Cryostats and Cryomodules: Design Choices and Implications

Transcription

1 SPL Meeting CERN July : Design Choices and Implications Paolo Pierini INFN Sezione di Milano LASA Paolo.Pierini@mi.infn.it

2 Outline Cryomodule design, is it starting to be a hot topic? of course it is within ILC (three regional facilities for testing) Invited Oral at PAC05, C. Rode Invited Oral at HPSL2005, C. Pagani Tutorial Talk at SRF05, C. Pagani Comment on existing cryomodule designs, trying to sort out the rationales behind them TESLA [ borrowing lots of transparencies from Carlo] What we asked for What we got and how we obtained it What can we do better SNS [ commenting published/unpublished material] Few words about others What are the design rationales for each design? what do we want to buy for SPL? Trade-off? July PP - SPL Meeting 2

3 SRF Cavities and Ancillaries - 1 Cavities and ancillaries design are chosen on the basis of a complex optimization that depends on: Accelerated particles Beam energy Variety of resonator shapes Beam current High current asks for consistent HOM damping Low current produces high external Q and tight resonance Beam quality requirements Alignment tolerances High Order Mode damping July PP - SPL Meeting 3

4 SRF Cavities and Ancillaries - 2 Pulsed operation High field is dominant with respect to minimum losses Lorentz Force Detuning impact the cavity/tuner design Active fast tuner required for high field High peak power coupler for high current CW operation High Q, low losses, dominant with respect to maximum field Microphonics can be crucial Active fast tuner considered for low current High average power coupler for high current Other machine dependent features High filling factor : interconnections, tuner, magnets, etc Very low static losses : long cryo-strings July PP - SPL Meeting 4

5 General Considerations on design New generation cryomodules are more and more integrated in the concept/optimization of the accelerator Past cryomodules can be viewed as the combination of a cavity system and an independently designed cryostat to contain it with minimum losses The cryostat is one of the cryomodule components and its optimization can affects the cavity system design In a SRF machine the overall cryomodule cost and performances dominate that of individual components Components and systems reliability, and the accelerator availability, are concepts that are now included in the large accelerator design from the beginning Redundancy or MTTR (mean time to repair)? Improve QC (quality control) for MTBF (mean time between failures) July PP - SPL Meeting 5

6 Large project impact and effects on cryomodule In 1985 the successful test of a pair of SC cavities in CERN started the large scale application of SRF for e - HEP laboratories invested in R&D, infrastructures and QC TJNAF and CERN played the major role in SRF development The need of building hundreds of cavities pushed the transfer to Industry of a large part of the production R&D and basic research on SRF had also a jump thanks to the work of many groups distributed worldwide The large programs forced a move to a new approach Independent insulation vacuum Clean room assembly of the cavity string Large dedicated handling infrastructure New level of system integration and Quality Control Cost driven issues Long cryo-strings when possible July PP - SPL Meeting 6

7 LEP II Cavity and Cryomodule 4.5 K Accessibility July PP - SPL Meeting 7

8 Clean Assembly of a CEBAF Cavity Pair 2 K Cleanliness & Handling procedures July PP - SPL Meeting 8

9 TTF for TTF = TESLA Test Facility TTF as operated for SASE FEL TTF Goals: Demonstrate that Superconducting RF technology is suitable for LC Operate TTF at E acc > 15 MV/m Develop cavity technology for Eacc > 25 MV/m July PP - SPL Meeting 9

10 TESLA Cryomodule Design Rationales High Performance Cryomodule was central for the TESLA Mission More then one order of magnitude to be gained in term of capital and operational cost Low static losses High filling factor: to maximize real estate gradient Long sub-units with many cavities (and quad): cryomodules (12-17 m) Sub-units connected in longer strings (2-3 km) Cooling and return pipes integrated into a unique cryomodule Low cost per meter: to be compatible with a long TeV Collider Cryomodule used also for feeding and return pipes Minimize the number of cold to warm connections for static losses Minimize the use of special components and materials Modular design using the simplest possible solution Easy to be aligned and stable: to fullfil beam requirements July PP - SPL Meeting 10

11 Three cryomodule generations to: improve simplicity and performances minimize costs Performing Cryomodules Reliable Alignment Strategy Finger Welded Shields Sliding 2 K 2 thermal shields (4 K and K) Required plug power for static losses < 5 kw/(12 m module) July PP - SPL Meeting 11

12 Cryomodules installed in TTF II ACC 5 ACC 3 ACC MeV 800 MeV ACC 4 & ACC 5 July ACC 1 ACC MeV RF gun 4 MeV ACC 2 & ACC 3 PP - SPL Meeting 12

13 Three Generation Cryomodules in TTF ACC 5 ACC 4 ACC 3 ACC 2 ACC 1 RF gun 800 MeV 400 MeV 120 MeV Cry 1 Cry 2 Cry 3 4 MeV Module 1 Module 2 & 3 Module 4 & Simplification of fabrication (tolerances), assembling & alignment strategy 2 3 Longitudinal references, redistribution of cross section (42 38 ) HeGRP is dominant in the cross-section July PP - SPL Meeting 13

14 Extensive Operation Experience Type Installation date Cold time [months] CryoCap Oct M1 1 Mar 97 5 M1 rep. 2 Jan M2 2 Sep M3 2 Jun M1* MSS M3* M4 M5 M2* July 2005 Jun 02 Apr 03 Feb July PP - SPL Meeting 14

15 From Prototype to Cry 3 Braid-cooled Cry 1 Extensive FEA modeling (ANSYS ) of the cryomodule Transient thermal analysis during cooldown/warmup cycles, Coupled structural/thermal simulations Full nonlinear material properties Detailed sub-modeling of new components and Laboratory tests Finger-welding tests at ZANON Cryogenic tests of the sliding supports at INFN-LASA July PP - SPL Meeting 15

16 Finger-Welded Shield Behavior Maximum shield temperature Cooldown simulation of the 4.2 K and 70 K aluminum thermal shields. We used a simultaneous 12 hour linear cooldown. The maximal thermal gradient on the shields (lower left graph) is below 60 K, a safe value. The temperature fields show that the gradient is concentrated in the welding region, where the fingers unload the structure Temperature fields during cooldown Maximum temperature gradient July PP - SPL Meeting 16

17 Thermo-mechanical analysis of Shields Applying the computed temperature field, deformations and stress distribution can be easily computed Maximum stresses are within acceptable limits Maximum deformations due to asymmetric cooling is below 10 mm. Maximum stresses during cool-down Maximum shield displacement July PP - SPL Meeting 17

18 Sliding Supports and Invar Rod Four C-Shaped stainless steel elements clamp a Ti pad welded to the helium tank. Rolling needles for longitudinal friction Cavities longitudinal position independent from the HeGRP contraction x and y defined by reference screws Longitudinal position defined through Invar Rods (and SS connections for compensation) A model has been developed to measure real friction force and test extreme conditions Friction force: < 1 N July PP - SPL Meeting 18

19 Summary of Cryo 3 TESLA improvements Reduce the Cross Section and use a standard pipeline tube (38 ) - Redistribute the internal components - Reduce the distances to the minimum Improve the connection of the active elements to the HeGRP - Sliding fixtures to allow Semi Rigid Coupler and Superstructures Reduce alignment sensitivity to the forces on the HeGRP edges - Move the external posts longitudinal position closer to the edges Further simplify the assembling procedure - Simplify coupler cones and braids - Reduce by a factor two the shield components System thought for mass production cost cutting - Tolerances reduced to the required ones - Simpler components and standard tubes wherever possible July PP - SPL Meeting 19

20 Diagnostics: WPMs to qualify alignment strategy On line monitoring of cold mass movements during cool-down, warm-up and operation 2 WPM lines with 2 x 18 sensors 4 sensors per active element 8 mm bore radius 1 WPM lines 1 sensors per active element 25 mm bore radius 1 WPM line 7 sensors/module 25 mm bore radius Cry 1 Cry 2 Cry 3 Module 1 Module 2 & 3 Module 4 & 5 July PP - SPL Meeting 20

21 Large Bending in First Cooldown During the first fast cooldown a large bending was observed with the WPM, as expected A new procedure was suggested and used in the following cooldowns The Big Banana July PP - SPL Meeting 21

22 Safe Cooldown of ACC4 and ACC5 X Y Still some work at the module interconnection Cavity axis to be properly defined July PP - SPL Meeting 22

23 ACC4 & ACC5 Met Specs July PP - SPL Meeting 23

24 WPM as Vibration Sensors ACC4 ACC5 WPM1 WPM4 WPM7 WPM8 WPM11 WPM14 Post V Dx= V x= a y= a A WPM is a microstrip four channel directional coupler. A 140 MHz RF signal is applied on a stretched wire in its center The low frequency vibrations of the cold mass, modulate in amplitude the RF signals picked up by the microstrips The microphonics (and the submicrophonics) can be recovered demodulating the microstrip RF signal B B V x y D + V D D + a 30 D + a 03 V Dy = V D + a 3 x 3 y 12 D + a A A 21 V D D x 2 x C + V C 2 y D D y Post July PP - SPL Meeting 24

25 Vibration spectrum [μm] Wire Oscillations In The Vertical Plane The wire proper vibration spectral lines (fundamental and harmonics) overcome the cold mass mechanical vibration lines On the other hand, being their frequencies and pattern are well predicted by VSE, it s easy to filter them when processing the data z [m] Vibrating String Equation (VSE) f n Wire parameters n F = = n 2 l ρ A Wire: (CuBe) (BERYLCO 25) 6.4 Hz Density (ρ): 8.25 g/cm 3 = 8250 kg/m 3 Cross Section (A): mm 2 Stretched Wire Length (l) m Tensile Strength: 18 kgw = N July PP - SPL Meeting 25

26 Preliminary analysis WPMs 4 and 11 are close to the central post: cold mass fix point WPMs 7 and 14 are at the end of the corresponding cryomodules Vertical ACC4 Vertical ACC5 Wire oscillation lines are not completely filtered (not to suppress useful information) Horizontal ACC4 Horizontal ACC5 Vibrations which are not wire harmonics can be clearly seen in the spectrum due to vacuum pumps cryogenic system The vibration signal is clearly higher at the cryomodule end, whereas the central post signal is primarily due to residual wire eigenfrequencies July PP - SPL Meeting 26

27 TTF Cryomodule Performances ~ 70 W ~ 13 W < 3.5 W July PP - SPL Meeting 27

28 Module assembly picture gallery - 1 String inside the Clean Room July PP - SPL Meeting 28

29 Module assembly picture gallery - 2 String in the assembly area July PP - SPL Meeting 29

30 Module assembly picture gallery - 3 Cavity interconnection detail July PP - SPL Meeting 30

31 Module assembly picture gallery - 4 String hanged to he HeGRP July PP - SPL Meeting 31

32 Module assembly picture gallery - 5 String on the cantilevers July PP - SPL Meeting 32

33 Module assembly picture gallery - 6 Close internal shield MLI July PP - SPL Meeting 33

34 Module assembly picture gallery - 7 External shield in place July PP - SPL Meeting 34

35 Module assembly picture gallery - 8 Welding fingers July PP - SPL Meeting 35

36 Module assembly picture gallery - 9 Sliding the Vacuum Vessel July PP - SPL Meeting 36

37 Module assembly picture gallery - 10 Complete module moved for storage July PP - SPL Meeting 37

38 Proven design, still few details to clean up XFEL/ILC would benefit from small enhancements Quad sliding fixture (as for cavities) planned for X-FEL Various braids for heat sinking (all coupler sinking style) Cables, Cabling, Connectors and Feed-through Composite post diameter (and fixture for transportation) Warm fixtures of cold mass on Vacuum Vessel (fixed and sliding) LMI Blankets for the K shield (LHC Style) Module interconnection: Vacuum Vessel sealing, pipe welds, etc. Coupler provisional fixtures and assembly Important actions for ILC Move quadrupole to center (vibrations) Short cavity design (decrease cutoff tube) Cavity interconnections: flanges and bellows (Reliability) Fast tuner (if coaxial filling factor can be further increased!) Longer modules (10-12 cavities) July PP - SPL Meeting 38

39 TESLA Cryomodule Concept Peculiarities Positive Very low static losses Very good filling factor: Best real estate gradient Low cost per meter in term both of fabrication and assembly Project Dependent Long cavity strings, few warm to cold transitions Large gas return pipe inside the cryomodule Cavities and Quads position settable at ± 300 μm (rms) Reliability and redundancy for longer MTTR (mean time to repair) Lateral access and cold window natural for the coupler Negative Long MTTR in case of non scheduled repair Moderate (± 1 mm) coupler flexibility required July PP - SPL Meeting 39

40 LHC and TESLA Cryomodule Comparison = 38 From an LHC Status Report by Lyndon R. Evans ACC 4 & ACC 5 in TTF ACC 2 & ACC 3 in TTF = 38 = 42 July PP - SPL Meeting 40

41 Different design: SNS Cryomodule July PP - SPL Meeting 41

42 Design Rationales Fast module exchange and independent cryogenics (bayonet connections) 1 day 2K production in CM Warm quad doublet Moderate filling factor Designed for shipment 800 km from TJNAF to ORNL No need to achieve small static losses single thermal shield July PP - SPL Meeting 42

43 Design for shipment (TJNAF to ORNL) 4 g 5 g g/2 Spaceframe concept July PP - SPL Meeting 43

44 Around the cold mass Helium to cool the SRF linac is provided by the central helium liquefier He then piped to the 4.5K cold box and sent through cryogenic transfer lines to the cryomodules Joule Thomson valves on the cryomodules produce 2.1 K (0.041 bar) LHe for cavity cooling, and 4.5 K He for fundamental power coupler cooling Boil-off goes to four cold-compressors recompressing the stream to 1.05 bar and 30 K for counter-flow cooling in the 4.5K cold box Magnetic shields 50 K thermal shield Vacuum chamber Tank End Plate July PP - SPL Meeting 44

45 4 BAR SNS He Flow HELIUM SUPPLY T.L. HELIUM RETURN T.L. He Supply He Return SUPPLY HP HELIUM COOLDOWN & POWER COUPLER RETURN GUARD VACUUM & RELIEF 20 BAR 20 BAR PHASE SEPARATOR LHe POWER COUPLER OUTER CONDUCTOR HEATER AT WINDOW Coupler and flange thermalization with 4.5 K flow END FLANGE HEAT STATION COUPLER HEAT STATIONS (2) 50K SHIELD COUPLER HEAT STATIONS (2) END FLANGE HEAT STATION 50 K Shield July PP - SPL Meeting BAR SURGE TANK 2K SUBCOOLER 2 K

46 SNS β=0.61 Cryomodule Assembly -1 Cavity string in Clean Room July PP - SPL Meeting 46

47 SNS β=0.61 Cryomodule Assembly -2 Internal magnetic shield and MLI July PP - SPL Meeting 47

48 SNS β=0.61 Cryomodule Assembly -3 Thermal shield and spaceframe July PP - SPL Meeting 48

49 SNS β=0.61 Cryomodule Assembly -4 String in spaceframe July PP - SPL Meeting 49

50 SNS β=0.61 Cryomodule Assembly -5 Vacuum vessel July PP - SPL Meeting 50

51 SNS β=0.61 Cryomodule Assembly -6 Spaceframe with external magnetic shield July PP - SPL Meeting 51

52 SNS β=0.61 Cryomodule Assembly -7 Sliding the Vacuum Vessel July PP - SPL Meeting 52

53 Alignment strategy Indexing off of the beamline flanges at either end of each cavity Nitronic support rods used to move the cavity into alignment Targets on rods on two sides of each flange. Cavity string is supported by the spaceframe Each target sighted along a line between set monuments (2 ends and sides) The nitronic rods are adjusted until all the targets are within 0.5 mm of the line set by the monuments Cavity string in the vacuum vessel: the alignment is verified and transferred (fiducialized) to the shell of the vacuum vessel. July PP - SPL Meeting 53

54 Other solutions are possible, according to needs APT Coupler-dominated case Huge power specs 100 ma CW, 1.7 GeV beam! Everything built around coupler! TRASCO/ADS Short module, interchangeable, sliding supports, G10 frame Alignment transferred to rails in vessel Slide-in from side Efficiency is not a concern (spacewise) Conceptual Designs July PP - SPL Meeting 54

55 Trade offs & choices for SPL Main decision: Filling factor vs. fast module exchange Linac length vs. activation/reliability concern Real estate gradient is more strongly influenced by module length or ancillaries (tuner!) than from cavity accelerating gradient Are static losses a real concern for SPL? Dynamic losses at 10% duty cycle, at 19/25 MV/m gradient are in the range from K per module No need to aim at 3.5 W per module 4.2 kw for all the 6*8+7*20=48+140=188 cavities High power needs anyhow many openings Can t buy a single design, as it is Can surely transfer design ideas and subcomponents TESLA attractive for filling factor SNS for module exchange LEP has easy access to cold mass, possibly not compatible with 2 K July PP - SPL Meeting 55