm. quench limit Two phase pipe size –5 meters/sec vapor “speed limit” over liquid –Not smaller than nozzle from helium vessel Gas return pipe (also serves as the support pipe in TESLA- style CM) –Pressure drop < 10% of total pressure in normal operation –Support structure considerations Loss of vacuum venting P < cold MAWP at cavity –Path includes nozzle from helium vessel, 2-phase pipe, may include gas return pipe, and any external vent lines

15. Two-phase pipe size, cavity tune, pressure, microphonics Tom Peterson Fermilab

16. Vapor velocity limit for separated flow I use a 5 meter/sec “speed limit” for helium vapor flow "Latest Developments on He II Co-current Two-phase Flow Studies," by B. Rousset, A. Gauthier, L. Grimaud, and R. van Weelderen, in Advances Vol 43B (1997 Cryogenic Engineering Conference).

17. Cryomodule Pipe Sizing Criteria Heat transport from cavity to 2-phase pipe –1 Watt/sq.cm. is a conservative rule for a vertical pipe (less heat flux with horizontal offset, i.e., if not vertical) I calculated 1.4 W/sq.cm. in 1993 for TESLA BESSY (Berlin) measured 1.5 W/sq.cm. quench limit Two phase pipe size –5 meters/sec vapor “speed limit” over liquid –Not smaller than nozzle from helium vessel Gas return pipe (also serves as the support pipe in TESLA- style CM) –Pressure drop < 10% of total pressure in normal operation –Support structure considerations Loss of vacuum venting P < cold MAWP at cavity –Path includes nozzle from helium vessel, 2-phase pipe, may include gas return pipe, and any external vent lines

18. CW cryomodule system schematic

19. 650 MHz 2-phase pipe size 33.6 W per cavity -- note the large pipe needed for even just a few cryomodules in series TTF is 72 mm

20. Air inflow heat flux limit Atmospheric air flowing into a vacuum via a round hole –~23 grams/sec air per cm 2 hole size Heat deposition by air condensing on cold surface –~470 J/g, so 10.8 kW per cm 2 hole size Helium heat input per gram helium ejected for typical (2.5 - 4 bar) pressures –~13 J/g Helium mass flow per air inlet area –~830 grams/sec helium ejected per cm 2 air inlet hole size

21. Note: a 3-inch air inlet hole results in a mass flow equivalent to ~ 8 beta=0.9 650 MHz cavities. Checking the feasibility of venting a CM string of cavities with a large 2-phase pipe. Looks OK but still need frequent cross-connects to a larger pipe.

22. 650 MHz Changes from TTF Pipe Sizes Larger surface area and steady-state heat loads result in larger pipes Nozzle from helium vessel to 2-phase pipe –TTF -- 55 mm –For venting need about 75 mm ID 2-phase pipe –TTF -- 72 mm –Steady-state vapor velocity >100 mm Gas return pipe –TTF -- 300 mm –Structure and emergency venting set the size for multiple cryomodules in series. Structure is main criterion for single cryomodules.

23. Further considerations Support structure –Stiffness of pipe if used as support backbone –Or other support structure options Emergency venting scenarios drive pipe sizes and influence segmentation –Cold MAWP may be low for 650 MHz, driving up pipe sizes and/or reducing spacing between relief vent ports –Trade-off of pipe size with vent spacing requires further work –Thermal shield pipe may also require frequent venting 5 K has a large surface area for large heat flux 70 K starts at a high pressure Liquid management length needs further work –May want to subdivide strings as finely as individual cryomodules for liquid management due to large heat load and specific helium flow rate per cavity

24. Liquid management length The 2 K to 4.5 K heat exchanger needs to be divided (not one large heat exchanger) in order to be a practical size, which means distributing multiple heat exchangers in the tunnel. 2 K to 4.5 K heat exchanger size which fits in the Project X tunnel will be roughly 10 - 15 grams/sec (about 200 - 300 Watts of 2 K heat) With 650 MHz and 1.3 GHz CW heat loads of ~ 200 W per cryomodule, this implies liquid management lengths of one (or possibly two) cryomodule as limited by JT heat exchanger practical size limits

25. From Matthias Liepe (Cornell) “ Microphonics and Frequency Control A Collection of Results and Thoughts”, presented at the Project X collaboration meeting, Sep, 2010 Ground vibrations and other mechanical vibrations do not strongly couple to the SRF cavities! Main contribution to cavity microphonics comes from fast fluctuations in the He-pressure and the cryogenic system

26. Cavity tune and pressure Need +/- 20 Hz from 1.3 GHz for TESLA-type linac Tuning motion for TESLA/ILC type cavities is 200 – 530 kHz/mm –Variation perhaps due to differences among tuners and cavity support structures as well as the cavity conditions after processing –400 Khz/mm tuning from DESY TTF CDR –So tune cavity length to < micron –Very stiff structure Pressure sensitivity for 1.3 GHz TESLA/ILC type cavities is about 7 to 10 Hz/mbar. So pressure variations < +/- 2 mbar Tighter constraints for CW cavities Design goal is +/- 0.1 mbar for project X

27. From “Controlling Cavity Detuning for Project X” by Ruben Carcagno, et. al. SNS –Long term pressure regulation to better than 100 ubar (Fabio Casagrande) Regulates flow and maintains constant heat load using bath heaters Distributes He at 4K with a cold box on each cryomodule CEBAF –25 ubar when everything is ‘quiet’ Also regulates flow and uses heaters to maintain constant load Distributes He at 2K –Transients of up to several 1-2 mbar about once a day Identified a control system problem not a cryogenic problem HoBiCaT (BESSY) –15-30 ubar steady state –Large non-gaussian tails in microphonics distribution ~17  fluctuation every one or two hours Origin not completely understood but cryogenic likely a large component Cornell –Heat leak at dead end pipe in cryo system creates gas bubbles which ‘pop’ about once per second

28. Pressure stability Consider short duration pulses (~ a few seconds or less) for which the vapor and liquid are not in equilibrium. –Treat the vapor volume as a closed volume of ideal gas –Heat or vapor flow change results in a direct change of stored gas –Pressure changes in proportion to net mass flow in or out –Pressure changes inversely with total vapor volume –Using total mass flow, if a TTF cryomodule with TTF-sized pipes and 16 W total heat (dynamic plus static), pressure change due to total heat flow into a closed volume is dP/dt =0.023 mbar/sec SSR0 dP/dt = 0.10 mbar/sec SSR1 dP/dt= 0.14 mbar/sec SSR2 dP/dt = 0.33 mbar/sec 650 MHz CW dP/dt = 0.31 - 0.36 mbar/sec

29. Pressure stability Consider pulses of energy or steady heat addition for which the 2 K liquid and vapor are always in complete thermal equilibrium. –The process is the addition of heat or transfer of mass (to or from) a closed 2-phase system –Liquid volume is the primary buffer in absorbing heat Liquid heat capacity (not just latent heat) –Vapor volume is the primary buffer in absorbing mass flow fluctuations

30. Example of volume effect on pressure change with heat load Consider TESLA cavity and with piping volumes somewhat increased for CW operation (very approximate here). Add a pulse of 10 J of heat. The result is a very small pressure rise of 40 micro-bar, corresponding to a temperature change of less than 0.5 mK. Most of the heat is absorbed by the liquid. The balance is essentially all latent heat of vaporization. Liquid volume serves as an effective buffer against pressure rise.

31. Example of volume effect on pressure change with flow change From Project X analysis for a 650 MHz CW cryomodule, calculate the new flow rate which produces the maximum allowed pressure change due to a new backpressure from flow resistance out via the JT heat exchanger. The result is a very small percentage change of flow, which is created by a relatively small heat load. It appears that a small pulse of energy is not much problem, but a small amount of steady heat added results in a new mass flow, producing a new pressure equilibrium which may easily exceed the allowable change in pressure for cavity frequency stability.

32. Pressure stability conclusion Project X CW linac goal of 0.1 mbar pressure stability 0.1 mbar = just 7 mm of liquid helium head Heat pulse buffered well by liquid mass –Latent heat of liquid absorbs energy –Vaporization takes most of the rest of the energy Flow change –Buffered in short term by vapor volume –Longer term equilibrium for a few % change easily exceeds 0.1 mbar Look at experience at existing SRF systems –Pressure stability, long-term and short-term excursions –How system volumes and features may correlate with their experience

33. Helium vessels SNS/CEBAF style dressed cavity with large helium vessel. SNS dressed cavity image From                by  , et. al. May provide pressure stability advantage 1.3 GHz dressed cavity with blade tuner, showing close- fitting helium vessel with separate 2-phase helium supply pipe. Developed for TESLA to reduce helium inventory and improve tuner efficiency.

34. Conclusions of a cavity detuning study at Fermilab Control of cavity detuning is manageable But –Crucial to minimize pressure transients –Crucial to minimize pressure variations –Crucial to minimize cavity vibration –Crucial to minimize cavity sensitivity –Crucial to provide adequate power for all cavities –Can not rely on active compensation alone

35. 650MHz Cavity Design & Fabrication Status at Fermilab Chuck Grimm, Serena Barbanotti, Mike Foley Fermilab

36. Introduction Single Cell Cavities –Design Progress - β= 0.90 & β= 0.61 –Fabrication Specification –Mechanical Analysis –Current Cavity Order 5-Cell Cavities –Design Progress – β= 0.90 –Thermal Analysis –Mechanical Analysis –Resolution of df/dP –Spin Form Seamless Beam Tube R & D Helium Vessel –Design Progress –Spin Form Seamless Pipe R & D –Automatic TIG Welding

37. Single Cell Cavities Design of Both Beta Versions Complete –Drawing Packages Complete –Fabrication Specification http://tdserver1.fnal.gov/grimm/371090-8pg.pdf Cavity Hardware Design Completed –Blank-off Flanges - Ordered –Aluminum Hex Seals – On-Hand –Electro-Polished Studs and Si-Bronze Nuts - Ordered β= 0.90β= 0.61

38. Single Cell Mechanical Analysis For our preliminary design studies we adopted the following basic premises: –Cavities will be tested in a vertical cryostat –Working pressure differential will be 1.0 bar –Von Mises equivalent stresses for niobium < 50 MPa [7250 psi] – Preliminary closer to 40 MPa For all analysis the material thickness was reduced by 0.3 mm to account for post-processing. End flanges fixed. Initial design assumed 4.0 mm half-cell wall and 3.0 mm beam tube wall. Why 4.0 mm? Material is used in similar cavity designs (e.g., SNS), and FNAL has enough in stock to fabricate two single-cell cavities.

39. Single Cell Mechanical Analysis 650 MHz  =0.9 single-cell cavity: 3 mm beam tube wall

40. Single Cell Mechanical Analysis Note that the maximum stress occurs at the iris to end tube joint. This result suggests that 4.0 mm wall beam tubes would be appropriate. For 4 mm beam tube wall maximum von Mises equivalent stress  32 MPa. Pressure differential for collapse is > 7.0 bar.

41. Single Cell Mechanical Analysis 650 MHz  =0.9 single-cell cavity: 4 mm beam tube wall

42. Single Cell Fabrication Fermilab is in the process of awarding a contract to build (6) single cell β= 0.9 cavities. –Fermilab will supply material to the vendor Delivery of first (2) single cells will be in 4-6 months ARO. –Material on-hand for (2) cavities Remaining (4) cavities to be delivered 8-9 months ARO. –An additional order for niobium has been placed with Wah-Chang, expected delivery Q1 – 2011 At this time no order will be placed for β= 0.61 single cell cavities.

43. 5-Cell Cavities Design of β= 0.9 Nearly Complete - Key Issues –Location of stiffening rings to mitigate influence of helium bath pressure fluctuations (df/dP) –Finalize power coupler design –HOM requirement….or not – order of first (2) without HOM’s –Interface between cavity and helium vessel Functional Requirement Specification – Draft V3 –http://tdserver1.fnal.gov/grimm/FRS_650_MHz_Cavity_draft-v3.pdf β= 0.90

44. 5-Cell Cavities Cavity Cross-Section

45. 5-Cell Thermal Analysis The solid end disk was chosen for ease of fabrication. A thermal analysis was conducted to determine if an unacceptable thermal gradient exists between the inner surface where RF power is dissipated, and the outer surface exposed to the liquid helium bath. The thermal analysis indicated that the power deposited on the wall during operation increases the inner surface temperature by a negligible amount. This was considered acceptable.

46. 5-Cell Thermal Analysis

47. 5-Cell Mechanical Analysis Various radial positions for the iris stiffening ring were studied. RF engineers would like the df/dP to be as close to 0 as possible. Our initial design target value was 75Hz / Torr Manufacturability and cavity tuning may limit how low a frequency we can go.

48. 5-Cell Mechanical Analysis

49. 5-Cell Cavity Beam Tubes Current manufacturing processes of cavity beam tubes. –Rolled and seam welded –Deep drawing seamless tube Fermilab has an order for (2) spun formed 100mm ID niobium tubes for R & D study. –Working with a Chicago area vendor to develop a spin process with niobium –First seamless tubes received with a 50mm ID, much tougher to produce than the 100mm. –Initial leak check and ID measurements performed –More tests required when 100mm ID tubes arrive, including pull-out tests.

50. 5-Cell Cavity Beam Tubes

51. Helium Vessel Design of helium vessel in preliminary stages. –Interface between cavity and helium vessel –Vessel wall thickness study to help with df/dP –Bladetuner vs. End Tuner –2-phase pipe vs. series connection of He vessels –Spin forming dished heads and seamless pipe Automatic TIG welding machine proposal for titanium weld joints. –Moving out of the box

52. Helium Vessel Current Vessel Cross-Section

53. Helium Vessel Current Vessel Cross-Section Field Probe End

54. Helium Vessel Current Vessel Cross-Section Main Coupler End

55. Helium Vessel Analysis Preliminary analysis at 2 Bar pressure show no stresses above 30 MPa