overview of cryomodules for proton accelerators

Transcription

1 overview of cryomodules for proton accelerators Paolo Pierini INFN Sezione di Milano Laboratorio Acceleratori e Superconduttività Applicata Paolo.Pierini@mi.infn.it 19 March 2009 Bilbao

2 outline discuss cryogenics & cryomodules design rationales intent limited to modules for elliptical cavities and few considerations for spoke cavities not covering other structures, especially QWR case often not completely relevant (common vacuum, 4 K operation, small scale,...) trying not to concentrate on design details, rather explore interplay with the design choices/requirements of the machine / supporting systems March essbilbao initiative workshop - Paolo Pierini 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 velocity profile beam energy variety of resonator shapes beam current high current asks for consistent HOM damping low current CW implies high external Q and tight resonance beam quality requirements alignment tolerances High Order Mode damping requirements March essbilbao initiative workshop - Paolo Pierini 3

4 pulsed operation SRF cavities and ancillaries - 2 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 h average power coupler for high h current other machine dependent features high filling factor: interconnections, tuner, magnets, etc minimization of static losses : long cryo-strings March essbilbao initiative workshop - Paolo Pierini 4

5 general considerations cryomodules are now more and more integrated in the concept/optimization of the accelerator no longer viewed as the combination of a cavity system and an independently designed cryostat to contain it with minimum losses modules are especially one (important) part of the overall cryogenic system the cryostat is one of the cryomodule components and its optimization can affect the cavity package design in a large size 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 for MTBF March essbilbao initiative workshop - Paolo Pierini 5

6 cryogenic plant: duties primary maintain cavities at normal operation temperature below 2K for elliptical below 4.5 K for spokes provide fluid flow for thermal intercepts and shields at multiple temperature levels supply liquefaction flow for power leads cool-down and fill (and empty and warm-up) the accelerator efficiently supports transient operating modes and off-nominal operation including RF on/rf off and beam commissioning secondary allow cool-down and warm-up of limited-length strings for repair or exchange of superconducting accelerating components to which h extent t is an important t design choice (unit module, strings...) March essbilbao initiative workshop - Paolo Pierini 6

7 cryogenic distribution system functions supports operation of the linac within cooldown and warm-up rate limits and other constraints imposed by accelerating SRF components time duration of cooldowns, transient thermal gradients,... guarantees safety All cryo component and circuits should be guaranteed not to ever exceed their MAWP (Maximum Allowable Working Pressure) during fault conditions guarantees machine protection RF cavities from over pressurization under faulty conditions that can hinder performance substantial difference with respect to SC magnets! March essbilbao initiative workshop - Paolo Pierini 7

8 cryogenic distribution system design design should be independent of cooldown rates, cooldown sequences, or pressurization rates includes many components to be designed/engineered feed boxes cryogenic transfer lines bayonet cans string/modules feed and end caps string connecting and segmentation boxes gas headers... cryogenic distribution system and cryomodules are not engineered independently March essbilbao initiative workshop - Paolo Pierini 8

9 the cryomodule environment: a cartoon view to He production and distribution system 2 K 5-8 K sup pports K all spurious sources of heat losses to the 2 K circuits need to be properly managed and intercepted at higher temperatures (e.g. conduction from penetration and supports, thermal radiation) these are the accelerator active devices with tight alignment constraints for beam quality RF cavities RF penetrations March essbilbao initiative workshop - Paolo Pierini 9

10 the efficiency of the thermal cycle thermal cycle efficiency efficiency of the thermal cycle, to extract heat Q deposited at T c we need a work W at temperature T h always greater than the Carnot cycle Th Tc W = Q ηth T c including the efficiency of the thermal machine (20% for T c = 2 K) we need 750 W at room temperature for 1 W at 2 K all sources of parasitical heat loads need to be carefully avoided if we do not want to pay such a high price! accurate thermal design in order to minimize the heat losses Static: Always present, needed to keep the module cold. Dynamic: Only when RF is on. Due to power deposition by RF fields. N.B. at different intercept temperatures when Tc=42Kwe 4.2 have ~ 250 W/W when Tc = K we have ~ W/W March essbilbao initiative workshop - Paolo Pierini 10

11 heat removal by He heat is removed by increasing the energy content of the cooling fluid (liquid or vapor) heating the vapor spending the energy into the phase transition from liquid to vapor cooling capacity is then related to the enthalpy difference between the input and output helium ( to mass flow) the rest is piping design to ensure the proper mass flow, convective thermal exchange coefficient, pressure drop, P removed [ W] = m [g/s] K 20 J/g latent heat 40 K to 80 K 5 K to 8 K 2 K flow Temperature level Temperature level Temperature level (module) (module) (module) Temp in (K) 40,00 5,0 2,4 Press in (bar) 16,0 5,0 1,2 Enthalpy in (J/g) 223,8 14,7 4,383 Entropy in (J/gK) 15,3 3,9 1,862 Temp out (K) 80, ,0 20 2,0 Press out (bar) 14,0 4,0 saturated vapor Enthalpy out (J/g) 432,5 46,7 25,04 Entropy out (J/gK) 19,2 9,1 12,58 March essbilbao initiative workshop - Paolo Pierini 11

12 isothermal saturated bath to operate the cavities the heat load is ultimately carried away by evaporation in an isothermal bath either saturated bath of LHe at ambient pressure (4.2 K) or saturated bath of subatmospheric superfluid LHe (< 2.1 K) March essbilbao initiative workshop - Paolo Pierini 12

13 state of the art two main different solutions the TESLA cryostring concept developed for a superconducting linear collider tested in the TTF (now FLASH) used for the European XFEL linac construction (1.7 km) assumed for the ILC design (~30 km) concept studied also for proton machines SPL at CERN, Project X at FNAL the SNS linac short & independent units fast replacement of a single faulty unit concept used for ADS linac March essbilbao initiative workshop - Paolo Pierini 13

14 high filling factor TESLA cryomodule design rationales maximize ratio between real estate gradient and cavity gradient long cryomodules/cryo-unitsunits and short interconnections moderate cost per unit length simple functional design based on reliable technologies use the cheapest allowable material that respect requirements minimum machining steps per component minimum number of different components low static heat losses in operation effective cold mass alignment strategy room temperature alignment preserved at cold effective and reproducible assembling procedure class 100/10 clean room assembly just for the cavity string minimize time consuming operations for cost and reliability March essbilbao initiative workshop - Paolo Pierini 14

15 consequences/i The combined request for a high filling factor [machine size] and the necessity to minimize static heat losses [operation cost] leads to integrate the cryomodule concept into the design of the whole cryogenic infrastructure Each cold-warm transition or cryogenic feed require space and introduces additional static losses Thus, long cryomodules, containing many cavities (and the necessary beam focusing elements) are preferred, and they should be cryogenically connected, to form cryo- strings, in order to minimize the number of cryogenic feeds Limit to each cryomodule unit is set by fabrication (and cost) issues, module handling, and capabilities to provide and guarantee alignment March essbilbao initiative workshop - Paolo Pierini 15

16 consequences/ii The cryogenic distribution for the cryo-string is integrated into the cryomodule, again to minimize static losses several cryogenic circuits running along the cold mass to provide the coolant for the cavities and for the heat interception at several temperatures To take out the RF power dissipated along the long cryostring formed by many cryomodules connected together a large mass flow of 2 K He gas is needed, leading to a large diameter He Gas Return Pipe (HeGRP) to reduce the pressure drop This pipe pp was made large and stiff enough so that it can act as the main structural backbone for the module cold mass cavities (and magnet package) can be supported by the HeGRP The HeGRP (and the whole cold mass) hangs from the vacuum vessel by means of low thermal conduction composite suspension posts March essbilbao initiative workshop - Paolo Pierini 16

17 the TESLA module provides cryogenic environment for the cold mass operation cavities/magnets in their vessels filled with sub atmospheric He at 2 K contains He coolant distribution lines at required temperatures collect large flow of return gas from the module string without pressure increase Low losses penetrations for RF, cryogenics and instrumentation shield parasitical heat transfer double thermal shield structural support of the cold mass different thermal contractions of materials precise alignment capabilities and reproducibility with thermal cycling cavity size 12 m, 38 diameter, string of 8 cavities and magnet March essbilbao initiative workshop - Paolo Pierini 17

18 TESLA/ILC/(XFEL) modular cryogenic concept each module contains all cryo piping each cavity tank in module connected to two phase line vapor is collected from 2 phase line once per module in the GRP several modules are connected in strings single two phase line along the string a JT valve once per string fills two phase line via subcooled 2.2 K line strings are connected into units each unit is fed by a single cryogenic plant ILC scheme for segmentation and distribution modules without with without quad quad quad RF unit (lengths in meters) three modules RF unit RF unit RF unit RF unit end box string (vacuum length) twelve modules plus string end box string string string string possible segmentation unit modules (segmentation box is the same as string end box (2.5 m) and all contain vacuum breaks) service service box end segment segment segment segment box end Cryogenic Unit (16 strings) (1 cryogenic unit = 192 modules = 4 segments*48 CM with string end boxes plus service boxes.) meters Line F Line E Line D Line C Line A Line B unit length limited by size of cryo plant needed (25 kw equivalent at 4.5 K seems max reasonable extrapolation of 18 kw LHC) 75 K return 50 K supply 8 K return 5 K supply Sub-cooled LHe supply Pumping return Cryo-string Cryo-string Cryo-string Cryo-string Cryogenic distribution box March essbilbao initiative workshop - Paolo Pierini 18 Cryo-unit

19 schematically All li ines in mo odule outer shield inner shield subcooled forward line GRP March essbilbao initiative workshop - Paolo Pierini 19

20 cryostrings in TTF&FLASH March essbilbao initiative workshop - Paolo Pierini 20

21 The cross section Low thermal conduction composite supports Cryogenic support Pressurized helium feeding Helium GRP (large because of Shield gas pressure drop, used feeding as structural backbone) Thermal shields RF Penetration cavity Two phase flow Coupler port Helium tank Sliding support March essbilbao initiative workshop - Paolo Pierini 21

22 three generations of cryomodules in TTF 1 2 Simplification of fabrication (tolerances), assembling & alignment strategy 2 3 Longitudinal references, redistribution of cross section (42 38 ) Module 1 Module 2 & 3 Module 4, 5 & 6 March essbilbao initiative workshop - Paolo Pierini 22

23 from prototype to Cry 3 Braid-cooled Cry 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 and testing of new components Finger-welding for stress-relief Cryogenic tests of the sliding supports March essbilbao initiative workshop - Paolo Pierini 23

24 Cold mass alignment strategy The Helium Gas Return Pipe (HeGRP) is the system backbone 3Taylor-Hobson spheres are aligned wrt the HeGRP axis, as defined by the machined interconnecting edge flanges Cavities are aligned and transferred to the T-H spheres Cavity (and Quad) sliding planes are parallel to the HeGRP axis by machining (milling machine) Longitudinal cavity movement is not affecting alignment Sliding supports and invar rod preserve the alignment while disconnecting the cavities from the huge SS HeGRP contraction 36 mm over the 12 m module length cooling from 300 K to 2 K Variation of axis distances by differential contraction are fully predictable and taken into account March essbilbao initiative workshop - Paolo Pierini 24

25 cooldown behavior Tin(CMTB) T out (CMTB) Delta T (CMTB) DeltaT (ANSYS) T in (ANSYS) T out (ANSYS) re (K) Temperatu K shield Fairly sophisticated non linear transient FEM models 240 reproduce with good accuracy 210 the cooldown behavior assess max thermal gradients 120 and stresses during transients 90 allow to identify suitable 60 cooldown rates to keep thermal 30 stresses below safe limits 0 re (K) Temperatur Time (h) comparison FEM with CMTB cooldown 5 K shield T out (CMTB) T in (CMTB) Delta T (CMTB) T in (ANSYS) T out (ANSYS) Delta T (ANSYS) Time (h) March essbilbao initiative workshop - Paolo Pierini 25

26 linac performances, low static load budget ~ 70 W ~ 13 W < 3.5 W March essbilbao initiative workshop - Paolo Pierini 26

27 proven design, still few details to clean up XFEL introduced small enhancements quad sliding fixture (as for cavities) better heat sinking (all coupler sinking style) cables, cabling, connectors and feed-through module interconnection: vacuum vessel sealing, pipe welds, etc. coupler provisional i fixtures and assembly preparing large production at qualified industries important actions for ILC move quadrupole to center (vibrations) short cavity design (decrease cutoff tube) cavity interconnections: flanges and bellows (Reliability) fast tuner (need coaxial so that filling factor can be further increased!) March essbilbao initiative workshop - Paolo Pierini 27

28 very low static losses TESLA cryomodule concept summary positive 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 pp pipe inside the cryomodule cavities and quads position controlled 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 March essbilbao initiative workshop - Paolo Pierini 28

29 different design: SNS cryomodule cryo distribution feed/end boxes March essbilbao initiative workshop - Paolo Pierini 29

30 SNS He flow He Supply 5 K, 3 bar He Return Cryo lines outs side module Coupler and flange thermalization with 4.5 K flow 2 K Counterflow HEX 50 K Shield/thermalization March essbilbao initiative workshop - Paolo Pierini 30

31 design rationales Fast module exchange and independent d cryogenics (bayonet connections) 1 day exchange 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 March essbilbao initiative workshop - Paolo Pierini 31

32 design for shipment (TJNAF to ORNL) 4 g 5 g g/2 spaceframe concept March essbilbao initiative workshop - Paolo Pierini 32

33 Around the cold mass Helium to cool the SRF linac is provided by the central helium liquefier He from (8 kw) 4.5K cold box sent through cryogenic transfer lines to the cryomodules Joule Thomson valves on the cryomodules produce 2.1 K ( 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 March essbilbao initiative workshop - Paolo Pierini 33

34 Alignment strategy indexing i 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. March essbilbao initiative workshop - Paolo Pierini 34

35 Project-X baseline cryogenics 2-phase He at 4.5 K Strings are fed in parallel first string SC solenoids, warm RF second string SSR/TSR modules Cryomodules are fed in series Revised TESLA cryo string concept 2 phase He line at 2 K concurrent liquid supply and vapor return flow in the string Double thermal shielding in strings to limit radiation flow at 2 K March essbilbao initiative workshop - Paolo Pierini 35

36 Project-X head load table Project X ICD 25 MV/m, 1.5 msec, Heat Load Qty 5Hz, 20 ma, 1.25 FT [# ] 2K 4.5K 5K 40K or 80K Item Static Dynamic Static Dynamic Static Dynamic Static Dynamic WRF Solenoid SSR SSR TSR S-ILC ILC ,226 ILC ,260 3,813 SCB, End Boxes, etc Auxiliary Load Estimated, [W] Design Capacity, [kw] K Eqv [kw] 8.2 Plug Power, [MW] 2.3 March essbilbao initiative workshop - Paolo Pierini 36

37 cryo distribution and segmentation Project-X cryo r&d plan study existing cryomodules thermal cycling experience stationary, transient, fault, maintenance and commissioning scenarios component over pressure protection study define cryogenic string size limits and segments liquid helium level control strategy development development of tunnel ODH mitigation strategy capital and operational cost optimization lifecycle cost optimization & Cryogenic Plant Cycle heat shields operating parameter optimization heat load analysis static and dynamic loads analysis for components/sub systems define overcapacity and uncertainty factors fault scenarios heat flux study March essbilbao initiative workshop - Paolo Pierini 37

38 HINS - SSR1 conceptual cryomodule layout string on strongback, dressed, aligned, shielded vessel replicates assembly table supports March essbilbao initiative workshop - Paolo Pierini 38

39 Support post pockets strongback concept Support lugs March essbilbao initiative workshop - Paolo Pierini 39

40 spoke/solenoid mounting scheme Analysis of the strongback deflections unders dead loads with support optimization March essbilbao initiative workshop - Paolo Pierini 40

41 Vacuum vessel with internal strongback supports March essbilbao initiative workshop - Paolo Pierini 41

42 EUROTRANS prototype module short, single cavity module under fabrication for the European program on ADS assisted nuclear waste transmutation EUROTRANS (CW) based on the SNS concept of short independently fed and rapidly exchangeable units will be used for long testing for the reliability characterization of components reliability/beam availability is the key goal for ADS linacs, rather than performance INFN MI & IPN Orsay March essbilbao initiative workshop - Paolo Pierini 42

43 emerging issues pressure vessel regulation (in a EU contest) will big machines in the near future require formal certification of components as pressure vessels? non standard materials, welds & T ranges, not in PV codes XFEL effort in collaboration with German TÜV Crash tests performed in Cryomodule Test Bench slow and fast loss of all vacuum spaces (coupler, iso, beam) very successful hydraulic testing of HeTank space at 1.43 MAWP=6 bar, according to safety regulations although ok for beta=1 cavities, treacherous issue for low beta structures resolving issues of integrating different components contributed inkind from several partner into a single object worldwide approach from ILC GDE how can a truly worldwide project deal with many different regulations across the three regions (Europe, Asia, America) also linked to plug-compatibility approach on components March essbilbao initiative workshop - Paolo Pierini 43

44 XFEL crash tests No major damage cavities unchanged pressure behavior in circuits confirmed beam pipe venting shows that pressure drop needs s to propagate to other side of module - Good March essbilbao initiative workshop - Paolo Pierini 44

45 trade offs & choices for cryomodule design Main decision: Filling factor vs. fast module exchange Linac length vs. availability/reliability concerns Real estate gradient is more strongly influenced by module length constraints or cavity ancillaries than from intrinsic cavity accelerating gradient Heat load balances and cryo system layout need in iterations to estabilish layout 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 capabilities LEP has easy access to cold mass, but not compatible with 2 K March essbilbao initiative workshop - Paolo Pierini 45

46 Acknowlegments I want to thank many colleagues, since I have been using their material from privately and publicly available presentations and tutorials, in particular (but not limited to...) Tom Peterson, Arkadiy Klebaner, Tom Nicol, Don Mitchell, Vittorio Parma, Joe Preble,... Whole TTF/XFEL colleagues in DESY & INFN Milano March essbilbao initiative workshop - Paolo Pierini 46